diff --git a/.dev-lib b/.dev-lib
index cb76151efe1..ac8f8ff8bb4 100644
--- a/.dev-lib
+++ b/.dev-lib
@@ -1 +1,2 @@
-SYNC_README_MD=0
\ No newline at end of file
+SYNC_README_MD=0
+PATH_EXCLUDES_PATTERN=includes/lib/*
diff --git a/amp.php b/amp.php
index db1abae1f12..1707e70499a 100644
--- a/amp.php
+++ b/amp.php
@@ -17,16 +17,12 @@
 define( 'AMP__DIR__', dirname( __FILE__ ) );
 define( 'AMP__VERSION', '0.6.0-alpha' );
 
+require_once AMP__DIR__ . '/includes/class-amp-autoloader.php';
+AMP_Autoloader::register();
+
 require_once AMP__DIR__ . '/back-compat/back-compat.php';
 require_once AMP__DIR__ . '/includes/amp-helper-functions.php';
-require_once AMP__DIR__ . '/includes/class-amp-post-type-support.php';
 require_once AMP__DIR__ . '/includes/admin/functions.php';
-require_once AMP__DIR__ . '/includes/admin/class-amp-customizer.php';
-require_once AMP__DIR__ . '/includes/admin/class-amp-post-meta-box.php';
-require_once AMP__DIR__ . '/includes/settings/class-amp-customizer-settings.php';
-require_once AMP__DIR__ . '/includes/settings/class-amp-customizer-design-settings.php';
-require_once AMP__DIR__ . '/includes/actions/class-amp-frontend-actions.php';
-require_once AMP__DIR__ . '/includes/actions/class-amp-paired-post-actions.php';
 
 register_activation_hook( __FILE__, 'amp_activate' );
 function amp_activate() {
@@ -150,7 +146,7 @@ function amp_maybe_add_actions() {
 }
 
 function amp_load_classes() {
-	require_once( AMP__DIR__ . '/includes/class-amp-post-template.php' ); // this loads everything else
+	_deprecated_function( __FUNCTION__, '0.6.0' );
 }
 
 function amp_add_frontend_actions() {
@@ -196,8 +192,6 @@ function amp_render_post( $post ) {
 	}
 	$post_id = $post->ID;
 
-	amp_load_classes();
-
 	/**
 	 * Fires before rendering a post in AMP.
 	 *
@@ -246,9 +240,3 @@ function amp_redirect_old_slug_to_new_url( $link ) {
 
 	return $link;
 }
-
-// Unconditionally load code required when running unit tests.
-if ( function_exists( 'tests_add_filter' ) ) {
-	amp_load_classes();
-	require_once dirname( __FILE__ ) . '/tests/stubs.php';
-}
diff --git a/composer.json b/composer.json
new file mode 100644
index 00000000000..8cbf0c809f4
--- /dev/null
+++ b/composer.json
@@ -0,0 +1,7 @@
+{
+    "name": "automattic/amp-wp",
+    "description": "WordPress plugin for adding AMP support.",
+    "homepage": "https://github.com/Automattic/amp-wp",
+    "type": "wordpress-plugin",
+    "license": "GPL-2.0"
+}
diff --git a/dev-lib b/dev-lib
index 52874e44d9e..4ac4bd20b5a 160000
--- a/dev-lib
+++ b/dev-lib
@@ -1 +1 @@
-Subproject commit 52874e44d9e737dfc8f1e9ca33381f45deb9b636
+Subproject commit 4ac4bd20b5a4b3953eb81a8d2536baef2bf60dcb
diff --git a/includes/actions/class-amp-frontend-actions.php b/includes/actions/class-amp-frontend-actions.php
index baf9a5d441e..8fa8ef1cfab 100644
--- a/includes/actions/class-amp-frontend-actions.php
+++ b/includes/actions/class-amp-frontend-actions.php
@@ -1,14 +1,27 @@
 <?php
-// Callbacks for adding AMP-related things to the main theme
-
-require_once( AMP__DIR__ . '/includes/actions/class-amp-actions.php' );
+/**
+ * Class AMP_Frontend_Actions
+ *
+ * @package AMP
+ */
 
+/**
+ * Class AMP_Frontend_Actions
+ *
+ * Callbacks for adding AMP-related things to the main theme
+ */
 class AMP_Frontend_Actions {
-	
+
+	/**
+	 * Register hooks.
+	 */
 	public static function register_hooks() {
 		add_action( 'wp_head', 'AMP_Frontend_Actions::add_canonical' );
 	}
-	
+
+	/**
+	 * Add canonical link.
+	 */
 	public static function add_canonical() {
 		if ( false === apply_filters( 'add_canonical_link', true ) ) {
 			return;
diff --git a/includes/actions/class-amp-paired-post-actions.php b/includes/actions/class-amp-paired-post-actions.php
index 7ce9b60e02c..a544ac878f6 100644
--- a/includes/actions/class-amp-paired-post-actions.php
+++ b/includes/actions/class-amp-paired-post-actions.php
@@ -1,9 +1,18 @@
 <?php
+/**
+ * Class AMP_Paired_Post_Actions
+ *
+ * @package AMP
+ */
 
-require_once( AMP__DIR__ . '/includes/actions/class-amp-actions.php' );
-
+/**
+ * Class AMP_Paired_Post_Actions
+ */
 class AMP_Paired_Post_Actions extends AMP_Actions {
-	
+
+	/**
+	 * Register hooks.
+	 */
 	public static function register_hooks() {
 		add_action( 'amp_post_template_head', 'AMP_Paired_Post_Actions::add_generator_metadata' );
 		add_action( 'amp_post_template_head', 'AMP_Paired_Post_Actions::add_title' );
@@ -16,42 +25,75 @@ public static function register_hooks() {
 		add_action( 'amp_post_template_data', 'AMP_Paired_Post_Actions::add_analytics_scripts' );
 		add_action( 'amp_post_template_footer', 'AMP_Paired_Post_Actions::add_analytics_data' );
 	}
- 
+
+	/**
+	 * Add title.
+	 *
+	 * @param AMP_Post_Template $amp_template template.
+	 */
 	public static function add_title( $amp_template ) {
 		?>
 		<title><?php echo esc_html( $amp_template->get( 'document_title' ) ); ?></title>
 		<?php
 	}
 
+	/**
+	 * Add canonical link.
+	 *
+	 * @param AMP_Post_Template $amp_template Template.
+	 */
 	public static function add_canonical_link( $amp_template ) {
 		?>
 		<link rel="canonical" href="<?php echo esc_url( $amp_template->get( 'canonical_url' ) ); ?>" />
 		<?php
 	}
 
+	/**
+	 * Print scripts.
+	 *
+	 * @param AMP_Post_Template $amp_template Template.
+	 */
 	public static function add_scripts( $amp_template ) {
 		$scripts = $amp_template->get( 'amp_component_scripts', array() );
 		foreach ( $scripts as $element => $script ) :
-			$custom_type = ($element == 'amp-mustache') ? 'template' : 'element'; ?>
+			$custom_type = ( 'amp-mustache' === $element ) ? 'template' : 'element';
+			?>
 			<script custom-<?php echo esc_attr( $custom_type ); ?>="<?php echo esc_attr( $element ); ?>" src="<?php echo esc_url( $script ); ?>" async></script>
 		<?php endforeach; ?>
 		<script src="<?php echo esc_url( $amp_template->get( 'amp_runtime_script' ) ); ?>" async></script>
 		<?php
 	}
 
+	/**
+	 * Print fonts.
+	 *
+	 * @param AMP_Post_Template $amp_template Template.
+	 */
 	public static function add_fonts( $amp_template ) {
 		$font_urls = $amp_template->get( 'font_urls', array() );
-		foreach ( $font_urls as $slug => $url ) : ?>
+		?>
+		<?php foreach ( $font_urls as $slug => $url ) : ?>
 			<link rel="stylesheet" href="<?php echo esc_url( $url ); ?>">
-		<?php endforeach;
+		<?php endforeach; ?>
+		<?php
 	}
 
+	/**
+	 * Print boilerplate CSS.
+	 *
+	 * @param AMP_Post_Template $amp_template Template.
+	 */
 	public static function add_boilerplate_css( $amp_template ) {
 		?>
 		<style amp-boilerplate>body{-webkit-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-moz-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-ms-animation:-amp-start 8s steps(1,end) 0s 1 normal both;animation:-amp-start 8s steps(1,end) 0s 1 normal both}@-webkit-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-moz-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-ms-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-o-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}</style><noscript><style amp-boilerplate>body{-webkit-animation:none;-moz-animation:none;-ms-animation:none;animation:none}</style></noscript>
 		<?php
 	}
 
+	/**
+	 * Print Schema.org metadata.
+	 *
+	 * @param AMP_Post_Template $amp_template Template.
+	 */
 	public static function add_schemaorg_metadata( $amp_template ) {
 		$metadata = $amp_template->get( 'metadata' );
 		if ( empty( $metadata ) ) {
@@ -62,54 +104,70 @@ public static function add_schemaorg_metadata( $amp_template ) {
 		<?php
 	}
 
+	/**
+	 * Print styles.
+	 *
+	 * @param AMP_Post_Template $amp_template Template.
+	 */
 	public static function add_styles( $amp_template ) {
 		$styles = $amp_template->get( 'post_amp_styles' );
 		if ( ! empty( $styles ) ) {
-			echo '/* Inline styles */' . PHP_EOL;
+			echo '/* Inline styles */' . PHP_EOL; // WPCS: XSS OK.
 			foreach ( $styles as $selector => $declarations ) {
 				$declarations = implode( ';', $declarations ) . ';';
-				printf( '%1$s{%2$s}', $selector, $declarations );
+				printf( '%1$s{%2$s}', $selector, $declarations ); // WPCS: XSS OK.
 			}
 		}
 	}
-	
+
+	/**
+	 * Add analytics scripts.
+	 *
+	 * @param array $data Data.
+	 * @return array Data.
+	 */
 	public static function add_analytics_scripts( $data ) {
 		if ( ! empty( $data['amp_analytics'] ) ) {
 			$data['amp_component_scripts']['amp-analytics'] = 'https://cdn.ampproject.org/v0/amp-analytics-0.1.js';
 		}
 		return $data;
 	}
-	
-	public static  function add_analytics_data( $amp_template ) {
+
+	/**
+	 * Print analytics data.
+	 *
+	 * @param AMP_Post_Template $amp_template Template.
+	 */
+	public static function add_analytics_data( $amp_template ) {
 		$analytics_entries = $amp_template->get( 'amp_analytics' );
 		if ( empty( $analytics_entries ) ) {
 			return;
 		}
-		
+
 		foreach ( $analytics_entries as $id => $analytics_entry ) {
 			if ( ! isset( $analytics_entry['type'], $analytics_entry['attributes'], $analytics_entry['config_data'] ) ) {
 				/* translators: %1$s is analytics entry ID, %2$s is actual entry keys. */
 				_doing_it_wrong( __FUNCTION__, sprintf( esc_html__( 'Analytics entry for %1$s is missing one of the following keys: `type`, `attributes`, or `config_data` (array keys: %2$s)', 'amp' ), esc_html( $id ), esc_html( implode( ', ', array_keys( $analytics_entry ) ) ) ), '0.3.2' );
 				continue;
 			}
-			
+
 			$script_element = AMP_HTML_Utils::build_tag( 'script', array(
 				'type' => 'application/json',
 			), wp_json_encode( $analytics_entry['config_data'] ) );
-			
+
 			$amp_analytics_attr = array_merge( array(
 				'id' => $id,
 				'type' => $analytics_entry['type'],
 			), $analytics_entry['attributes'] );
-			
-			echo AMP_HTML_Utils::build_tag( 'amp-analytics', $amp_analytics_attr, $script_element );
+
+			echo AMP_HTML_Utils::build_tag( 'amp-analytics', $amp_analytics_attr, $script_element ); // WPCS: XSS OK.
 		}
 	}
 
 	/**
-	 * Add AMP generator metadata.
+	 * Print AMP generator metadata.
 	 *
-	 * @param object $amp_template AMP_Post_Template object.
+	 * @param AMP_Post_Template $amp_template AMP_Post_Template object.
 	 * @since 0.6
 	 */
 	public static function add_generator_metadata( $amp_template ) {
diff --git a/includes/admin/functions.php b/includes/admin/functions.php
index 62bbc0e69f8..f033a620d0b 100644
--- a/includes/admin/functions.php
+++ b/includes/admin/functions.php
@@ -1,9 +1,6 @@
 <?php
 // Callbacks for adding AMP-related things to the admin.
 
-require_once AMP__DIR__ . '/includes/options/class-amp-options-menu.php';
-require_once AMP__DIR__ . '/includes/options/views/class-amp-options-manager.php';
-
 define( 'AMP_CUSTOMIZER_QUERY_VAR', 'customize_amp' );
 
 /**
@@ -31,6 +28,11 @@ function amp_maybe_init_customizer() {
 	add_action( 'admin_menu', 'amp_add_customizer_link' );
 }
 
+/**
+ * Get permalink for the first AMP-eligible post.
+ *
+ * @return string|null
+ */
 function amp_admin_get_preview_permalink() {
 	/**
 	 * Filter the post type to retrieve the latest for use in the AMP template customizer.
@@ -40,7 +42,7 @@ function amp_admin_get_preview_permalink() {
 	$post_type = (string) apply_filters( 'amp_customizer_post_type', 'post' );
 
 	if ( ! post_type_supports( $post_type, 'amp' ) ) {
-		return;
+		return null;
 	}
 
 	$post_ids = get_posts( array(
diff --git a/includes/amp-helper-functions.php b/includes/amp-helper-functions.php
index 1d909a99cd1..a1f104a4d99 100644
--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -1,11 +1,17 @@
 <?php
+/**
+ * AMP Helper Functions
+ *
+ * @package AMP
+ */
 
 /**
- * Get AMP permalink.
+ * Retrieves the full AMP-specific permalink for the given post ID.
  *
  * @since 0.1
  *
  * @param int $post_id Post ID.
+ *
  * @return string AMP permalink.
  */
 function amp_get_permalink( $post_id ) {
@@ -27,7 +33,7 @@ function amp_get_permalink( $post_id ) {
 	}
 
 	$parsed_url = wp_parse_url( get_permalink( $post_id ) );
-	$structure = get_option( 'permalink_structure' );
+	$structure  = get_option( 'permalink_structure' );
 	if ( empty( $structure ) || ! empty( $parsed_url['query'] ) || is_post_type_hierarchical( get_post_type( $post_id ) ) ) {
 		$amp_url = add_query_arg( AMP_QUERY_VAR, '', get_permalink( $post_id ) );
 	} else {
@@ -64,6 +70,8 @@ function post_supports_amp( $post ) {
  * Are we currently on an AMP URL?
  *
  * Note: will always return `false` if called before the `parse_query` hook.
+ *
+ * @return bool Whether it is the AMP endpoint.
  */
 function is_amp_endpoint() {
 	if ( 0 === did_action( 'parse_query' ) ) {
@@ -73,6 +81,12 @@ function is_amp_endpoint() {
 	return false !== get_query_var( AMP_QUERY_VAR, false );
 }
 
+/**
+ * Get AMP asset URL.
+ *
+ * @param string $file Relative path to file in assets directory.
+ * @return string URL.
+ */
 function amp_get_asset_url( $file ) {
 	return plugins_url( sprintf( 'assets/%s', $file ), AMP__FILE__ );
 }
diff --git a/includes/class-amp-autoloader.php b/includes/class-amp-autoloader.php
new file mode 100644
index 00000000000..74cdc586523
--- /dev/null
+++ b/includes/class-amp-autoloader.php
@@ -0,0 +1,137 @@
+<?php
+/**
+ * Class AMP_Autoloader
+ *
+ * @package AMP
+ */
+
+/**
+ * Autoload the classes used by the AMP plugin.
+ *
+ * Class AMP_Autoloader
+ */
+class AMP_Autoloader {
+
+	/**
+	 * Map of Classname to relative filepath sans extension.
+	 *
+	 * @note We omitted the leading slash and the .php extension from each
+	 *       relative filepath because they are redundant and to include
+	 *       them would take up unnecessary bytes of memory at runtime.
+	 *
+	 * @example Format (note no leading / and no .php extension):
+	 *
+	 *  array(
+	 *      'Class_Name1' =>  'subdir-of-includes/filename1',
+	 *      'Class_Name2' =>  '2nd-subdir-of-includes/filename2',
+	 *  );
+	 *
+	 * @var string[]
+	 */
+	private static $_classmap = array(
+		'AMP_Actions'                                                    => 'includes/actions/class-amp-actions',
+		'AMP_Frontend_Actions'                                           => 'includes/actions/class-amp-frontend-actions',
+		'AMP_Paired_Post_Actions'                                        => 'includes/actions/class-amp-paired-post-actions',
+		'AMP_Template_Customizer'                                        => 'includes/admin/class-amp-customizer',
+		'AMP_Post_Meta_Box'                                              => 'includes/admin/class-amp-post-meta-box',
+		'AMP_Post_Type_Support'                                          => 'includes/class-amp-post-type-support',
+		'AMP_Base_Embed_Handler'                                         => 'includes/embeds/class-amp-base-embed-handler',
+		'AMP_DailyMotion_Embed_Handler'                                  => 'includes/embeds/class-amp-dailymotion-embed',
+		'AMP_Facebook_Embed_Handler'                                     => 'includes/embeds/class-amp-facebook-embed',
+		'AMP_Gallery_Embed_Handler'                                      => 'includes/embeds/class-amp-gallery-embed',
+		'AMP_Instagram_Embed_Handler'                                    => 'includes/embeds/class-amp-instagram-embed',
+		'AMP_Pinterest_Embed_Handler'                                    => 'includes/embeds/class-amp-pinterest-embed',
+		'AMP_SoundCloud_Embed_Handler'                                   => 'includes/embeds/class-amp-soundcloud-embed',
+		'AMP_Twitter_Embed_Handler'                                      => 'includes/embeds/class-amp-twitter-embed',
+		'AMP_Vimeo_Embed_Handler'                                        => 'includes/embeds/class-amp-vimeo-embed',
+		'AMP_Vine_Embed_Handler'                                         => 'includes/embeds/class-amp-vine-embed',
+		'AMP_YouTube_Embed_Handler'                                      => 'includes/embeds/class-amp-youtube-embed',
+		'FastImage'                                                      => 'includes/lib/fastimage/class-fastimage',
+		'WillWashburn\\Stream\\Exception\\StreamBufferTooSmallException' => 'includes/lib/fasterimage/Stream/Exception/StreamBufferTooSmallException',
+		'WillWashburn\\Stream\\StreamableInterface'                      => 'includes/lib/fasterimage/Stream/StreamableInterface',
+		'WillWashburn\\Stream\\Stream'                                   => 'includes/lib/fasterimage/Stream/Stream',
+		'FasterImage\\Exception\\InvalidImageException'                  => 'includes/lib/fasterimage/Exception/InvalidImageException',
+		'FasterImage\\ExifParser'                                        => 'includes/lib/fasterimage/ExifParser',
+		'FasterImage\\ImageParser'                                       => 'includes/lib/fasterimage/ImageParser',
+		'FasterImage\\FasterImage'                                       => 'includes/lib/fasterimage/FasterImage',
+		'AMP_Analytics_Options_Submenu'                                  => 'includes/options/class-amp-analytics-options-submenu',
+		'AMP_Options_Menu'                                               => 'includes/options/class-amp-options-menu',
+		'AMP_Options_Manager'                                            => 'includes/options/class-amp-options-manager',
+		'AMP_Analytics_Options_Submenu_Page'                             => 'includes/options/views/class-amp-analytics-options-submenu-page',
+		'AMP_Options_Menu_Page'                                          => 'includes/options/views/class-amp-options-menu-page',
+		'AMP_Rule_Spec'                                                  => 'includes/sanitizers/class-amp-rule-spec',
+		'AMP_Allowed_Tags_Generated'                                     => 'includes/sanitizers/class-amp-allowed-tags-generated',
+		'AMP_Audio_Sanitizer'                                            => 'includes/sanitizers/class-amp-audio-sanitizer',
+		'AMP_Base_Sanitizer'                                             => 'includes/sanitizers/class-amp-base-sanitizer',
+		'AMP_Blacklist_Sanitizer'                                        => 'includes/sanitizers/class-amp-blacklist-sanitizer',
+		'AMP_Iframe_Sanitizer'                                           => 'includes/sanitizers/class-amp-iframe-sanitizer',
+		'AMP_Img_Sanitizer'                                              => 'includes/sanitizers/class-amp-img-sanitizer',
+		'AMP_Playbuzz_Sanitizer'                                         => 'includes/sanitizers/class-amp-playbuzz-sanitizer',
+		'AMP_Style_Sanitizer'                                            => 'includes/sanitizers/class-amp-style-sanitizer',
+		'AMP_Tag_And_Attribute_Sanitizer'                                => 'includes/sanitizers/class-amp-tag-and-attribute-sanitizer',
+		'AMP_Video_Sanitizer'                                            => 'includes/sanitizers/class-amp-video-sanitizer',
+		'AMP_Customizer_Design_Settings'                                 => 'includes/settings/class-amp-customizer-design-settings',
+		'AMP_Customizer_Settings'                                        => 'includes/settings/class-amp-customizer-settings',
+		'AMP_Content'                                                    => 'includes/templates/class-amp-content',
+		'AMP_Content_Sanitizer'                                          => 'includes/templates/class-amp-content-sanitizer',
+		'AMP_Post_Template'                                              => 'includes/templates/class-amp-post-template',
+		'AMP_DOM_Utils'                                                  => 'includes/utils/class-amp-dom-utils',
+		'AMP_HTML_Utils'                                                 => 'includes/utils/class-amp-html-utils',
+		'AMP_Image_Dimension_Extractor'                                  => 'includes/utils/class-amp-image-dimension-extractor',
+		'AMP_String_Utils'                                               => 'includes/utils/class-amp-string-utils',
+		'AMP_WP_Utils'                                                   => 'includes/utils/class-amp-wp-utils',
+		'WPCOM_AMP_Polldaddy_Embed'                                      => 'includes/wpcom/class-amp-polldaddy-embed',
+		'AMP_Test_Stub_Sanitizer'                                        => 'tests/stubs',
+		'AMP_Test_World_Sanitizer'                                       => 'tests/stubs',
+	);
+
+	/**
+	 * Is registered.
+	 *
+	 * @var bool
+	 */
+	public static $is_registered = false;
+
+	/**
+	 * Perform the autoload on demand when requested by PHP runtime.
+	 *
+	 * Design Goal: Execute as few lines of code as possible each call.
+	 *
+	 * @since 0.6.0
+	 *
+	 * @param string $class_name Class name.
+	 */
+	protected static function autoload( $class_name ) {
+		if ( ! isset( self::$_classmap[ $class_name ] ) ) {
+			return;
+		}
+		$filepath = self::$_classmap[ $class_name ];
+		require AMP__DIR__ . "/{$filepath}.php";
+	}
+
+	/**
+	 * Registers this autoloader to PHP.
+	 *
+	 * @since 0.6.0
+	 *
+	 * Called at the end of this file; calling a second time has no effect.
+	 */
+	public static function register() {
+		if ( ! self::$is_registered ) {
+			spl_autoload_register( array( __CLASS__, 'autoload' ) );
+			self::$is_registered = true;
+		}
+	}
+
+	/**
+	 * Allows an extensions plugin to register a class and its file for autoloading
+	 *
+	 * @since 0.6.0
+	 *
+	 * @param string $class_name Full classname (include namespace if applicable).
+	 * @param string $filepath   Absolute filepath to class file, including .php extension.
+	 */
+	public static function register_autoload_class( $class_name, $filepath ) {
+		self::$_classmap[ $class_name ] = '!' . $filepath;
+	}
+}
diff --git a/includes/embeds/class-amp-base-embed-handler.php b/includes/embeds/class-amp-base-embed-handler.php
index d9152d0a420..8e3133e62d2 100644
--- a/includes/embeds/class-amp-base-embed-handler.php
+++ b/includes/embeds/class-amp-base-embed-handler.php
@@ -1,8 +1,15 @@
 <?php
+/**
+ * Class AMP_Base_Embed_Handler
+ *
+ * Used by some children.
+ *
+ * @package  AMP
+ */
 
-// Used by some children
-require_once( AMP__DIR__ . '/includes/utils/class-amp-html-utils.php' );
-
+/**
+ * Class AMP_Base_Embed_Handler
+ */
 abstract class AMP_Base_Embed_Handler {
 	protected $DEFAULT_WIDTH = 600;
 	protected $DEFAULT_HEIGHT = 480;
diff --git a/includes/embeds/class-amp-dailymotion-embed.php b/includes/embeds/class-amp-dailymotion-embed.php
index be28426551c..1ddac3a07a2 100644
--- a/includes/embeds/class-amp-dailymotion-embed.php
+++ b/includes/embeds/class-amp-dailymotion-embed.php
@@ -1,8 +1,15 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
-// Much of this class is borrowed from Jetpack embeds
+/**
+ * Class AMP_DailyMotion_Embed_Handler
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_DailyMotion_Embed_Handler
+ *
+ * Much of this class is borrowed from Jetpack embeds
+ */
 class AMP_DailyMotion_Embed_Handler extends AMP_Base_Embed_Handler {
 
 	const URL_PATTERN = '#https?:\/\/(www\.)?dailymotion\.com\/video\/.*#i';
diff --git a/includes/embeds/class-amp-facebook-embed.php b/includes/embeds/class-amp-facebook-embed.php
index 6761444759b..230429268ce 100644
--- a/includes/embeds/class-amp-facebook-embed.php
+++ b/includes/embeds/class-amp-facebook-embed.php
@@ -1,7 +1,13 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
+/**
+ * Class AMP_Facebook_Embed_Handler
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Facebook_Embed_Handler
+ */
 class AMP_Facebook_Embed_Handler extends AMP_Base_Embed_Handler {
 	const URL_PATTERN = '#https?://(www\.)?facebook\.com/.*#i';
 
diff --git a/includes/embeds/class-amp-gallery-embed.php b/includes/embeds/class-amp-gallery-embed.php
index 2e51bb426e0..199912b41b1 100644
--- a/includes/embeds/class-amp-gallery-embed.php
+++ b/includes/embeds/class-amp-gallery-embed.php
@@ -1,7 +1,13 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
+/**
+ * Class AMP_Gallery_Embed_Handler
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Gallery_Embed_Handler
+ */
 class AMP_Gallery_Embed_Handler extends AMP_Base_Embed_Handler {
 	private static $script_slug = 'amp-carousel';
 	private static $script_src = 'https://cdn.ampproject.org/v0/amp-carousel-0.1.js';
diff --git a/includes/embeds/class-amp-instagram-embed.php b/includes/embeds/class-amp-instagram-embed.php
index 1a03e2ad384..adb002680c5 100644
--- a/includes/embeds/class-amp-instagram-embed.php
+++ b/includes/embeds/class-amp-instagram-embed.php
@@ -1,8 +1,15 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
-// Much of this class is borrowed from Jetpack embeds
+/**
+ * Class AMP_Instagram_Embed_Handler
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Instagram_Embed_Handler
+ *
+ * Much of this class is borrowed from Jetpack embeds
+ */
 class AMP_Instagram_Embed_Handler extends AMP_Base_Embed_Handler {
 	const SHORT_URL_HOST = 'instagr.am';
 	const URL_PATTERN = '#http(s?)://(www\.)?instagr(\.am|am\.com)/p/([^/?]+)#i';
diff --git a/includes/embeds/class-amp-pinterest-embed.php b/includes/embeds/class-amp-pinterest-embed.php
index 1600cc5d41f..aedf6c66e69 100644
--- a/includes/embeds/class-amp-pinterest-embed.php
+++ b/includes/embeds/class-amp-pinterest-embed.php
@@ -1,7 +1,13 @@
 <?php
+/**
+ * Class AMP_Pinterest_Embed_Handler
+ *
+ * @package AMP
+ */
 
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php');
-
+/**
+ * Class AMP_Pinterest_Embed_Handler
+ */
 class AMP_Pinterest_Embed_Handler extends AMP_Base_Embed_Handler {
 
 	const URL_PATTERN = '#https?://(www\.)?pinterest\.com/pin/.*#i';
diff --git a/includes/embeds/class-amp-soundcloud-embed.php b/includes/embeds/class-amp-soundcloud-embed.php
index ea89d375823..4e8b757e235 100644
--- a/includes/embeds/class-amp-soundcloud-embed.php
+++ b/includes/embeds/class-amp-soundcloud-embed.php
@@ -1,7 +1,13 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
+/**
+ * Class AMP_SoundCloud_Embed_Handler
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_SoundCloud_Embed_Handler
+ */
 class AMP_SoundCloud_Embed_Handler extends AMP_Base_Embed_Handler {
 	const URL_PATTERN = '#https?://api\.soundcloud\.com/tracks/.*#i';
 	protected $DEFAULT_HEIGHT = 200;
diff --git a/includes/embeds/class-amp-twitter-embed.php b/includes/embeds/class-amp-twitter-embed.php
index 71cb0fd69fa..c97284032ec 100644
--- a/includes/embeds/class-amp-twitter-embed.php
+++ b/includes/embeds/class-amp-twitter-embed.php
@@ -1,8 +1,15 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
-// Much of this class is borrowed from Jetpack embeds
+/**
+ * Class AMP_Twitter_Embed_Handler
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Twitter_Embed_Handler
+ *
+ *  Much of this class is borrowed from Jetpack embeds
+ */
 class AMP_Twitter_Embed_Handler extends AMP_Base_Embed_Handler {
 	const URL_PATTERN = '#http(s|):\/\/twitter\.com(\/\#\!\/|\/)([a-zA-Z0-9_]{1,20})\/status(es)*\/(\d+)#i';
 
diff --git a/includes/embeds/class-amp-vimeo-embed.php b/includes/embeds/class-amp-vimeo-embed.php
index aca63a723a5..b37e3fb7f84 100644
--- a/includes/embeds/class-amp-vimeo-embed.php
+++ b/includes/embeds/class-amp-vimeo-embed.php
@@ -1,8 +1,15 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
-// Much of this class is borrowed from Jetpack embeds
+/**
+ * Class AMP_Vimeo_Embed_Handler
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Vimeo_Embed_Handler
+ *
+ * Much of this class is borrowed from Jetpack embeds
+ */
 class AMP_Vimeo_Embed_Handler extends AMP_Base_Embed_Handler {
 
 	const URL_PATTERN = '#https?:\/\/(www\.)?vimeo\.com\/.*#i';
diff --git a/includes/embeds/class-amp-vine-embed.php b/includes/embeds/class-amp-vine-embed.php
index bfa0e175aba..9e14b037bd4 100644
--- a/includes/embeds/class-amp-vine-embed.php
+++ b/includes/embeds/class-amp-vine-embed.php
@@ -1,7 +1,13 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
+/**
+ * Class AMP_Vine_Embed_Handler
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Vine_Embed_Handler
+ */
 class AMP_Vine_Embed_Handler extends AMP_Base_Embed_Handler {
 	const URL_PATTERN = '#https?://vine\.co/v/([^/?]+)#i';
 
diff --git a/includes/embeds/class-amp-youtube-embed.php b/includes/embeds/class-amp-youtube-embed.php
index a4d5e8648de..8ff81cc84c5 100644
--- a/includes/embeds/class-amp-youtube-embed.php
+++ b/includes/embeds/class-amp-youtube-embed.php
@@ -1,8 +1,15 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
-// Much of this class is borrowed from Jetpack embeds
+/**
+ * Class AMP_YouTube_Embed_Handler
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_YouTube_Embed_Handler
+ *
+ * Much of this class is borrowed from Jetpack embeds.
+ */
 class AMP_YouTube_Embed_Handler extends AMP_Base_Embed_Handler {
 	const SHORT_URL_HOST = 'youtu.be';
 	// Only handling single videos. Playlists are handled elsewhere.
diff --git a/includes/lib/fasterimage/amp-fasterimage.php b/includes/lib/fasterimage/amp-fasterimage.php
index a0aa536de29..0f9f808657f 100644
--- a/includes/lib/fasterimage/amp-fasterimage.php
+++ b/includes/lib/fasterimage/amp-fasterimage.php
@@ -1,25 +1,27 @@
 <?php
+/**
+ * Functions for FasterImage.
+ *
+ * @package AMP
+ */
 
+/**
+ * Load classes for FasterImage.
+ *
+ * This is obsolete now that there is an autoloader.
+ *
+ * @deprecated
+ */
 function amp_load_fasterimage_classes() {
-	// We're not using composer to pull in FasterImage so we need to load the files manually
-	$fasterimage__DIR__ = dirname( __FILE__ );
-
-	// Stream files
-	require_once( $fasterimage__DIR__ . '/Stream/Exception/StreamBufferTooSmallException.php' );
-	require_once( $fasterimage__DIR__ . '/Stream/StreamableInterface.php' );
-	require_once( $fasterimage__DIR__ . '/Stream/Stream.php' );
-
-	// FasterImage files
-	require_once( $fasterimage__DIR__ . '/Exception/InvalidImageException.php' );
-	require_once( $fasterimage__DIR__ . '/ExifParser.php' );
-	require_once( $fasterimage__DIR__ . '/ImageParser.php' );
-	require_once( $fasterimage__DIR__ . '/FasterImage.php' );
+	_deprecated_function( __FUNCTION__, '0.6.0' );
 }
 
+/**
+ * Get FasterImage client for user agent.
+ *
+ * @param string $user_agent User Agent.
+ * @return \FasterImage\FasterImage Instance.
+ */
 function amp_get_fasterimage_client( $user_agent ) {
-	if ( ! class_exists( 'FasterImage\FasterImage' ) ) {
-		amp_load_fasterimage_classes();
-	}
-
 	return new FasterImage\FasterImage( $user_agent );
 }
diff --git a/includes/options/class-amp-analytics-options-submenu.php b/includes/options/class-amp-analytics-options-submenu.php
index 862c173f72a..82da7dba0b0 100644
--- a/includes/options/class-amp-analytics-options-submenu.php
+++ b/includes/options/class-amp-analytics-options-submenu.php
@@ -1,8 +1,13 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/options/views/class-amp-analytics-options-submenu-page.php' );
-require_once( AMP__DIR__ . '/includes/utils/class-amp-html-utils.php' );
-
+/**
+ * Class AMP_Analytics_Options_Submenu
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Analytics_Options_Submenu
+ */
 class AMP_Analytics_Options_Submenu {
 
 	private $parent_menu_slug;
diff --git a/includes/options/views/class-amp-options-manager.php b/includes/options/class-amp-options-manager.php
similarity index 97%
rename from includes/options/views/class-amp-options-manager.php
rename to includes/options/class-amp-options-manager.php
index 0a2c8594844..f754c63eec5 100644
--- a/includes/options/views/class-amp-options-manager.php
+++ b/includes/options/class-amp-options-manager.php
@@ -1,13 +1,10 @@
 <?php
 /**
- * AMP_Options_Manager class.
+ * Class AMP_Options_Manager.
  *
- * @todo This file is located in the wrong directory.
  * @package AMP
  */
 
-require_once AMP__DIR__ . '/includes/utils/class-amp-html-utils.php';
-
 /**
  * Class AMP_Options_Manager
  */
diff --git a/includes/options/class-amp-options-menu.php b/includes/options/class-amp-options-menu.php
index 88d299f3947..0db894b48e5 100644
--- a/includes/options/class-amp-options-menu.php
+++ b/includes/options/class-amp-options-menu.php
@@ -5,10 +5,6 @@
  * @package AMP
  */
 
-// Includes.
-require_once AMP__DIR__ . '/includes/options/class-amp-analytics-options-submenu.php';
-require_once AMP__DIR__ . '/includes/options/views/class-amp-options-manager.php';
-
 /**
  * AMP_Options_Menu class.
  */
diff --git a/includes/options/views/class-amp-analytics-options-submenu-page.php b/includes/options/views/class-amp-analytics-options-submenu-page.php
index 06787f86ac6..f1efc645c9b 100644
--- a/includes/options/views/class-amp-analytics-options-submenu-page.php
+++ b/includes/options/views/class-amp-analytics-options-submenu-page.php
@@ -1,13 +1,18 @@
 <?php
+/**
+ * Class AMP_Analytics_Options_Submenu_Page
+ *
+ * @package AMP
+ */
 
-require_once( AMP__DIR__ . '/includes/options/views/class-amp-options-manager.php' );
-
+/**
+ * Class AMP_Analytics_Options_Submenu_Page
+ */
 class AMP_Analytics_Options_Submenu_Page {
 
 	private function render_entry( $id = '', $type = '', $config = '' ) {
 		$is_existing_entry = ! empty( $id );
 
-		$analytics_title = false;
 		if ( $is_existing_entry ) {
 			$entry_slug = sprintf( '%s%s', ( $type ? $type . '-' : '' ), substr( $id, -6 ) );
 			$analytics_title = sprintf( __( 'Analytics: %s', 'amp' ), $entry_slug );
diff --git a/includes/sanitizers/class-amp-audio-sanitizer.php b/includes/sanitizers/class-amp-audio-sanitizer.php
index a90040a7cce..26d506d51b0 100644
--- a/includes/sanitizers/class-amp-audio-sanitizer.php
+++ b/includes/sanitizers/class-amp-audio-sanitizer.php
@@ -1,53 +1,121 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-base-sanitizer.php' );
-
+/**
+ * Class AMP_Audio_Sanitizer
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Audio_Sanitizer
+ *
+ * Converts <audio> tags to <amp-audio>
+ */
 class AMP_Audio_Sanitizer extends AMP_Base_Sanitizer {
+
+	/**
+	 * Tag.
+	 *
+	 * @var string HTML audio tag to identify and replace with AMP version.
+	 * @since 0.2
+	 */
 	public static $tag = 'audio';
 
+	/**
+	 * Script slug.
+	 *
+	 * @var string AMP HTML audio tag to use in place of HTML's 'audio' tag.
+	 *
+	 * @since 0.2
+	 */
 	private static $script_slug = 'amp-audio';
+
+	/**
+	 * Script src.
+	 *
+	 * @var string URL to AMP Project's Audio element javascript file found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 */
 	private static $script_src = 'https://cdn.ampproject.org/v0/amp-audio-0.1.js';
 
+	/**
+	 * Return one element array containing AMP HTML audio tag and respective Javascript URL
+	 *
+	 * HTML tags and Javascript URLs found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 *
+	 * @return string[] Returns AMP HTML audio tag as array key and Javascript URL as array value,
+	 *                  respectively. Will return an empty array if sanitization has yet to be run
+	 *                  or if it did not find any HTML audio elements to convert to AMP equivalents.
+	 */
 	public function get_scripts() {
 		if ( ! $this->did_convert_elements ) {
 			return array();
 		}
-
 		return array( self::$script_slug => self::$script_src );
 	}
 
+	/**
+	 * Sanitize the <audio> elements from the HTML contained in this instance's DOMDocument.
+	 *
+	 * @since 0.2
+	 */
 	public function sanitize() {
-		$nodes = $this->dom->getElementsByTagName( self::$tag );
+		$nodes     = $this->dom->getElementsByTagName( self::$tag );
 		$num_nodes = $nodes->length;
 		if ( 0 === $num_nodes ) {
 			return;
 		}
 
 		for ( $i = $num_nodes - 1; $i >= 0; $i-- ) {
-			$node = $nodes->item( $i );
+			$node           = $nodes->item( $i );
 			$old_attributes = AMP_DOM_Utils::get_node_attributes_as_assoc_array( $node );
 
 			$new_attributes = $this->filter_attributes( $old_attributes );
 
 			$new_node = AMP_DOM_Utils::create_node( $this->dom, 'amp-audio', $new_attributes );
 
-			// TODO: `source` does not have closing tag, and DOMDocument doesn't handle it well.
 			foreach ( $node->childNodes as $child_node ) {
+
+				/**
+				 * Child node.
+				 *
+				 * @todo: Fix when `source` has no closing tag as DOMDocument does not handle well.
+				 *
+				 * @var DOMNode $child_node
+				 */
+
 				$new_child_node = $child_node->cloneNode( true );
+				if ( ! $new_child_node instanceof DOMElement ) {
+					continue;
+				}
+
 				$old_child_attributes = AMP_DOM_Utils::get_node_attributes_as_assoc_array( $new_child_node );
 				$new_child_attributes = $this->filter_attributes( $old_child_attributes );
 
-				// Only append source tags with a valid src attribute
-				if ( ! empty( $new_child_attributes['src'] ) && 'source' === $new_child_node->tagName ) {
-					$new_node->appendChild( $new_child_node );
+				if ( empty( $new_child_attributes['src'] ) ) {
+					continue;
+				}
+				if ( 'source' !== $new_child_node->tagName ) {
+					continue;
 				}
+
+				// The textContent is invalid for `source` nodes.
+				$new_child_node->textContent = null;
+
+				// Only append source tags with a valid src attribute.
+				$new_node->appendChild( $new_child_node );
+
 			}
 
-			// If the node has at least one valid source, replace the old node with it.
-			// Otherwise, just remove the node.
-			//
-			// TODO: Add a fallback handler.
-			// See: https://github.com/ampproject/amphtml/issues/2261
+			/**
+			 * If the node has at least one valid source, replace the old node with it.
+			 * Otherwise, just remove the node.
+			 *
+			 * @todo: Add a fallback handler.
+			 * @see: https://github.com/ampproject/amphtml/issues/2261
+			 */
 			if ( 0 === $new_node->childNodes->length && empty( $new_attributes['src'] ) ) {
 				$node->parentNode->removeChild( $node );
 			} else {
@@ -58,6 +126,24 @@ public function sanitize() {
 		}
 	}
 
+	/**
+	 * "Filter" HTML attributes for <amp-audio> elements.
+	 *
+	 * @since 0.2
+	 *
+	 * @param string[] $attributes {
+	 *      Attributes.
+	 *
+	 *      @type string $src Audio URL - Empty if HTTPS required per $this->args['require_https_src']
+	 *      @type int $width <audio> attribute - Set to numeric value if px or %
+	 *      @type int $height <audio> attribute - Set to numeric value if px or %
+	 *      @type string $class <audio> attribute - Pass along if found
+	 *      @type bool $loop <audio> attribute - Convert 'false' to empty string ''
+	 *      @type bool $muted <audio> attribute - Convert 'false' to empty string ''
+	 *      @type bool $autoplay <audio> attribute - Convert 'false' to empty string ''
+	 * }
+	 * @return array Returns HTML attributes; removes any not specifically declared above from input.
+	 */
 	private function filter_attributes( $attributes ) {
 		$out = array();
 
@@ -83,7 +169,7 @@ private function filter_attributes( $attributes ) {
 					}
 					break;
 
-				default;
+				default:
 					break;
 			}
 		}
diff --git a/includes/sanitizers/class-amp-base-sanitizer.php b/includes/sanitizers/class-amp-base-sanitizer.php
index 3bd7f7ba2b0..4e421ed6ea6 100644
--- a/includes/sanitizers/class-amp-base-sanitizer.php
+++ b/includes/sanitizers/class-amp-base-sanitizer.php
@@ -1,36 +1,138 @@
 <?php
+/**
+ * Class AMP_Base_Sanitizer
+ *
+ * @package AMP
+ */
 
+/**
+ * Class AMP_Base_Sanitizer
+ */
 abstract class AMP_Base_Sanitizer {
+
+	/**
+	 * Value used with the height attribute in an $attributes parameter is empty.
+	 *
+	 * @since 0.3.3
+	 *
+	 * @const int
+	 */
 	const FALLBACK_HEIGHT = 400;
 
+	/**
+	 * Placeholder for default args, to be set in child classes.
+	 *
+	 * @since 0.2
+	 *
+	 * @var array
+	 */
 	protected $DEFAULT_ARGS = array();
 
+	/**
+	 * DOM.
+	 *
+	 * @var DOMDocument A standard PHP representation of an HTML document in object form.
+	 *
+	 * @since 0.2
+	 */
 	protected $dom;
+
+	/**
+	 * Array of flags used to control sanitization.
+	 *
+	 * @var array {
+	 *      @type int $content_max_width
+	 *      @type bool $add_placeholder
+	 *      @type bool $require_https_src
+	 *      @type string[] $amp_allowed_tags
+	 *      @type string[] $amp_globally_allowed_attributes
+	 *      @type string[] $amp_layout_allowed_attributes
+	 * }
+	 */
 	protected $args;
+
+	/**
+	 * Flag to be set in child class' sanitize() method indicating if the
+	 * HTML contained in the DOMDocument has been santized yet or not.
+	 *
+	 * @since 0.2
+	 *
+	 * @var bool
+	 */
 	protected $did_convert_elements = false;
 
+	/**
+	 * AMP_Base_Sanitizer constructor.
+	 *
+	 * @since 0.2
+	 *
+	 * @param DOMDocument $dom Represents the HTML document to sanitize.
+	 * @param array       $args array {
+	 *      Args.
+	 *
+	 *      @type int $content_max_width
+	 *      @type bool $add_placeholder
+	 *      @type bool $require_https_src
+	 *      @type string[] $amp_allowed_tags
+	 *      @type string[] $amp_globally_allowed_attributes
+	 *      @type string[] $amp_layout_allowed_attributes
+	 * }
+	 */
 	public function __construct( $dom, $args = array() ) {
-		$this->dom = $dom;
+		$this->dom  = $dom;
 		$this->args = array_merge( $this->DEFAULT_ARGS, $args );
 	}
 
+	/**
+	 * Sanitize the HTML contained in the DOMDocument received by the constructor
+	 */
 	abstract public function sanitize();
 
+	/**
+	 * Return array of values that would be valid as an HTML `script` element.
+	 *
+	 * Array keys are AMP element names and array values are their respective
+	 * Javascript URLs from https://cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 *
+	 * @return string[] This are empty in this the base class.
+	 */
 	public function get_scripts() {
 		return array();
 	}
 
+	/**
+	 * Return array of values that would be valid as an HTML `style` attribute.
+	 *
+	 * @since 0.4
+	 *
+	 * @return string[] This are empty in this the base class.
+	 */
 	public function get_styles() {
 		return array();
 	}
 
+	/**
+	 * Get HTML body as DOMElement from DOMDocument received by the constructor.
+	 *
+	 * @return DOMElement
+	 */
 	protected function get_body_node() {
 		return $this->dom->getElementsByTagName( 'body' )->item( 0 );
 	}
 
+	/**
+	 * Sanitizes a CSS dimension specifier while being sensitive to dimension context.
+	 *
+	 * @param string $value A valid CSS dimension specifier; e.g. 50, 50px, 50%.
+	 * @param string $dimension 'width' or ignored. 'width' only affects $values ending in '%'.
+	 *
+	 * @return float|int|string Returns a numeric dimension value, or an empty string.
+	 */
 	public function sanitize_dimension( $value, $dimension ) {
 		if ( empty( $value ) ) {
-			return $value;
+			return '';
 		}
 
 		if ( false !== filter_var( $value, FILTER_VALIDATE_INT ) ) {
@@ -51,6 +153,20 @@ public function sanitize_dimension( $value, $dimension ) {
 		return '';
 	}
 
+	/**
+	 * Enforce fixed height.
+	 *
+	 * @param string[] $attributes {
+	 *      Attributes.
+	 *
+	 *      @type int $height
+	 *      @type int $width
+	 *      @type string $sizes
+	 *      @type string $class
+	 *      @type string $layout
+	 * }
+	 * @return string[]
+	 */
 	public function enforce_fixed_height( $attributes ) {
 		if ( empty( $attributes['height'] ) ) {
 			unset( $attributes['width'] );
@@ -71,6 +187,17 @@ public function enforce_fixed_height( $attributes ) {
 	 *
 	 * See https://github.com/ampproject/amphtml/issues/1280#issuecomment-171533526
 	 * See https://github.com/Automattic/amp-wp/issues/101
+	 *
+	 * @param string[] $attributes {
+	 *      Attributes.
+	 *
+	 *      @type int $height
+	 *      @type int $width
+	 *      @type string $sizes
+	 *      @type string $class
+	 *      @type string $layout
+	 * }
+	 * @return string[]
 	 */
 	public function enforce_sizes_attribute( $attributes ) {
 		if ( ! isset( $attributes['width'], $attributes['height'] ) ) {
@@ -89,6 +216,25 @@ public function enforce_sizes_attribute( $attributes ) {
 		return $attributes;
 	}
 
+	/**
+	 * Adds or appends key and value to list of attributes
+	 *
+	 * Adds key and value to list of attributes, or if the key already exists in the array
+	 * it concatenates to existing attribute separator by a space or other supplied separator.
+	 *
+	 * @param string[] $attributes {
+	 *      Attributes.
+	 *
+	 *      @type int $height
+	 *      @type int $width
+	 *      @type string $sizes
+	 *      @type string $class
+	 *      @type string $layout
+	 * }
+	 * @param string   $key       Valid associative array index to add.
+	 * @param string   $value     Value to add or append to array indexed at the key.
+	 * @param string   $separator Optional; defaults to space but some other separator if needed.
+	 */
 	public function add_or_append_attribute( &$attributes, $key, $value, $separator = ' ' ) {
 		if ( isset( $attributes[ $key ] ) ) {
 			$attributes[ $key ] .= $separator . $value;
@@ -99,22 +245,23 @@ public function add_or_append_attribute( &$attributes, $key, $value, $separator
 
 	/**
 	 * Decide if we should remove a src attribute if https is required.
+	 *
 	 * If not required, the implementing class may want to try and force https instead.
 	 *
-	 * @param string $src
-	 * @param boolean $force_https
-	 * @return string
+	 * @param string  $src         URL to convert to HTTPS if forced, or made empty if $args['require_https_src'].
+	 * @param boolean $force_https Force setting of HTTPS if true.
+	 * @return string URL which may have been updated with HTTPS, or may have been made empty.
 	 */
 	public function maybe_enforce_https_src( $src, $force_https = false ) {
 		$protocol = strtok( $src, ':' );
 		if ( 'https' !== $protocol ) {
-			// Check if https is required
+			// Check if https is required.
 			if ( isset( $this->args['require_https_src'] ) && true === $this->args['require_https_src'] ) {
 				// Remove the src. Let the implementing class decide what do from here.
 				$src = '';
 			} elseif ( ( ! isset( $this->args['require_https_src'] ) || false === $this->args['require_https_src'] )
 				&& true === $force_https ) {
-				// Don't remove the src, but force https instead
+				// Don't remove the src, but force https instead.
 				$src = set_url_scheme( $src, 'https' );
 			}
 		}
diff --git a/includes/sanitizers/class-amp-blacklist-sanitizer.php b/includes/sanitizers/class-amp-blacklist-sanitizer.php
index 059912a7b78..bc833b8005a 100644
--- a/includes/sanitizers/class-amp-blacklist-sanitizer.php
+++ b/includes/sanitizers/class-amp-blacklist-sanitizer.php
@@ -1,6 +1,9 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-base-sanitizer.php' );
+/**
+ * Class AMP_Blacklist_Sanitizer
+ *
+ * @package AMP
+ */
 
 /**
  * Strips blacklisted tags and attributes from content.
@@ -8,28 +11,43 @@
  * See following for blacklist:
  *     https://github.com/ampproject/amphtml/blob/master/spec/amp-html-format.md#html-tags
  *
- * As of AMP 0.5 this has been replaced by AMP_Tag_And_Attribute_Sanitizer but is kept around for back-compat.
- *
+ * @since 0.5 This has been replaced by AMP_Tag_And_Attribute_Sanitizer but is kept around for back-compat.
+ * @deprecated
  */
 class AMP_Blacklist_Sanitizer extends AMP_Base_Sanitizer {
 	const PATTERN_REL_WP_ATTACHMENT = '#wp-att-([\d]+)#';
 
+	/**
+	 * Default args.
+	 *
+	 * @var array
+	 */
 	protected $DEFAULT_ARGS = array(
 		'add_blacklisted_protocols' => array(),
 		'add_blacklisted_tags' => array(),
 		'add_blacklisted_attributes' => array(),
 	);
 
+	/**
+	 * Sanitize.
+	 */
 	public function sanitize() {
-		$blacklisted_tags = $this->get_blacklisted_tags();
+		$blacklisted_tags       = $this->get_blacklisted_tags();
 		$blacklisted_attributes = $this->get_blacklisted_attributes();
-		$blacklisted_protocols = $this->get_blacklisted_protocols();
+		$blacklisted_protocols  = $this->get_blacklisted_protocols();
 
 		$body = $this->get_body_node();
 		$this->strip_tags( $body, $blacklisted_tags );
 		$this->strip_attributes_recursive( $body, $blacklisted_attributes, $blacklisted_protocols );
 	}
 
+	/**
+	 * Strip attributes recursively.
+	 *
+	 * @param DOMNode $node DOM Node.
+	 * @param array   $bad_attributes Bad attributes.
+	 * @param array   $bad_protocols  Bad protocols.
+	 */
 	private function strip_attributes_recursive( $node, $bad_attributes, $bad_protocols ) {
 		if ( XML_ELEMENT_NODE !== $node->nodeType ) {
 			return;
@@ -50,14 +68,14 @@ private function strip_attributes_recursive( $node, $bad_attributes, $bad_protoc
 		if ( $node->hasAttributes() ) {
 			$length = $node->attributes->length;
 			for ( $i = $length - 1; $i >= 0; $i-- ) {
-				$attribute = $node->attributes->item( $i );
+				$attribute      = $node->attributes->item( $i );
 				$attribute_name = strtolower( $attribute->name );
 				if ( in_array( $attribute_name, $bad_attributes, true ) ) {
 					$node->removeAttribute( $attribute_name );
 					continue;
 				}
 
-				// on* attributes (like onclick) are a special case
+				// The on* attributes (like onclick) are a special case.
 				if ( 0 === stripos( $attribute_name, 'on' ) && 'on' !== $attribute_name ) {
 					$node->removeAttribute( $attribute_name );
 					continue;
@@ -75,16 +93,22 @@ private function strip_attributes_recursive( $node, $bad_attributes, $bad_protoc
 		}
 	}
 
+	/**
+	 * Strip tags.
+	 *
+	 * @param DOMElement $node      Node.
+	 * @param string[]   $tag_names Tag names.
+	 */
 	private function strip_tags( $node, $tag_names ) {
 		foreach ( $tag_names as $tag_name ) {
 			$elements = $node->getElementsByTagName( $tag_name );
-			$length = $elements->length;
+			$length   = $elements->length;
 			if ( 0 === $length ) {
 				continue;
 			}
 
 			for ( $i = $length - 1; $i >= 0; $i-- ) {
-				$element = $elements->item( $i );
+				$element     = $elements->item( $i );
 				$parent_node = $element->parentNode;
 				$parent_node->removeChild( $element );
 
@@ -95,6 +119,12 @@ private function strip_tags( $node, $tag_names ) {
 		}
 	}
 
+	/**
+	 * Sanitize attribute.
+	 *
+	 * @param DOMElement $node      Node.
+	 * @param DOMAttr    $attribute Attribute.
+	 */
 	private function sanitize_a_attribute( $node, $attribute ) {
 		$attribute_name = strtolower( $attribute->name );
 
@@ -117,23 +147,31 @@ private function sanitize_a_attribute( $node, $attribute ) {
 				// _new is not allowed; swap with _blank
 				$node->setAttribute( $attribute_name, '_blank' );
 			} else {
-				// only _blank is allowed
+				// Only _blank is allowed.
 				$node->removeAttribute( $attribute_name );
 			}
 		}
 	}
 
+	/**
+	 * Validate node.
+	 *
+	 * @param DOMElement $node Node.
+	 * @return bool
+	 */
 	private function validate_a_node( $node ) {
-		// Get the href attribute
+		// Get the href attribute.
 		$href = $node->getAttribute( 'href' );
 
 		if ( empty( $href ) ) {
-			// If no href, check that a is an anchor or not.
-			// We don't need to validate anchors any further.
+			/*
+			 * If no href, check that a is an anchor or not.
+			 * We don't need to validate anchors any further.
+			 */
 			return $node->hasAttribute( 'name' ) || $node->hasAttribute( 'id' );
 		}
 
-		// If this is an anchor link, just return true
+		// If this is an anchor link, just return true.
 		if ( 0 === strpos( $href, '#' ) ) {
 			return true;
 		}
@@ -143,9 +181,9 @@ private function validate_a_node( $node ) {
 			$href = untrailingslashit( get_home_url() ) . $href;
 		}
 
-		$valid_protocols = array( 'http', 'https', 'mailto', 'sms', 'tel', 'viber', 'whatsapp' );
-		$special_protocols = array( 'tel', 'sms' ); // these ones don't valid with `filter_var+FILTER_VALIDATE_URL`
-		$protocol = strtok( $href, ':' );
+		$valid_protocols   = array( 'http', 'https', 'mailto', 'sms', 'tel', 'viber', 'whatsapp' );
+		$special_protocols = array( 'tel', 'sms' ); // These ones don't valid with `filter_var+FILTER_VALIDATE_URL`.
+		$protocol          = strtok( $href, ':' );
 
 		if ( false === filter_var( $href, FILTER_VALIDATE_URL )
 			&& ! in_array( $protocol, $special_protocols, true ) ) {
@@ -159,6 +197,13 @@ private function validate_a_node( $node ) {
 		return true;
 	}
 
+	/**
+	 * Replace node with children.
+	 *
+	 * @param DOMElement $node           Node.
+	 * @param array      $bad_attributes Bad attributes.
+	 * @param array      $bad_protocols  Bad protocols.
+	 */
 	private function replace_node_with_children( $node, $bad_attributes, $bad_protocols ) {
 		// If the node has children and also has a parent node,
 		// clone and re-add all the children just before current node.
@@ -176,8 +221,15 @@ private function replace_node_with_children( $node, $bad_attributes, $bad_protoc
 		}
 	}
 
+	/**
+	 * Merge defaults with args.
+	 *
+	 * @param string $key    Key.
+	 * @param array  $values Values.
+	 * @return array Merged args.
+	 */
 	private function merge_defaults_with_args( $key, $values ) {
-		// Merge default values with user specified args
+		// Merge default values with user specified args.
 		if ( ! empty( $this->args[ $key ] )
 			&& is_array( $this->args[ $key ] ) ) {
 			$values = array_merge( $values, $this->args[ $key ] );
@@ -186,12 +238,22 @@ private function merge_defaults_with_args( $key, $values ) {
 		return $values;
 	}
 
+	/**
+	 * Get blacklisted protocols.
+	 *
+	 * @return array Protocols.
+	 */
 	private function get_blacklisted_protocols() {
 		return $this->merge_defaults_with_args( 'add_blacklisted_protocols', array(
 			'javascript',
 		) );
 	}
 
+	/**
+	 * Get blacklisted tags.
+	 *
+	 * @return array Tags.
+	 */
 	private function get_blacklisted_tags() {
 		return $this->merge_defaults_with_args( 'add_blacklisted_tags', array(
 			'script',
@@ -215,17 +277,16 @@ private function get_blacklisted_tags() {
 			'embed',
 			'embedvideo',
 
-			// Other weird ones
+			// Other weird ones.
 			'comments-count',
-
-			// These are converted into amp-* versions
-			//'img',
-			//'video',
-			//'audio',
-			//'iframe',
 		) );
 	}
 
+	/**
+	 * Get blacklisted attributes.
+	 *
+	 * @return array Attributes.
+	 */
 	private function get_blacklisted_attributes() {
 		return $this->merge_defaults_with_args( 'add_blacklisted_attributes', array(
 			'style',
diff --git a/includes/sanitizers/class-amp-iframe-sanitizer.php b/includes/sanitizers/class-amp-iframe-sanitizer.php
index e1f0275f105..daee7e9bf8b 100644
--- a/includes/sanitizers/class-amp-iframe-sanitizer.php
+++ b/includes/sanitizers/class-amp-iframe-sanitizer.php
@@ -1,50 +1,114 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-base-sanitizer.php' );
+/**
+ * Class AMP_Iframe_Sanitizer
+ *
+ * @package AMP
+ */
 
 /**
+ * Class AMP_Iframe_Sanitizer
+ *
  * Converts <iframe> tags to <amp-iframe>
  */
 class AMP_Iframe_Sanitizer extends AMP_Base_Sanitizer {
+
+	/**
+	 * Value used for height attribute when $attributes['height'] is empty.
+	 *
+	 * @since 0.2
+	 *
+	 * @const int
+	 */
 	const FALLBACK_HEIGHT = 400;
+
+	/**
+	 * Default values for sandboxing IFrame.
+	 *
+	 * @since 0.2
+	 *
+	 * @const int
+	 */
 	const SANDBOX_DEFAULTS = 'allow-scripts allow-same-origin';
 
+	/**
+	 * Tag.
+	 *
+	 * @var string HTML <iframe> tag to identify and replace with AMP version.
+	 *
+	 * @since 0.2
+	 */
 	public static $tag = 'iframe';
 
+	/**
+	 * Script slug.
+	 *
+	 * @var string AMP HTML tag to use in place of HTML's <iframe> tag.
+	 *
+	 * @since 0.2
+	 */
 	private static $script_slug = 'amp-iframe';
+
+	/**
+	 * Script src.
+	 *
+	 * @var string URL to AMP Project's IFrame element's JavaScript file found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 */
 	private static $script_src = 'https://cdn.ampproject.org/v0/amp-iframe-0.1.js';
 
+	/**
+	 * Default args.
+	 *
+	 * @var array
+	 */
 	protected $DEFAULT_ARGS = array(
 		'add_placeholder' => false,
 	);
 
+	/**
+	 * Return one element array containing AMP HTML iframe tag and respective Javascript URL
+	 *
+	 * HTML tags and Javascript URLs found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 *
+	 * @return string[] Returns AMP HTML iframe tag as array key and Javascript URL as array value,
+	 *                  respectively. Will return an empty array if sanitization has yet to be run
+	 *                  or if it did not find any HTML iframe elements to convert to AMP equivalents.
+	 */
 	public function get_scripts() {
 		if ( ! $this->did_convert_elements ) {
 			return array();
 		}
-
 		return array( self::$script_slug => self::$script_src );
 	}
 
+	/**
+	 * Sanitize the <iframe> elements from the HTML contained in this instance's DOMDocument.
+	 *
+	 * @since 0.2
+	 */
 	public function sanitize() {
-		$nodes = $this->dom->getElementsByTagName( self::$tag );
+		$nodes     = $this->dom->getElementsByTagName( self::$tag );
 		$num_nodes = $nodes->length;
 		if ( 0 === $num_nodes ) {
 			return;
 		}
 
 		for ( $i = $num_nodes - 1; $i >= 0; $i-- ) {
-			$node = $nodes->item( $i );
+			$node           = $nodes->item( $i );
 			$old_attributes = AMP_DOM_Utils::get_node_attributes_as_assoc_array( $node );
 
 			$new_attributes = $this->filter_attributes( $old_attributes );
 
-			// If the src doesn't exist, remove the node.
-			// This means that it never existed or was invalidated
-			// while filtering attributes above.
-			//
-			// TODO: add a filter to allow for a fallback element in this instance.
-			// See: https://github.com/ampproject/amphtml/issues/2261
+			/**
+			 * If the src doesn't exist, remove the node. Either it never
+			 * existed or was invalidated while filtering attributes above.
+			 *
+			 * @todo: add a filter to allow for a fallback element in this instance.
+			 * @see: https://github.com/ampproject/amphtml/issues/2261
+			 */
 			if ( empty( $new_attributes['src'] ) ) {
 				$node->parentNode->removeChild( $node );
 				continue;
@@ -63,20 +127,40 @@ public function sanitize() {
 			}
 
 			$parent_node = $node->parentNode;
-			if ( 'p' === strtolower( $parent_node->tagName ) ) {
-				// AMP does not like iframes in p tags
+			if ( 'p' !== strtolower( $parent_node->tagName ) ) {
+				$parent_node->replaceChild( $new_node, $node );
+			} else {
+				// AMP does not like iframes in <p> tags.
 				$parent_node->removeChild( $node );
 				$parent_node->parentNode->insertBefore( $new_node, $parent_node->nextSibling );
 
 				if ( AMP_DOM_Utils::is_node_empty( $parent_node ) ) {
 					$parent_node->parentNode->removeChild( $parent_node );
 				}
-			} else {
-				$parent_node->replaceChild( $new_node, $node );
 			}
 		}
 	}
 
+	/**
+	 * "Filter" HTML attributes for <amp-iframe> elements.
+	 *
+	 * @since 0.2
+	 *
+	 * @param string[] $attributes {
+	 *      Attributes.
+	 *
+	 *      @type string $src IFrame URL - Empty if HTTPS required per $this->args['require_https_src']
+	 *      @type int $width <iframe> width attribute - Set to numeric value if px or %
+	 *      @type int $height <iframe> width attribute - Set to numeric value if px or %
+	 *      @type string $sandbox <iframe> `sandbox` attribute - Pass along if found; default to value of self::SANDBOX_DEFAULTS
+	 *      @type string $class <iframe> `class` attribute - Pass along if found
+	 *      @type string $sizes <iframe> `sizes` attribute - Pass along if found
+	 *      @type int $frameborder <iframe> `frameborder` attribute - Filter to '0' or '1'; default to '0'
+	 *      @type bool $allowfullscreen <iframe> `allowfullscreen` attribute - Convert 'false' to empty string ''
+	 *      @type bool $allowtransparency <iframe> `allowtransparency` attribute - Convert 'false' to empty string ''
+	 * }
+	 * @return array Returns HTML attributes; removes any not specifically declared above from input.
+	 */
 	private function filter_attributes( $attributes ) {
 		$out = array();
 
@@ -111,7 +195,7 @@ private function filter_attributes( $attributes ) {
 					}
 					break;
 
-				default;
+				default:
 					break;
 			}
 		}
@@ -123,6 +207,19 @@ private function filter_attributes( $attributes ) {
 		return $out;
 	}
 
+	/**
+	 * Builds a DOMElement to use as a placeholder for an <iframe>.
+	 *
+	 * @since 0.2
+	 *
+	 * @param string[] $parent_attributes {
+	 *      Attributes.
+	 *
+	 *      @type string $placeholder AMP HTML <amp-iframe> `placeholder` attribute; default to 'amp-wp-iframe-placeholder'
+	 *      @type string $class AMP HTML <amp-iframe> `class` attribute; default to 'amp-wp-iframe-placeholder'
+	 * }
+	 * @return DOMElement|false
+	 */
 	private function build_placeholder( $parent_attributes ) {
 		$placeholder_node = AMP_DOM_Utils::create_node( $this->dom, 'div', array(
 			'placeholder' => '',
diff --git a/includes/sanitizers/class-amp-img-sanitizer.php b/includes/sanitizers/class-amp-img-sanitizer.php
index 889bf92e62e..853a546867e 100644
--- a/includes/sanitizers/class-amp-img-sanitizer.php
+++ b/includes/sanitizers/class-amp-img-sanitizer.php
@@ -1,25 +1,101 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-base-sanitizer.php' );
-require_once( AMP__DIR__ . '/includes/utils/class-amp-image-dimension-extractor.php' );
+/**
+ * Class AMP_Img_Sanitizer.
+ *
+ * @package AMP
+ */
 
 /**
+ * Class AMP_Img_Sanitizer
+ *
  * Converts <img> tags to <amp-img> or <amp-anim>
  */
 class AMP_Img_Sanitizer extends AMP_Base_Sanitizer {
+
+	/**
+	 * Value used for width attribute when $attributes['width'] is empty.
+	 *
+	 * @since 0.2
+	 *
+	 * @const int
+	 */
 	const FALLBACK_WIDTH = 600;
+
+	/**
+	 * Value used for height attribute when $attributes['height'] is empty.
+	 *
+	 * @since 0.2
+	 *
+	 * @const int
+	 */
 	const FALLBACK_HEIGHT = 400;
 
+	/**
+	 * Tag.
+	 *
+	 * @var string HTML <img> tag to identify and replace with AMP version.
+	 *
+	 * @since 0.2
+	 */
 	public static $tag = 'img';
 
+	/**
+	 * Animation extension.
+	 *
+	 * @var string
+	 */
 	private static $anim_extension = '.gif';
 
+	/**
+	 * Script slug.
+	 *
+	 * @var string AMP HTML tag to use in place of HTML's <img> tag.
+	 *
+	 * @since 0.2
+	 */
 	private static $script_slug = 'amp-anim';
+
+	/**
+	 * Script src.
+	 *
+	 * @var string URL to AMP Project's Image element's JavaScript file found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 */
 	private static $script_src = 'https://cdn.ampproject.org/v0/amp-anim-0.1.js';
 
+	/**
+	 * Return one element array containing AMP HTML image tag and respective Javascript URL
+	 *
+	 * HTML tags and Javascript URLs found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 *
+	 * @return string[] Returns AMP HTML image tag as array key and Javascript URL
+	 *                  as array value, respectively. Will return an empty array
+	 *                  if sanitization has yet to be run or if it did not find any
+	 *                  HTML image elements to convert to AMP equivalents.
+	 */
+	public function get_scripts() {
+		if ( ! $this->did_convert_elements ) {
+			return array();
+		}
+		return array( self::$script_slug => self::$script_src );
+	}
+
+	/**
+	 * Sanitize the <img> elements from the HTML contained in this instance's DOMDocument.
+	 *
+	 * @since 0.2
+	 */
 	public function sanitize() {
 
-		$nodes = $this->dom->getElementsByTagName( self::$tag );
+		/**
+		 * Node list.
+		 *
+		 * @var DOMNodeList $node
+		 */
+		$nodes           = $this->dom->getElementsByTagName( self::$tag );
 		$need_dimensions = array();
 
 		$num_nodes = $nodes->length;
@@ -30,6 +106,9 @@ public function sanitize() {
 
 		for ( $i = $num_nodes - 1; $i >= 0; $i-- ) {
 			$node = $nodes->item( $i );
+			if ( ! $node instanceof DOMElement ) {
+				continue;
+			}
 
 			if ( ! $node->hasAttribute( 'src' ) || '' === $node->getAttribute( 'src' ) ) {
 				$node->parentNode->removeChild( $node );
@@ -49,36 +128,103 @@ public function sanitize() {
 	}
 
 	/**
-	 * Figure out width and height attribute values for images that don't have them by
-	 * attempting to determine actual dimensions and setting reasonable defaults otherwise.
+	 * "Filter" HTML attributes for <amp-anim> elements.
 	 *
-	 * @param array $need_dimensions List of Img src url to node mappings corresponding to images that need dimensions.
+	 * @since 0.2
+	 *
+	 * @param string[] $attributes {
+	 *      Attributes.
+	 *
+	 *      @type string $src Image URL - Pass along if found
+	 *      @type string $alt <img> `alt` attribute - Pass along if found
+	 *      @type string $class <img> `class` attribute - Pass along if found
+	 *      @type string $srcset <img> `srcset` attribute - Pass along if found
+	 *      @type string $sizes <img> `sizes` attribute - Pass along if found
+	 *      @type string $on <img> `on` attribute - Pass along if found
+	 *      @type string $attribution <img> `attribution` attribute - Pass along if found
+	 *      @type int $width <img> width attribute - Set to numeric value if px or %
+	 *      @type int $height <img> width attribute - Set to numeric value if px or %
+	 * }
+	 * @return array Returns HTML attributes; removes any not specifically declared above from input.
+	 */
+	private function filter_attributes( $attributes ) {
+		$out = array();
+
+		foreach ( $attributes as $name => $value ) {
+			switch ( $name ) {
+				case 'src':
+				case 'alt':
+				case 'class':
+				case 'srcset':
+				case 'sizes':
+				case 'on':
+				case 'attribution':
+					$out[ $name ] = $value;
+					break;
+
+				case 'width':
+				case 'height':
+					$out[ $name ] = $this->sanitize_dimension( $value, $name );
+					break;
+
+				default:
+					break;
+			}
+		}
+
+		return $out;
+	}
+
+	/**
+	 * Determine width and height attribute values for images without them.
+	 *
+	 * Attempt to determine actual dimensions, otherwise set reasonable defaults.
+	 *
+	 * @param DOMElement[][] $need_dimensions Map <img> @src URLs to node for images with missing dimensions.
 	 */
 	private function determine_dimensions( $need_dimensions ) {
+
 		$dimensions_by_url = AMP_Image_Dimension_Extractor::extract( array_keys( $need_dimensions ) );
 
 		foreach ( $dimensions_by_url as $url => $dimensions ) {
 			foreach ( $need_dimensions[ $url ] as $node ) {
+				if ( ! $node instanceof DOMElement ) {
+					continue;
+				}
+
 				// Provide default dimensions for images whose dimensions we couldn't fetch.
-				if ( false === $dimensions ) {
-					$width = isset( $this->args['content_max_width'] ) ? $this->args['content_max_width'] : self::FALLBACK_WIDTH;
+				if ( false !== $dimensions ) {
+					$node->setAttribute( 'width', $dimensions['width'] );
+					$node->setAttribute( 'height', $dimensions['height'] );
+				} else {
+					$width  = isset( $this->args['content_max_width'] ) ? $this->args['content_max_width'] : self::FALLBACK_WIDTH;
 					$height = self::FALLBACK_HEIGHT;
 					$node->setAttribute( 'width', $width );
 					$node->setAttribute( 'height', $height );
 					$class = $node->hasAttribute( 'class' ) ? $node->getAttribute( 'class' ) . ' amp-wp-unknown-size' : 'amp-wp-unknown-size';
 					$node->setAttribute( 'class', $class );
-				} else {
-					$node->setAttribute( 'width', $dimensions['width'] );
-					$node->setAttribute( 'height', $dimensions['height'] );
 				}
 			}
 		}
 	}
 
+	/**
+	 * Now that all images have width and height attributes, make final tweaks and replace original image nodes
+	 *
+	 * @param DOMNodeList[] $node_lists Img DOM nodes (now with width and height attributes).
+	 */
+	private function adjust_and_replace_nodes_in_array_map( $node_lists ) {
+		foreach ( $node_lists as $node_list ) {
+			foreach ( $node_list as $node ) {
+				$this->adjust_and_replace_node( $node );
+			}
+		}
+	}
+
 	/**
 	 * Make final modifications to DOMNode
 	 *
-	 * @param DOMNode $node The DOMNode to adjust and replace
+	 * @param DOMNode $node The DOMNode to adjust and replace.
 	 */
 	private function adjust_and_replace_node( $node ) {
 		$old_attributes = AMP_DOM_Utils::get_node_attributes_as_assoc_array( $node );
@@ -86,6 +232,7 @@ private function adjust_and_replace_node( $node ) {
 		$new_attributes = $this->enforce_sizes_attribute( $new_attributes );
 		if ( $this->is_gif_url( $new_attributes['src'] ) ) {
 			$this->did_convert_elements = true;
+
 			$new_tag = 'amp-anim';
 		} else {
 			$new_tag = 'amp-img';
@@ -95,55 +242,16 @@ private function adjust_and_replace_node( $node ) {
 	}
 
 	/**
-	 * Now that all images have width and height attributes, make final tweaks and replace original image nodes
+	 * Determines is a URL is considered a GIF URL
+	 *
+	 * @since 0.2
+	 *
+	 * @param string $url URL to inspect for GIF vs. JPEG or PNG.
 	 *
-	 * @param array $node_lists Img DOM nodes (now with width and height attributes).
+	 * @return bool Returns true if $url ends in `.gif`
 	 */
-	private function adjust_and_replace_nodes_in_array_map( $node_lists ) {
-		foreach ( $node_lists as $node_list ) {
-			foreach ( $node_list as $node ) {
-				$this->adjust_and_replace_node( $node );
-			}
-		}
-	}
-
-	public function get_scripts() {
-		if ( ! $this->did_convert_elements ) {
-			return array();
-		}
-
-		return array( self::$script_slug => self::$script_src );
-	}
-
-	private function filter_attributes( $attributes ) {
-		$out = array();
-
-		foreach ( $attributes as $name => $value ) {
-			switch ( $name ) {
-				case 'src':
-				case 'alt':
-				case 'class':
-				case 'srcset':
-				case 'sizes':
-				case 'on':
-					$out[ $name ] = $value;
-					break;
-
-				case 'width':
-				case 'height':
-					$out[ $name ] = $this->sanitize_dimension( $value, $name );
-					break;
-
-				default;
-					break;
-			}
-		}
-
-		return $out;
-	}
-
 	private function is_gif_url( $url ) {
-		$ext = self::$anim_extension;
+		$ext  = self::$anim_extension;
 		$path = AMP_WP_Utils::parse_url( $url, PHP_URL_PATH );
 		return substr( $path, -strlen( $ext ) ) === $ext;
 	}
diff --git a/includes/sanitizers/class-amp-playbuzz-sanitizer.php b/includes/sanitizers/class-amp-playbuzz-sanitizer.php
index 4400b6fc1b3..aeddfdc876f 100644
--- a/includes/sanitizers/class-amp-playbuzz-sanitizer.php
+++ b/includes/sanitizers/class-amp-playbuzz-sanitizer.php
@@ -1,46 +1,100 @@
 <?php
-
-require_once AMP__DIR__ . '/includes/sanitizers/class-amp-base-sanitizer.php';
+/**
+ * Class AMP_Playbuzz_Sanitizer
+ *
+ * @package AMP
+ */
 
 /**
+ * Class AMP_Playbuzz_Sanitizer
+ *
  * Converts Playbuzz embed to <amp-playbuzz>
+ *
+ * @see https://www.playbuzz.com/
  */
 class AMP_Playbuzz_Sanitizer extends AMP_Base_Sanitizer {
 
-
+	/**
+	 * Tag.
+	 *
+	 * @var string HTML tag to identify and replace with AMP version.
+	 * @since 0.2
+	 */
 	public static $tag = 'div';
+
+	/**
+	 * PlayBuzz class.
+	 *
+	 * @var string CSS class to identify Playbuzz <div> to replace with AMP version.
+	 *
+	 * @since 0.2
+	 */
 	public static $pb_class = 'pb_feed';
+
+	/**
+	 * Script slug.
+	 *
+	 * @var string AMP HTML audio tag to use in place of HTML's 'audio' tag.
+	 *
+	 * @since 0.2
+	 */
 	private static $script_slug = 'amp-playbuzz';
-	private static $height = '500';
-	private static $script_src = 'https://cdn.ampproject.org/v0/amp-playbuzz-0.1.js';
 
+	/**
+	 * Script src.
+	 *
+	 * @var string URL to AMP Project's Playbuzz element javascript file found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 */
+	private static $script_src = 'https://cdn.ampproject.org/v0/amp-playbuzz-0.1.js';
 
+	/**
+	 * Hardcoded height to set for Playbuzz elements.
+	 *
+	 * @var string
+	 *
+	 * @since 0.2
+	 */
+	private static $height = '500';
 
+	/**
+	 * Return one element array containing AMP HTML audio tag and respective Javascript URL
+	 *
+	 * HTML tags and Javascript URLs found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 *
+	 * @return string[] Returns AMP Playbuzz tag as array key and Javascript URL as array value,
+	 *                  respectively. Will return an empty array if sanitization has yet to be run
+	 *                  or if it did not find any HTML Playbuzz elements to convert to AMP equivalents.
+	 */
 	public function get_scripts() {
 		if ( ! $this->did_convert_elements ) {
 			return array();
 		}
-		return array(
-			self::$script_slug => self::$script_src,
-		);
+		return array( self::$script_slug => self::$script_src );
 	}
 
 
+	/**
+	 * Sanitize the Playbuzz elements from the HTML contained in this instance's DOMDocument.
+	 *
+	 * @since 0.2
+	 */
 	public function sanitize() {
 
-		$nodes = $this->dom->getElementsByTagName( self::$tag );
+		$nodes     = $this->dom->getElementsByTagName( self::$tag );
 		$num_nodes = $nodes->length;
 
 		if ( 0 === $num_nodes ) {
-
 			return;
-
 		}
 
 		for ( $i = $num_nodes - 1; $i >= 0; $i-- ) {
 			$node = $nodes->item( $i );
 
-			if ( self::$pb_class !== $node -> getAttribute( 'class' ) ) {
+			if ( self::$pb_class !== $node->getAttribute( 'class' ) ) {
 				continue;
 			}
 
@@ -62,7 +116,23 @@ public function sanitize() {
 
 	}
 
-
+	/**
+	 * "Filter" HTML attributes for <amp-audio> elements.
+	 *
+	 * @since 0.2
+	 *
+	 * @param string[] $attributes {
+	 *      Attributes.
+	 *
+	 *      @type string $data-item Playbuzz <div> attribute - Pass along if found and not empty.
+	 *      @type string $data-game Playbuzz <div> attribute - Assign to its value to $attributes['src'] if found and not empty.
+	 *      @type string $data-game-info Playbuzz <div> attribute - Assign to its value to $attributes['data-item-info'] if found.
+	 *      @type string $data-shares Playbuzz <div> attribute - Assign to its value to $attributes['data-share-buttons'] if found.
+	 *      @type string $data-comments Playbuzz <div> attribute - Pass along if found.
+	 *      @type int $height Playbuzz <div> attribute - Set to hardcoded value of 500.
+	 * }
+	 * @return array Returns HTML attributes; removes any not specifically declared above from input.
+	 */
 	private function filter_attributes( $attributes ) {
 		$out = array();
 
@@ -72,28 +142,28 @@ private function filter_attributes( $attributes ) {
 					if ( ! empty( $value ) ) {
 						$out['data-item'] = $value;
 					}
-				break;
+					break;
 
 				case 'data-game':
 					if ( ! empty( $value ) ) {
 						$out['src'] = $value;
 					}
-				break;
+					break;
 
 				case 'data-game-info':
 					$out['data-item-info'] = $value;
-				break;
+					break;
 
 				case 'data-shares':
 					$out['data-share-buttons'] = $value;
-				break;
+					break;
 
 				case 'data-comments':
 					$out['data-comments'] = $value;
-				break;
+					break;
 
-				default;
-				break;
+				default:
+					break;
 			}
 		}
 
diff --git a/includes/sanitizers/class-amp-rule-spec.php b/includes/sanitizers/class-amp-rule-spec.php
new file mode 100644
index 00000000000..19320fd2262
--- /dev/null
+++ b/includes/sanitizers/class-amp-rule-spec.php
@@ -0,0 +1,111 @@
+<?php
+/**
+ * Class AMP_Rule_Spec
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Rule_Spec
+ *
+ * Set of constants used throughout the sanitizer.
+ */
+abstract class AMP_Rule_Spec {
+
+	/**
+	 * AMP rule_spec types
+	 */
+	const ATTR_SPEC_LIST = 'attr_spec_list';
+	const TAG_SPEC       = 'tag_spec';
+
+	/**
+	 * AMP attr_spec value check results
+	 */
+	const PASS           = 'pass';
+	const FAIL           = 'fail';
+	const NOT_APPLICABLE = 'not_applicable';
+
+	/**
+	 * HTML Element Tag rule names
+	 */
+	const DISALLOWED_ANCESTOR = 'disallowed_ancestor';
+	const MANDATORY_ANCESTOR  = 'mandatory_ancestor';
+	const MANDATORY_PARENT    = 'mandatory_parent';
+
+	/**
+	 * HTML Element Attribute rule names
+	 */
+	const ALLOW_EMPTY             = 'allow_empty';
+	const ALLOW_RELATIVE          = 'allow_relative';
+	const ALLOWED_PROTOCOL        = 'allowed_protocol';
+	const ALTERNATIVE_NAMES       = 'alternative_names';
+	const BLACKLISTED_VALUE_REGEX = 'blacklisted_value_regex';
+	const DISALLOWED_DOMAIN       = 'disallowed_domain';
+	const MANDATORY               = 'mandatory';
+	const VALUE                   = 'value';
+	const VALUE_CASEI             = 'value_casei';
+	const VALUE_REGEX             = 'value_regex';
+	const VALUE_REGEX_CASEI       = 'value_regex_casei';
+
+	/**
+	 * If a node type listed here is invalid, it and it's subtree will be
+	 * removed if it is invalid. This is mainly  because any children will be
+	 * non-functional without this parent.
+	 *
+	 * If a tag is not listed here, it will be replaced by its children if it
+	 * is invalid.
+	 *
+	 * @todo There are other nodes that should probably be listed here as well.
+	 *
+	 * @var array
+	 */
+	public static $node_types_to_remove_if_invalid = array(
+		'form',
+		'input',
+		'link',
+		'meta',
+		'style',
+		// Include 'script' here?
+	);
+
+	/**
+	 *  It is mentioned in the documentation in several places that data-*
+	 *  is generally allowed, but there is no specific rule for it in the
+	 *  protoascii file, so we include it here.
+	 *
+	 * @var array
+	 */
+	public static $whitelisted_attr_regex = array(
+		'@^data-[a-zA-Z][\\w:.-]*$@uis',
+		'(update|item|pagination)', // Allowed for live reference points.
+	);
+
+	/**
+	 * Additional allowed tags.
+	 *
+	 * @var array
+	 */
+	public static $additional_allowed_tags = array(
+
+		/**
+		 * An experimental tag with no protoascii
+		 */
+		'amp-share-tracking' => array(
+			'attr_spec_list' => array(),
+			'tag_spec' => array(),
+		),
+
+		/**
+		 *  Needed for some tags such as analytics
+		 */
+		'script' => array(
+			'attr_spec_list' => array(
+				'type' => array(
+					'mandatory' => true,
+					'value_casei' => 'text/javascript',
+				),
+			),
+			'tag_spec' => array(),
+		),
+	);
+}
diff --git a/includes/sanitizers/class-amp-style-sanitizer.php b/includes/sanitizers/class-amp-style-sanitizer.php
index 4f31f487cdb..9cb33aab8a8 100644
--- a/includes/sanitizers/class-amp-style-sanitizer.php
+++ b/includes/sanitizers/class-amp-style-sanitizer.php
@@ -1,22 +1,64 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-base-sanitizer.php' );
+/**
+ * Class AMP_Style_Sanitizer
+ *
+ * @package AMP
+ */
 
 /**
+ * Class AMP_Style_Sanitizer
+ *
  * Collects inline styles and outputs them in the amp-custom stylesheet.
  */
 class AMP_Style_Sanitizer extends AMP_Base_Sanitizer {
+
+	/**
+	 * Styles.
+	 *
+	 * @var string[] List of CSS styles in HTML content of DOMDocument ($this->dom).
+	 *
+	 * @since 0.4
+	 */
 	private $styles = array();
 
+	/**
+	 * Get list of CSS styles in HTML content of DOMDocument ($this->dom).
+	 *
+	 * @since 0.4
+	 *
+	 * @return string[]
+	 */
 	public function get_styles() {
+		if ( ! $this->did_convert_elements ) {
+			return array();
+		}
 		return $this->styles;
 	}
 
+	/**
+	 * Sanitize CSS styles within the HTML contained in this instance's DOMDocument.
+	 *
+	 * @since 0.4
+	 */
 	public function sanitize() {
 		$body = $this->get_body_node();
 		$this->collect_styles_recursive( $body );
+		$this->did_convert_elements = true;
 	}
 
+	/**
+	 * Collect and store all CSS styles.
+	 *
+	 * Collects the CSS styles from within the HTML contained in this instance's DOMDocument.
+	 *
+	 * @see Retrieve array of styles using $this->get_styles() after calling this method.
+	 *
+	 * @since 0.4
+	 *
+	 * @note Uses recursion to traverse down the tree of DOMDocument nodes.
+	 *
+	 * @param DOMNode $node Node.
+	 */
 	private function collect_styles_recursive( $node ) {
 		if ( XML_ELEMENT_NODE !== $node->nodeType ) {
 			return;
@@ -35,7 +77,6 @@ private function collect_styles_recursive( $node ) {
 					$node->setAttribute( 'class', $new_class );
 					$this->styles[ '.' . $class_name ] = $style;
 				}
-
 				$node->removeAttribute( 'style' );
 			}
 		}
@@ -43,29 +84,42 @@ private function collect_styles_recursive( $node ) {
 		$length = $node->childNodes->length;
 		for ( $i = $length - 1; $i >= 0; $i -- ) {
 			$child_node = $node->childNodes->item( $i );
-
 			$this->collect_styles_recursive( $child_node );
 		}
 	}
 
+	/**
+	 * Sanitize and convert individual styles.
+	 *
+	 * @since 0.4
+	 *
+	 * @param string $string Style string.
+	 * @return array
+	 */
 	private function process_style( $string ) {
-		// Filter properties
+
+		/**
+		 * Filter properties
+		 */
 		$string = safecss_filter_attr( esc_html( $string ) );
 
 		if ( ! $string ) {
 			return array();
 		}
 
-		// safecss returns a string but we want individual rules.
-		// Using preg_split to break up rules by `;` but only if the semi-colon is not inside parens (like a data-encoded image).
-		$styles = array_map( 'trim', preg_split( "/;(?![^(]*\))/", $string ) );
+		/*
+		 * safecss returns a string but we want individual rules.
+		 * Use preg_split to break up rules by `;` but only if the
+		 * semi-colon is not inside parens (like a data-encoded image).
+		 */
+		$styles = array_map( 'trim', preg_split( '/;(?![^(]*\))/', $string ) );
 
-		// Normalize the order of the styles
+		// Normalize the order of the styles.
 		sort( $styles );
 
 		$processed_styles = array();
 
-		// Normalize whitespace and filter rules
+		// Normalize whitespace and filter rules.
 		foreach ( $styles as $index => $rule ) {
 			$arr2 = array_map( 'trim', explode( ':', $rule, 2 ) );
 			if ( 2 !== count( $arr2 ) ) {
@@ -77,26 +131,43 @@ private function process_style( $string ) {
 				continue;
 			}
 
-			$processed_styles[ $index ] = $property . ':' . $value;
+			$processed_styles[ $index ] = "{$property}:{$value}";
 		}
 
 		return $processed_styles;
 	}
 
+	/**
+	 * Filter individual CSS name/value pairs.
+	 *
+	 *   - Remove overflow if value is `auto` or `scroll`
+	 *   - Change `width` to `max-width`
+	 *   - Remove !important
+	 *
+	 * @since 0.4
+	 *
+	 * @param string $property Property.
+	 * @param string $value    Value.
+	 * @return array
+	 */
 	private function filter_style( $property, $value ) {
-		// Handle overflow rule
-		// https://www.ampproject.org/docs/reference/spec.html#properties
-		if ( 0 === strpos( $property, 'overflow' )
-			&& ( false !== strpos( $value, 'auto' ) || false !== strpos( $value, 'scroll' ) )
-		) {
-			return false;
+
+		/**
+		 * Remove overflow if value is `auto` or `scroll`; not allowed in AMP
+		 *
+		 * @see https://www.ampproject.org/docs/reference/spec.html#properties
+		 */
+		if ( preg_match( '#^overflow#i', $property ) && preg_match( '#^(auto|scroll)$#i', $value ) ) {
+			return array( false, false );
 		}
 
 		if ( 'width' === $property ) {
 			$property = 'max-width';
 		}
 
-		// !important is not allowed
+		/**
+		 * Remove `!important`; not allowed in AMP
+		 */
 		if ( false !== strpos( $value, 'important' ) ) {
 			$value = preg_replace( '/\s*\!\s*important$/', '', $value );
 		}
@@ -104,6 +175,16 @@ private function filter_style( $property, $value ) {
 		return array( $property, $value );
 	}
 
+	/**
+	 * Generate a unique class name
+	 *
+	 * Use the md5() of the $data parameter
+	 *
+	 * @since 0.4
+	 *
+	 * @param string $data Data.
+	 * @return string Class name.
+	 */
 	private function generate_class_name( $data ) {
 		$string = maybe_serialize( $data );
 		return 'amp-wp-inline-' . md5( $string );
diff --git a/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php b/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php
index b3d6d1287bd..715bf3c97dd 100644
--- a/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php
+++ b/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php
@@ -1,33 +1,84 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-base-sanitizer.php' );
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-allowed-tags-generated.php' );
+/**
+ * Class AMP_Tag_And_Attribute_Sanitizer
+ *
+ * @package AMP
+ */
 
 /**
- * Strips tags and attributes not allowed by the AMP sped from the content.
+ * Strips the tags and attributes from the content that are not allowed by the AMP spec.
  *
  * Allowed tags array is generated from this protocol buffer:
+ *
  *     https://github.com/ampproject/amphtml/blob/master/validator/validator-main.protoascii
  *     by the python script in amp-wp/bin/amp_wp_build.py. See the comment at the top
  *     of that file for instructions to generate class-amp-allowed-tags-generated.php.
  *
- * TODO: AMP Spec items not checked by this sanitizer -
- * - `if_value_regex` - if one attribute value matches, this places a restriction
- *		on another attribute/value.
- * - `also_requires_attr` - if one attribute is present, this requires another.
- * - `mandatory_oneof` -  Within the context of the tag, exactly one of the attributes
- *		must be present.
- * - `CdataSpec` - CDATA is not validated or sanitized.
- * - `ChildTagSpec` - Places restrictions on the number and type of child tags.
+ * @todo Need to check the following items that are not yet checked by this sanitizer:
+ *
+ *     - `also_requires_attr` - if one attribute is present, this requires another.
+ *     - `CdataSpec`          - CDATA is not validated or sanitized.
+ *     - `ChildTagSpec`       - Places restrictions on the number and type of child tags.
+ *     - `if_value_regex`     - if one attribute value matches, this places a restriction
+ *                              on another attribute/value.
+ *     - `mandatory_oneof`    - Within the context of the tag, exactly one of the attributes
+ *                              must be present.
  */
 class AMP_Tag_And_Attribute_Sanitizer extends AMP_Base_Sanitizer {
+
+	/**
+	 * Allowed tags.
+	 *
+	 * @since 0.5
+	 *
+	 * @var string[]
+	 */
 	protected $allowed_tags;
+
+	/**
+	 * Globally-allowed attributes.
+	 *
+	 * @since 0.5
+	 *
+	 * @var array[][]
+	 */
 	protected $globally_allowed_attributes;
+
+	/**
+	 * Layout-allowed attributes.
+	 *
+	 * @since 0.5
+	 *
+	 * @var string[]
+	 */
 	protected $layout_allowed_attributes;
+
+	/**
+	 * Stack.
+	 *
+	 * @since 0.5
+	 *
+	 * @var DOMElement[]
+	 */
 	private $stack = array();
 
+	/**
+	 * Default args.
+	 *
+	 * @since 0.5
+	 *
+	 * @var array
+	 */
 	protected $DEFAULT_ARGS = array();
 
+	/**
+	 * AMP_Tag_And_Attribute_Sanitizer constructor.
+	 *
+	 * @since 0.5
+	 *
+	 * @param DOMDocument $dom  DOM.
+	 * @param array       $args Args.
+	 */
 	public function __construct( $dom, $args = array() ) {
 		$this->DEFAULT_ARGS = array(
 			'amp_allowed_tags' => AMP_Allowed_Tags_Generated::get_allowed_tags(),
@@ -37,66 +88,94 @@ public function __construct( $dom, $args = array() ) {
 
 		parent::__construct( $dom, $args );
 
-		// Prepare whitelists
-		$this->allowed_tags = $this->args['amp_allowed_tags'];
+		/**
+		 * Prepare whitelists
+		 */
+		$this->allowed_tags                = $this->args['amp_allowed_tags'];
 		$this->globally_allowed_attributes = $this->args['amp_globally_allowed_attributes'];
-		$this->layout_allowed_attributes = $this->args['amp_layout_allowed_attributes'];
+		$this->layout_allowed_attributes   = $this->args['amp_layout_allowed_attributes'];
 	}
 
+	/**
+	 * Sanitize the <video> elements from the HTML contained in this instance's DOMDocument.
+	 *
+	 * @since 0.5
+	 */
 	public function sanitize() {
-		foreach( AMP_Rule_Spec::$additional_allowed_tags as $tag_name => $tag_rule_spec ) {
+
+		foreach ( AMP_Rule_Spec::$additional_allowed_tags as $tag_name => $tag_rule_spec ) {
 			$this->allowed_tags[ $tag_name ][] = $tag_rule_spec;
 		}
 
-		// Add root of content to the stack
+		/**
+		 * Add root of content to the stack.
+		 *
+		 * @var DOMElement $body
+		 */
 		$body = $this->get_body_node();
+
 		$this->stack[] = $body;
 
-		// This loop traverses through the DOM tree iteratively.
-		while ( 0 < count( $this->stack ) ) {
+		/**
+		 * This loop traverses through the DOM tree iteratively.
+		 */
+		while ( ! empty( $this->stack ) ) {
 
 			// Get the next node to process.
 			$node = array_pop( $this->stack );
 
-			// Process this node.
+			/**
+			 * Process this node.
+			 */
 			$this->process_node( $node );
 
-			// Push child nodes onto the stack, if any exist.
-			// Note: if the node was removed, then it's parentNode value is null.
+			/*
+			 * Push child nodes onto the stack, if any exist.
+			 * if node was removed, then it's parentNode value is null.
+			 */
 			if ( $node->parentNode ) {
 				$child = $node->firstChild;
 				while ( $child ) {
 					$this->stack[] = $child;
-					$child = $child->nextSibling;
+					$child         = $child->nextSibling;
 				}
 			}
 		}
 	}
 
+	/**
+	 * Process a node by sanitizing and/or validating it per.
+	 *
+	 * @param DOMNode $node Node.
+	 */
 	private function process_node( $node ) {
-		// Don't process text or comment nodes
-		if ( ( XML_TEXT_NODE == $node->nodeType ) ||
-			( XML_COMMENT_NODE == $node->nodeType ) ||
-			( XML_CDATA_SECTION_NODE == $node->nodeType ) ) {
+
+		// Don't process text or comment nodes.
+		if ( XML_TEXT_NODE === $node->nodeType || XML_COMMENT_NODE === $node->nodeType || XML_CDATA_SECTION_NODE === $node->nodeType ) {
 			return;
 		}
 
-		// Remove nodes with non-whitelisted tags.
+		// Remove nodes with tags that have not been whitelisted.
 		if ( ! $this->is_amp_allowed_tag( $node ) ) {
-			// If it's not an allowed tag, replace the node with it's children
+
+			// If it's not an allowed tag, replace the node with it's children.
 			$this->replace_node_with_children( $node );
+
 			// Return early since this node no longer exists.
 			return;
 		}
 
-		// Compile a list of rule_specs to validate for this node based on
-		// tag name of the node.
+		/*
+		 * Compile a list of rule_specs to validate for this node
+		 * based on tag name of the node.
+		 */
 		$rule_spec_list_to_validate = array();
+		$rule_spec_list             = array();
 		if ( isset( $this->allowed_tags[ $node->nodeName ] ) ) {
 			$rule_spec_list = $this->allowed_tags[ $node->nodeName ];
 		}
 		foreach ( $rule_spec_list as $id => $rule_spec ) {
-			if ( $this->validate_tag_spec_for_node( $node, $rule_spec[AMP_Rule_Spec::TAG_SPEC] ) ) {
+			if ( $this->validate_tag_spec_for_node( $node, $rule_spec[ AMP_Rule_Spec::TAG_SPEC ] ) ) {
 				$rule_spec_list_to_validate[ $id ] = $rule_spec;
 			}
 		}
@@ -110,52 +189,70 @@ private function process_node( $node ) {
 		// The remaining validations all have to do with attributes.
 		$attr_spec_list = array();
 
-		// If we have exactly one rule_spec, use it's attr_spec_list to
-		// validate the node's attributes.
-		if ( 1 == count( $rule_spec_list_to_validate ) ) {
-			$rule_spec = array_pop( $rule_spec_list_to_validate );
-			$attr_spec_list = $rule_spec[AMP_Rule_Spec::ATTR_SPEC_LIST];
-		} else {
-			// If there is more than one valid rule_spec for this node, then
-			// try to deduce which one is intended by inspecting the node's
-			// attributes.
+		/*
+		 * If we have exactly one rule_spec, use it's attr_spec_list
+		 * to validate the node's attributes.
+		 */
+		if ( 1 === count( $rule_spec_list_to_validate ) ) {
+			$rule_spec      = array_pop( $rule_spec_list_to_validate );
+			$attr_spec_list = $rule_spec[ AMP_Rule_Spec::ATTR_SPEC_LIST ];
 
-			// Get a score from each attr_spec_list by seeing how many
-			// attributes and values match the node.
+		} else {
+			/*
+			 * If there is more than one valid rule_spec for this node,
+			 * then try to deduce which one is intended by inspecting
+			 * the node's attributes.
+			 */
+
+			/*
+			 * Get a score from each attr_spec_list by seeing how many
+			 * attributes and values match the node.
+			 */
 			$attr_spec_scores = array();
 			foreach ( $rule_spec_list_to_validate as $spec_id => $rule_spec ) {
-				$attr_spec_scores[ $spec_id ] = $this->validate_attr_spec_list_for_node( $node, $rule_spec[AMP_Rule_Spec::ATTR_SPEC_LIST] );
+				$attr_spec_scores[ $spec_id ] = $this->validate_attr_spec_list_for_node( $node, $rule_spec[ AMP_Rule_Spec::ATTR_SPEC_LIST ] );
 			}
 
 			// Get the key(s) to the highest score(s).
-			$spec_ids_sorted = array_keys( $attr_spec_scores, max( $attr_spec_scores ) );
+			$spec_ids_sorted = array_keys( $attr_spec_scores, max( $attr_spec_scores ), true );
 
 			// If there is exactly one attr_spec with a max score, use that one.
-			if ( 1 == count( $spec_ids_sorted ) ) {
-				$attr_spec_list = $rule_spec_list_to_validate[ $spec_ids_sorted[0] ][AMP_Rule_Spec::ATTR_SPEC_LIST];
-			// Otherwise...
+			if ( 1 === count( $spec_ids_sorted ) ) {
+				$attr_spec_list = $rule_spec_list_to_validate[ $spec_ids_sorted[0] ][ AMP_Rule_Spec::ATTR_SPEC_LIST ];
 			} else {
-				// This should not happen very often, but...
-				// If we're here, then we're not sure which spec should
-				// be used. Let's use the top scoring ones.
-				foreach( $spec_ids_sorted as $id ) {
-					$attr_spec_list = array_merge( $attr_spec_list, $rule_spec_list_to_validate[ $id ][AMP_Rule_Spec::ATTR_SPEC_LIST] );
+				/*
+				 * This should not happen very often, but...
+				 * If we're here, then we're not sure which spec should be used.
+				 * Let's use the top scoring ones.
+				 */
+				foreach ( $spec_ids_sorted as $id ) {
+					$attr_spec_list = array_merge( $attr_spec_list, $rule_spec_list_to_validate[ $id ][ AMP_Rule_Spec::ATTR_SPEC_LIST ] );
 				}
 			}
 		}
 
-		// If an attribute is mandatory and we don't have it, just remove the node and move on.
+		/*
+		 * If an attribute is mandatory and we don't have it,
+		 * just remove the node and move on.
+		 */
 		foreach ( $attr_spec_list as $attr_name => $attr_spec_rule_value ) {
-			$is_mandatory =
-				isset( $attr_spec_rule_value[ AMP_Rule_Spec::MANDATORY ] )
+
+			$is_mandatory = isset( $attr_spec_rule_value[ AMP_Rule_Spec::MANDATORY ] )
 				? (bool) $attr_spec_rule_value[ AMP_Rule_Spec::MANDATORY ]
 				: false;
-			$attribute_exists = $node->hasAttribute( $attr_name );
+			if ( ! $is_mandatory ) {
+				continue;
+			}
 
-			if ( $is_mandatory && ! $attribute_exists ) {
-				$this->remove_node( $node );
-				return;
+			$attribute_exists = $node instanceof DOMElement && $node->hasAttribute( $attr_name );
+
+			if ( $attribute_exists ) {
+				continue;
 			}
+
+			$this->remove_node( $node );
+			return;
+
 		}
 
 		// Remove any remaining disallowed attributes.
@@ -166,30 +263,35 @@ private function process_node( $node ) {
 	}
 
 	/**
+	 * Determines is a node is currently valid per its tag specification.
+	 *
 	 * Checks to see if a node's placement with the DOM is be valid for the
 	 * given tag_spec. If there are restrictions placed on the type of node
 	 * that can be an immediate parent or an ancestor of this node, then make
 	 * sure those restrictions are met.
 	 *
-	 * If any of the tests on the restrictions fail, return false, otherwise
-	 *	return true.
+	 * @since 0.5
+	 *
+	 * @param DOMNode          $node     Node.
+	 * @param array[]|string[] $tag_spec Tag spec.
+	 *
+	 * @return bool If any of the tests on the restrictions fail return false,
+	 *              otherwise return true.
 	 */
 	private function validate_tag_spec_for_node( $node, $tag_spec ) {
-		if ( ! empty( $tag_spec[AMP_Rule_Spec::MANDATORY_PARENT] ) &&
-			! $this->has_parent( $node, $tag_spec[AMP_Rule_Spec::MANDATORY_PARENT] ) ) {
+		if ( ! empty( $tag_spec[ AMP_Rule_Spec::MANDATORY_PARENT ] ) && ! $this->has_parent( $node, $tag_spec[ AMP_Rule_Spec::MANDATORY_PARENT ] ) ) {
 			return false;
 		}
 
-		if ( ! empty( $tag_spec[AMP_Rule_Spec::DISALLOWED_ANCESTOR] ) ) {
-			foreach ( $tag_spec[AMP_Rule_Spec::DISALLOWED_ANCESTOR] as $disallowed_ancestor_node_name ) {
+		if ( ! empty( $tag_spec[ AMP_Rule_Spec::DISALLOWED_ANCESTOR ] ) ) {
+			foreach ( $tag_spec[ AMP_Rule_Spec::DISALLOWED_ANCESTOR ] as $disallowed_ancestor_node_name ) {
 				if ( $this->has_ancestor( $node, $disallowed_ancestor_node_name ) ) {
 					return false;
 				}
 			}
 		}
 
-		if ( ! empty( $tag_spec[AMP_Rule_Spec::MANDATORY_ANCESTOR] ) &&
-			! $this->has_ancestor( $node, $tag_spec[AMP_Rule_Spec::MANDATORY_ANCESTOR] ) ) {
+		if ( ! empty( $tag_spec[ AMP_Rule_Spec::MANDATORY_ANCESTOR ] ) && ! $this->has_ancestor( $node, $tag_spec[ AMP_Rule_Spec::MANDATORY_ANCESTOR ] ) ) {
 			return false;
 		}
 
@@ -197,144 +299,196 @@ private function validate_tag_spec_for_node( $node, $tag_spec ) {
 	}
 
 	/**
-	 * Checks to see if a spec is potentially valid for the given node based on
-	 * 	the attributes present in the node.
+	 * Checks to see if a spec is potentially valid.
+	 *
+	 * Checks the given node based on the attributes present in the node.
+	 *
+	 * @note This can be a very expensive function. Use it sparingly.
 	 *
-	 * Note: This can be a very expensive function. Use it sparingly.
+	 * @param DOMNode $node           Node.
+	 * @param array[] $attr_spec_list Attribute Spec list.
+	 *
+	 * @return int Validity.
 	 */
 	private function validate_attr_spec_list_for_node( $node, $attr_spec_list ) {
-		// If this node doesn't have any attributes, there is no point in
-		//	continuing.
+
+		/**
+		 * If node has no attributes there is no point in continuing.
+		 */
 		if ( ! $node->hasAttributes() ) {
 			return 0;
 		}
 
-		foreach( $node->attributes as $attr_name => $attr_node ) {
-			if ( isset( $attr_spec_list[ $attr_name ][AMP_Rule_Spec::ALTERNATIVE_NAMES] ) ) {
-				foreach( $attr_spec_list[ $attr_name ][AMP_Rule_Spec::ALTERNATIVE_NAMES] as $attr_alt_name ) {
-					$attr_spec_list[ $attr_alt_name ] = $attr_spec_list[ $attr_name ];
-				}
+		foreach ( $node->attributes as $attr_name => $attr_node ) {
+			if ( ! isset( $attr_spec_list[ $attr_name ][ AMP_Rule_Spec::ALTERNATIVE_NAMES ] ) ) {
+				continue;
+			}
+			foreach ( $attr_spec_list[ $attr_name ][ AMP_Rule_Spec::ALTERNATIVE_NAMES ] as $attr_alt_name ) {
+				$attr_spec_list[ $attr_alt_name ] = $attr_spec_list[ $attr_name ];
 			}
 		}
 
+		if ( ! $node instanceof DOMElement ) {
+			/*
+			 * A DOMNode is not valid for checks so might
+			 * as well bail here is not an DOMElement.
+			 */
+			return 0;
+		}
+
 		$score = 0;
 
-		// Iterate through each attribute rule in this attr spec list and run
-		// the series of tests. Each filter is given a `$node`, an `$attr_name`,
-		// and an `$attr_spec_rule`. If the `$attr_spec_rule` seems to be valid
-		// for the given node, then the filter should increment the score by one.
+		/*
+		 * Iterate through each attribute rule in this attr spec list and run
+		 * the series of tests. Each filter is given a `$node`, an `$attr_name`,
+		 * and an `$attr_spec_rule`. If the `$attr_spec_rule` seems to be valid
+		 * for the given node, then the filter should increment the score by one.
+		 */
 		foreach ( $attr_spec_list as $attr_name => $attr_spec_rule ) {
 
-		 	// If a mandatory attribute is required, and attribute exists, pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::MANDATORY] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_mandatory( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			// If a mandatory attribute is required, and attribute exists, pass.
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::MANDATORY ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_mandatory( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 
-			// Check 'value' - case sensitive
-			// Given attribute's value must exactly equal value of the rule to pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_value( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			/*
+			 * Check 'value' - case sensitive
+			 * Given attribute's value must exactly equal value of the rule to pass.
+			 */
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_value( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 
-			// Check 'value_regex' - case sensitive regex match
-			// Given attribute's value must be a case insensitive match to regex pattern
-			// specified by the value of rule to pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE_REGEX] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_value_regex( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			/*
+			 * Check 'value_regex' - case sensitive regex match
+			 * Given attribute's value must be a case insensitive match to regex pattern
+			 * specified by the value of rule to pass.
+			 */
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE_REGEX ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_value_regex( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 
-			// Check 'value_casei' - case insensitive
-			// Given attribute's value must be a case insensitive match to the value of
-			// the rule to pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE_CASEI] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_value_casei( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			/*
+			 * Check 'value_casei' - case insensitive
+			 * Given attribute's value must be a case insensitive match to the value of
+			 * the rule to pass.
+			 */
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE_CASEI ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_value_casei( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 
-			// Check 'value_regex_casei' - case insensitive regex match
-			// Given attribute's value must be a case insensitive match to the regex
-			// pattern specified by the value of the rule to pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE_REGEX_CASEI] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_value_regex_casei( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			/*
+			 * Check 'value_regex_casei' - case insensitive regex match
+			 * Given attribute's value must be a case insensitive match to the regex
+			 * pattern specified by the value of the rule to pass.
+			 */
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE_REGEX_CASEI ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_value_regex_casei( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 
-			// If given attribute's value is a URL with a protocol, the protocol must
-			// be in the array specified by the rule's value to pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::ALLOWED_PROTOCOL] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_allowed_protocol( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			/*
+			 * If given attribute's value is a URL with a protocol, the protocol must
+			 * be in the array specified by the rule's value to pass.
+			 */
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALLOWED_PROTOCOL ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_allowed_protocol( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 
-			// If the given attribute's value is *not* a relative path, and the rule's
-			// value is `false`, then pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::ALLOW_RELATIVE] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_disallowed_relative( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			/*
+			 * If the given attribute's value is *not* a relative path, and the rule's
+			 * value is `false`, then pass.
+			 */
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALLOW_RELATIVE ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_disallowed_relative( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 
-			// If the given attribute's value exists, is non-empty and the rule's value
-			// is false, then pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::ALLOW_EMPTY] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_disallowed_empty( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			/*
+			 * If the given attribute's value exists, is non-empty and the rule's value
+			 * is false, then pass.
+			 */
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALLOW_EMPTY ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_disallowed_empty( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 
-			// If the given attribute's value is a URL and does not match any of the list
-			// of domains in the value of the rule, then pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::DISALLOWED_DOMAIN] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_disallowed_domain( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			/*
+			 * If the given attribute's value is a URL and does not match any of the list
+			 * of domains in the value of the rule, then pass.
+			 */
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::DISALLOWED_DOMAIN ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_disallowed_domain( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 
-			// If the attribute's value exists and does not match the regex specified
-			// by the rule's value, then pass.
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::BLACKLISTED_VALUE_REGEX] ) ) {
-				if ( AMP_Rule_Spec::PASS == $this->check_attr_spec_rule_blacklisted_value_regex( $node, $attr_name, $attr_spec_rule ) ) {
-					$score += 1;
+			/*
+			 * If the attribute's value exists and does not match the regex specified
+			 * by the rule's value, then pass.
+			 */
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::BLACKLISTED_VALUE_REGEX ] ) ) {
+				if ( AMP_Rule_Spec::PASS === $this->check_attr_spec_rule_blacklisted_value_regex( $node, $attr_name, $attr_spec_rule ) ) {
+					$score++;
 				}
 			}
 		}
 
-		// return true;
 		return $score;
 	}
 
 	/**
-	 * If an attribute is not listed in $allowed_attrs, then it will be removed
-	 * from $node.
+	 * Remove attributes from $node that are not listed in $allowed_attrs.
+	 *
+	 * @param DOMNode $node           Node.
+	 * @param array[] $attr_spec_list Attribute spec list.
 	 */
 	private function sanitize_disallowed_attributes_in_node( $node, $attr_spec_list ) {
-		// Note: We can't remove the attributes inside the 'foreach' loop
-		//	because that breaks the process of iterating through the attrs. So,
-		//	we keep track of what needs to be removed in the first loop, then
-		//	actually remove the attributes in the second loop.
+
+		if ( ! $node instanceof DOMElement ) {
+			/**
+			 * If $node is only a DOMNode and not a DOMElement we can't
+			 * remove an attribute from it anyway.  So bail out now.
+			 */
+			return;
+		}
+
+		/*
+		 * We can't remove attributes inside the 'foreach' loop without
+		 * breaking the iteration. So we keep track of the attributes to
+		 * remove in the first loop, then remove them in the second loop.
+		 */
 		$attrs_to_remove = array();
 		foreach ( $node->attributes as $attr_name => $attr_node ) {
-			// see if this attribute is allowed for this node
 			if ( ! $this->is_amp_allowed_attribute( $attr_name, $attr_spec_list ) ) {
+				/**
+				 * This attribute is not allowed for this node; plan to remove it.
+				 */
 				$attrs_to_remove[] = $attr_name;
 			}
 		}
 
 		if ( ! empty( $attrs_to_remove ) ) {
-			// Make sure we're not removing an attribtue that is listed as an alternative
-			//	for some other allowed attribtue. ex. 'srcset' is an alternative for 'src'.
+			/*
+			 * Ensure we are not removing attributes listed as an alternate
+			 * or allowed attributes, e.g. 'srcset' is an alternate for 'src'.
+			 */
 			foreach ( $attr_spec_list as $attr_name => $attr_spec_rule_value ) {
-				if ( isset( $attr_spec_rule_value[AMP_Rule_Spec::ALTERNATIVE_NAMES] ) ) {
-					foreach ( $attr_spec_rule_value[AMP_Rule_Spec::ALTERNATIVE_NAMES] as $alternative_name ) {
+				if ( isset( $attr_spec_rule_value[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] ) ) {
+					foreach ( $attr_spec_rule_value[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] as $alternative_name ) {
 						$alt_name_keys = array_keys( $attrs_to_remove, $alternative_name, true );
 						if ( ! empty( $alt_name_keys ) ) {
 							unset( $attrs_to_remove[ $alt_name_keys[0] ] );
@@ -343,7 +497,7 @@ private function sanitize_disallowed_attributes_in_node( $node, $attr_spec_list
 				}
 			}
 
-			// Remove the disllowed attributes
+			// Remove the disallowed attributes.
 			foreach ( $attrs_to_remove as $attr_name ) {
 				$node->removeAttribute( $attr_name );
 			}
@@ -351,71 +505,91 @@ private function sanitize_disallowed_attributes_in_node( $node, $attr_spec_list
 	}
 
 	/**
-	 * If a node has an attribute value that is disallowed, then that value will
-	 * be removed from the attribute on this node.
+	 * Remove invalid AMP attributes values from $node that have been implicitly disallowed.
+	 *
+	 * Allowed values are found $this->globally_allowed_attributes and in parameter $attr_spec_list
+	 *
+	 * @param DOMNode   $node           Node.
+	 * @param array[][] $attr_spec_list Attribute spec list.
 	 */
 	private function sanitize_disallowed_attribute_values_in_node( $node, $attr_spec_list ) {
-		$this->_sanitize_disallowed_attribute_values_in_node( $node, $this->globally_allowed_attributes );
+
+		if ( ! $node instanceof DOMElement ) {
+			/*
+			 * If $node is only a DOMNode and not a DOMElement we can't
+			 * remove an attribute from it anyway.  So bail out now.
+			 */
+			return;
+		}
+
+		$this->delegated_sanitize_disallowed_attribute_values_in_node( $node, $this->globally_allowed_attributes );
 		if ( ! empty( $attr_spec_list ) ) {
-			$this->_sanitize_disallowed_attribute_values_in_node( $node, $attr_spec_list );
+			$this->delegated_sanitize_disallowed_attribute_values_in_node( $node, $attr_spec_list );
 		}
 	}
-	private function _sanitize_disallowed_attribute_values_in_node( $node, $attr_spec_list ) {
+
+	/**
+	 * Remove attributes values from $node that have been disallowed by AMP.
+	 *
+	 * @see $this->sanitize_disallowed_attribute_values_in_node() which delegates to this method
+	 *
+	 * @param DOMElement $node           Node.
+	 * @param array[][]  $attr_spec_list Attribute spec list.
+	 */
+	private function delegated_sanitize_disallowed_attribute_values_in_node( $node, $attr_spec_list ) {
 		$attrs_to_remove = array();
 
-		foreach( $attr_spec_list as $attr_name => $attr_val ) {
-			if ( isset( $attr_spec_list[ $attr_name ][AMP_Rule_Spec::ALTERNATIVE_NAMES] ) ) {
-				foreach( $attr_spec_list[ $attr_name ][AMP_Rule_Spec::ALTERNATIVE_NAMES] as $attr_alt_name ) {
+		foreach ( $attr_spec_list as $attr_name => $attr_val ) {
+			if ( isset( $attr_spec_list[ $attr_name ][ AMP_Rule_Spec::ALTERNATIVE_NAMES ] ) ) {
+				foreach ( $attr_spec_list[ $attr_name ][ AMP_Rule_Spec::ALTERNATIVE_NAMES ] as $attr_alt_name ) {
 					$attr_spec_list[ $attr_alt_name ] = $attr_spec_list[ $attr_name ];
 				}
 			}
 		}
 
-		$mandatory_attributes = array();
-
-		foreach( $node->attributes as $attr_name => $attr_node ) {
+		foreach ( $node->attributes as $attr_name => $attr_node ) {
 
-			if ( ! isset( $attr_spec_list[$attr_name] ) ) {
+			if ( ! isset( $attr_spec_list[ $attr_name ] ) ) {
 				continue;
 			}
 
 			$should_remove_node = false;
-			$attr_spec_rule = $attr_spec_list[ $attr_name ];
+			$attr_spec_rule     = $attr_spec_list[ $attr_name ];
 
-			if ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE] ) &&
-				AMP_Rule_Spec::FAIL == $this->check_attr_spec_rule_value( $node, $attr_name, $attr_spec_rule ) ) {
+			if ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE ] ) &&
+				AMP_Rule_Spec::FAIL === $this->check_attr_spec_rule_value( $node, $attr_name, $attr_spec_rule ) ) {
 				$should_remove_node = true;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE_CASEI] ) &&
-				AMP_Rule_Spec::FAIL == $this->check_attr_spec_rule_value_casei( $node, $attr_name, $attr_spec_rule ) ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE_CASEI ] ) &&
+				AMP_Rule_Spec::FAIL === $this->check_attr_spec_rule_value_casei( $node, $attr_name, $attr_spec_rule ) ) {
 				$should_remove_node = true;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE_REGEX] ) &&
-				AMP_Rule_Spec::FAIL == $this->check_attr_spec_rule_value_regex( $node, $attr_name, $attr_spec_rule ) ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE_REGEX ] ) &&
+					AMP_Rule_Spec::FAIL === $this->check_attr_spec_rule_value_regex( $node, $attr_name, $attr_spec_rule ) ) {
 				$should_remove_node = true;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE_REGEX_CASEI] ) &&
-				AMP_Rule_Spec::FAIL == $this->check_attr_spec_rule_value_regex_casei( $node, $attr_name, $attr_spec_rule ) ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE_REGEX_CASEI ] ) &&
+				AMP_Rule_Spec::FAIL === $this->check_attr_spec_rule_value_regex_casei( $node, $attr_name, $attr_spec_rule ) ) {
 				$should_remove_node = true;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::ALLOWED_PROTOCOL] ) &&
-				AMP_Rule_Spec::FAIL == $this->check_attr_spec_rule_allowed_protocol( $node, $attr_name, $attr_spec_rule ) ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALLOWED_PROTOCOL ] ) &&
+				AMP_Rule_Spec::FAIL === $this->check_attr_spec_rule_allowed_protocol( $node, $attr_name, $attr_spec_rule ) ) {
 				$should_remove_node = true;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::ALLOW_RELATIVE] ) &&
-				AMP_Rule_Spec::FAIL == $this->check_attr_spec_rule_disallowed_relative( $node, $attr_name, $attr_spec_rule ) ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALLOW_RELATIVE ] ) &&
+				AMP_Rule_Spec::FAIL === $this->check_attr_spec_rule_disallowed_relative( $node, $attr_name, $attr_spec_rule ) ) {
 				$should_remove_node = true;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::ALLOW_EMPTY] ) &&
-				AMP_Rule_Spec::FAIL == $this->check_attr_spec_rule_disallowed_empty( $node, $attr_name, $attr_spec_rule ) ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALLOW_EMPTY ] ) &&
+				AMP_Rule_Spec::FAIL === $this->check_attr_spec_rule_disallowed_empty( $node, $attr_name, $attr_spec_rule ) ) {
 				$should_remove_node = true;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::DISALLOWED_DOMAIN] ) &&
-				AMP_Rule_Spec::FAIL == $this->check_attr_spec_rule_disallowed_domain( $node, $attr_name, $attr_spec_rule ) ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::DISALLOWED_DOMAIN ] ) &&
+				AMP_Rule_Spec::FAIL === $this->check_attr_spec_rule_disallowed_domain( $node, $attr_name, $attr_spec_rule ) ) {
 				$should_remove_node = true;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::BLACKLISTED_VALUE_REGEX] ) &&
-				AMP_Rule_Spec::FAIL == $this->check_attr_spec_rule_blacklisted_value_regex( $node, $attr_name, $attr_spec_rule ) ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::BLACKLISTED_VALUE_REGEX ] ) &&
+				AMP_Rule_Spec::FAIL === $this->check_attr_spec_rule_blacklisted_value_regex( $node, $attr_name, $attr_spec_rule ) ) {
 				$should_remove_node = true;
 			}
 
 			if ( $should_remove_node ) {
 				$is_mandatory =
 					isset( $attr_spec_rule[ AMP_Rule_Spec::MANDATORY ] )
-					? (bool) $attr_spec_rule[ AMP_Rule_Spec::MANDATORY ]
-					: false;
+						? (bool) $attr_spec_rule[ AMP_Rule_Spec::MANDATORY ]
+						: false;
 
 				if ( $is_mandatory ) {
 					$this->remove_node( $node );
@@ -426,11 +600,10 @@ private function _sanitize_disallowed_attribute_values_in_node( $node, $attr_spe
 			}
 		}
 
-		// Remove the disallowed values
+		// Remove the disallowed values.
 		foreach ( $attrs_to_remove as $attr_name ) {
-			if ( isset( $attr_spec_list[$attr_name][AMP_Rule_Spec::ALLOW_EMPTY] ) &&
-				( true == $attr_spec_list[$attr_name][AMP_Rule_Spec::ALLOW_EMPTY] ) ) {
-				$attr = $node->attributes;
+			if ( isset( $attr_spec_list[ $attr_name ][ AMP_Rule_Spec::ALLOW_EMPTY ] ) && ( $attr_spec_list[ $attr_name ][ AMP_Rule_Spec::ALLOW_EMPTY ] ) ) {
+				$attr                      = $node->attributes;
 				$attr[ $attr_name ]->value = '';
 			} else {
 				$node->removeAttribute( $attr_name );
@@ -439,29 +612,32 @@ private function _sanitize_disallowed_attribute_values_in_node( $node, $attr_spe
 	}
 
 	/**
-	 * Checks to see if the given attribute is mandatory for the given node and
-	 * whether the attribute (or a specified alternate) exists.
+	 * Check if attribute is mandatory determine whether it exists in $node.
 	 *
-	 *	Returns:
-	 *		- AMP_Rule_Spec::PASS - $attr_name is mandatory and it exists
-	 *		- AMP_Rule_Spec::FAIL - $attr_name is mandatory, but doesn't exist
-	 *		- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name is not mandatory
+	 * When checking for the given attribute it also checks valid alternates.
+	 *
+	 * @param DOMElement $node           Node.
+	 * @param string     $attr_name      Attribute name.
+	 * @param array[]    $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name is mandatory and it exists
+	 *      - AMP_Rule_Spec::FAIL - $attr_name is mandatory, but doesn't exist
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name is not mandatory
 	 */
 	private function check_attr_spec_rule_mandatory( $node, $attr_name, $attr_spec_rule ) {
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::MANDATORY] ) &&
-			( true == $attr_spec_rule[AMP_Rule_Spec::MANDATORY] ) ) {
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::MANDATORY ] ) && ( $attr_spec_rule[ AMP_Rule_Spec::MANDATORY ] ) ) {
 			if ( $node->hasAttribute( $attr_name ) ) {
 				return AMP_Rule_Spec::PASS;
 			} else {
-				// check if an alternative name list is specified
-				if ( isset( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] ) ) {
-					foreach ( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] as $alt_name ) {
+				// Check if an alternative name list is specified.
+				if ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] ) ) {
+					foreach ( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] as $alt_name ) {
 						if ( $node->hasAttribute( $alt_name ) ) {
 							return AMP_Rule_Spec::PASS;
 						}
 					}
 				}
-
 				return AMP_Rule_Spec::FAIL;
 			}
 		}
@@ -469,27 +645,32 @@ private function check_attr_spec_rule_mandatory( $node, $attr_name, $attr_spec_r
 	}
 
 	/**
-	 * Checks to see if the given attribute exists, has a value rule, and whether
-	 * the attribute matches the value.
-	 *
-	 * Returns:
-	 * 	- AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
-	 * 	- AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
-	 * 	- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
-	 * 		is no rule for this attribute.
+	 * Check if attribute has a value rule determine if its value is valid.
+	 *
+	 * Checks for value validity by matches against valid values.
+	 *
+	 * @param DOMElement $node           Node.
+	 * @param string     $attr_name      Attribute name.
+	 * @param array[]    $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
+	 *      - AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
+	 *                                        is no rule for this attribute.
 	 */
 	private function check_attr_spec_rule_value( $node, $attr_name, $attr_spec_rule ) {
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE] ) ) {
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE ] ) ) {
 			if ( $node->hasAttribute( $attr_name ) ) {
-				if ( $node->getAttribute( $attr_name ) == $attr_spec_rule[AMP_Rule_Spec::VALUE] ) {
+				if ( $node->getAttribute( $attr_name ) === $attr_spec_rule[ AMP_Rule_Spec::VALUE ] ) {
 					return AMP_Rule_Spec::PASS;
 				} else {
 					return AMP_Rule_Spec::FAIL;
 				}
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] ) ) {
-				foreach ( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] as $alternative_name ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] ) ) {
+				foreach ( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] as $alternative_name ) {
 					if ( $node->hasAttribute( $alternative_name ) ) {
-						if ( $node->getAttribute( $alternative_name ) == $attr_spec_rule[AMP_Rule_Spec::VALUE] ) {
+						if ( $node->getAttribute( $alternative_name ) === $attr_spec_rule[ AMP_Rule_Spec::VALUE ] ) {
 							return AMP_Rule_Spec::PASS;
 						} else {
 							return AMP_Rule_Spec::FAIL;
@@ -502,31 +683,36 @@ private function check_attr_spec_rule_value( $node, $attr_name, $attr_spec_rule
 	}
 
 	/**
-	 * Checks to see if the given attribute exists, has a value rule, and whether
-	 * or not the value matches the rule without respect to case.
-	 *
-	 * Returns:
-	 * 	- AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
-	 * 	- AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
-	 * 	- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
-	 * 		is no rule for this attribute.
+	 * Check if attribute has a value rule determine if its value matches ignoring case.
+	 *
+	 * @param DOMElement       $node           Node.
+	 * @param string           $attr_name      Attribute name.
+	 * @param array[]|string[] $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
+	 *      - AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
+	 *                                        is no rule for this attribute.
 	 */
 	private function check_attr_spec_rule_value_casei( $node, $attr_name, $attr_spec_rule ) {
-		// check 'value_casei' - case insensitive
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE_CASEI] ) ) {
-			$rule_value = strtolower( $attr_spec_rule[AMP_Rule_Spec::VALUE_CASEI] );
+		/**
+		 * Check 'value_casei' - case insensitive
+		 */
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE_CASEI ] ) ) {
+			$rule_value = strtolower( $attr_spec_rule[ AMP_Rule_Spec::VALUE_CASEI ] );
 			if ( $node->hasAttribute( $attr_name ) ) {
 				$attr_value = strtolower( $node->getAttribute( $attr_name ) );
-				if ( $attr_value == $rule_value ) {
+				if ( $attr_value === (string) $rule_value ) {
 					return AMP_Rule_Spec::PASS;
 				} else {
 					return AMP_Rule_Spec::FAIL;
 				}
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] ) ) {
-				foreach ( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] as $alternative_name ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] ) ) {
+				foreach ( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] as $alternative_name ) {
 					if ( $node->hasAttribute( $alternative_name ) ) {
 						$attr_value = strtolower( $node->getAttribute( $alternative_name ) );
-						if ( $attr_value == $rule_value ) {
+						if ( $attr_value === (string) $rule_value ) {
 							return AMP_Rule_Spec::PASS;
 						} else {
 							return AMP_Rule_Spec::FAIL;
@@ -539,25 +725,30 @@ private function check_attr_spec_rule_value_casei( $node, $attr_name, $attr_spec
 	}
 
 	/**
-	 * Checks to see if the given attribute exists, has a value_regex rule, and
-	 * whether or not the value matches the rule.
-	 *
-	 * Returns:
-	 * 	- AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
-	 * 	- AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
-	 * 	- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
-	 * 		is no rule for this attribute.
+	 * Check if attribute has a regex value rule determine if it matches.
+	 *
+	 * @param DOMElement       $node           Node.
+	 * @param string           $attr_name      Attribute name.
+	 * @param array[]|string[] $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
+	 *      - AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
+	 *                                        is no rule for this attribute.
 	 */
 	private function check_attr_spec_rule_value_regex( $node, $attr_name, $attr_spec_rule ) {
-		// check 'value_regex' - case sensitive regex match
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE_REGEX] ) && $node->hasAttribute( $attr_name ) ) {
-			$rule_value = $attr_spec_rule[AMP_Rule_Spec::VALUE_REGEX];
-			// Note: I added in the '^' and '$' to the regex pattern even though
-			//	they weren't in the AMP spec. But leaving them out would allow
-			//	both '_blank' and 'yyy_blankzzz' to be matched by a regex rule of
-			//	'(_blank|_self|_top)'. The AMP JS validator only accepts '_blank',
-			//	so I'm leaving it this way for now.
-			if ( preg_match('@^' . $rule_value . '$@u', $node->getAttribute( $attr_name )) ) {
+		// Check 'value_regex' - case sensitive regex match.
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE_REGEX ] ) && $node->hasAttribute( $attr_name ) ) {
+			$rule_value = $attr_spec_rule[ AMP_Rule_Spec::VALUE_REGEX ];
+
+			/*
+			 * The regex pattern has '^' and '$' though they are not in the AMP spec.
+			 * Leaving them out would allow both '_blank' and 'yyy_blankzzz' to be
+			 * matched by a regex rule of '(_blank|_self|_top)'. As the AMP JS validator
+			 * only accepts '_blank' we leave it this way for now.
+			 */
+			if ( preg_match( '@^' . $rule_value . '$@u', $node->getAttribute( $attr_name ) ) ) {
 				return AMP_Rule_Spec::PASS;
 			} else {
 				return AMP_Rule_Spec::FAIL;
@@ -567,21 +758,27 @@ private function check_attr_spec_rule_value_regex( $node, $attr_name, $attr_spec
 	}
 
 	/**
-	 * Checks to see if the given attribute exists, has a value_regex_casei rule,
-	 * and whether or not the value matches the rule without respect to case.
-	 *
-	 * Returns:
-	 * 	- AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
-	 * 	- AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
-	 * 	- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
-	 * 		is no rule for this attribute.
+	 * Check if attribute has a case-insensitive regex value rule determine if it matches.
+	 *
+	 * @param DOMElement       $node           Node.
+	 * @param string           $attr_name      Attribute name.
+	 * @param array[]|string[] $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
+	 *      - AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
+	 *                                        is no rule for this attribute.
 	 */
 	private function check_attr_spec_rule_value_regex_casei( $node, $attr_name, $attr_spec_rule ) {
-		// check 'value_regex_casei' - case insensitive regex match
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::VALUE_REGEX_CASEI] ) && $node->hasAttribute( $attr_name ) ) {
-			$rule_value = $attr_spec_rule[AMP_Rule_Spec::VALUE_REGEX_CASEI];
+		/**
+		 * Check 'value_regex_casei' - case insensitive regex match
+		 */
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::VALUE_REGEX_CASEI ] ) && $node->hasAttribute( $attr_name ) ) {
+			$rule_value = $attr_spec_rule[ AMP_Rule_Spec::VALUE_REGEX_CASEI ];
+
 			// See note above regarding the '^' and '$' that are added here.
-			if ( preg_match('/^' . $rule_value . '$/ui', $node->getAttribute( $attr_name ) ) ) {
+			if ( preg_match( '/^' . $rule_value . '$/ui', $node->getAttribute( $attr_name ) ) ) {
 				return AMP_Rule_Spec::PASS;
 			} else {
 				return AMP_Rule_Spec::FAIL;
@@ -591,42 +788,51 @@ private function check_attr_spec_rule_value_regex_casei( $node, $attr_name, $att
 	}
 
 	/**
-	 * Checks to see if the given attribute exists, has a protocol rule, and
-	 * whether or not the value matches the rule.
-	 *
-	 * Returns:
-	 * 	- AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
-	 * 	- AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
-	 * 	- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
-	 * 		is no rule for this attribute.
+	 * Check if attribute has a protocol value rule determine if it matches.
+	 *
+	 * @param DOMElement       $node           Node.
+	 * @param string           $attr_name      Attribute name.
+	 * @param array[]|string[] $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
+	 *      - AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
+	 *                                        is no rule for this attribute.
 	 */
 	private function check_attr_spec_rule_allowed_protocol( $node, $attr_name, $attr_spec_rule ) {
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::ALLOWED_PROTOCOL] ) ) {
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALLOWED_PROTOCOL ] ) ) {
 			if ( $node->hasAttribute( $attr_name ) ) {
-				$attr_value = $node->getAttribute( $attr_name );
-				$attr_value = preg_replace( '/\s*,\s*/', ',', $attr_value );
+				$attr_value   = $node->getAttribute( $attr_name );
+				$attr_value   = preg_replace( '/\s*,\s*/', ',', $attr_value );
 				$urls_to_test = explode( ',', $attr_value );
 				foreach ( $urls_to_test as $url ) {
-					// This seems to be an acceptable check since the AMP validator
-					//	will allow a URL with no protocol to pass validation.
-					if ( $url_scheme = AMP_WP_Utils::parse_url( $url, PHP_URL_SCHEME ) ) {
-						if ( ! in_array( strtolower( $url_scheme ), $attr_spec_rule[AMP_Rule_Spec::ALLOWED_PROTOCOL] ) ) {
+					/*
+					 * This seems to be an acceptable check since the AMP validator
+					 * will allow a URL with no protocol to pass validation.
+					 */
+					$url_scheme = AMP_WP_Utils::parse_url( $url, PHP_URL_SCHEME );
+					if ( $url_scheme ) {
+						if ( ! in_array( strtolower( $url_scheme ), $attr_spec_rule[ AMP_Rule_Spec::ALLOWED_PROTOCOL ], true ) ) {
 							return AMP_Rule_Spec::FAIL;
 						}
 					}
 				}
 				return AMP_Rule_Spec::PASS;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] ) ) {
-				foreach ( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] as $alternative_name ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] ) ) {
+				foreach ( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] as $alternative_name ) {
 					if ( $node->hasAttribute( $alternative_name ) ) {
-						$attr_value = $node->getAttribute( $alternative_name );
-						$attr_value = preg_replace( '/\s*,\s*/', ',', $attr_value );
+						$attr_value   = $node->getAttribute( $alternative_name );
+						$attr_value   = preg_replace( '/\s*,\s*/', ',', $attr_value );
 						$urls_to_test = explode( ',', $attr_value );
 						foreach ( $urls_to_test as $url ) {
-							// This seems to be an acceptable check since the AMP validator
-							//	will allow a URL with no protocol to pass validation.
-							if ( $url_scheme = AMP_WP_Utils::parse_url( $url, PHP_URL_SCHEME ) ) {
-								if ( ! in_array( strtolower( $url_scheme ), $attr_spec_rule[AMP_Rule_Spec::ALLOWED_PROTOCOL] ) ) {
+							/*
+							 * This seems to be an acceptable check since the AMP validator
+							 *  will allow a URL with no protocol to pass validation.
+							 */
+							$url_scheme = AMP_WP_Utils::parse_url( $url, PHP_URL_SCHEME );
+							if ( $url_scheme ) {
+								if ( ! in_array( strtolower( $url_scheme ), $attr_spec_rule[ AMP_Rule_Spec::ALLOWED_PROTOCOL ], true ) ) {
 									return AMP_Rule_Spec::FAIL;
 								}
 							}
@@ -640,39 +846,44 @@ private function check_attr_spec_rule_allowed_protocol( $node, $attr_name, $attr
 	}
 
 	/**
-	 * Checks to see if the given attribute exists, has a disallowed_relative rule,
-	 * and whether or not the value matches the rule.
-	 *
-	 * Returns:
-	 * 	- AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
-	 * 	- AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
-	 * 	- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
-	 * 		is no rule for this attribute.
+	 * Check if attribute has disallowed relative value rule determine if disallowed relative value matches.
+	 *
+	 * @param DOMElement       $node           Node.
+	 * @param string           $attr_name      Attribute name.
+	 * @param array[]|string[] $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
+	 *      - AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
+	 *                                        is no rule for this attribute.
 	 */
 	private function check_attr_spec_rule_disallowed_relative( $node, $attr_name, $attr_spec_rule ) {
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::ALLOW_RELATIVE] ) &&
-			( false == $attr_spec_rule[AMP_Rule_Spec::ALLOW_RELATIVE] ) ) {
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALLOW_RELATIVE ] ) && ! ( $attr_spec_rule[ AMP_Rule_Spec::ALLOW_RELATIVE ] ) ) {
 			if ( $node->hasAttribute( $attr_name ) ) {
-				$attr_value = $node->getAttribute( $attr_name );
-				$attr_value = preg_replace( '/\s*,\s*/', ',', $attr_value );
+				$attr_value   = $node->getAttribute( $attr_name );
+				$attr_value   = preg_replace( '/\s*,\s*/', ',', $attr_value );
 				$urls_to_test = explode( ',', $attr_value );
 				foreach ( $urls_to_test as $url ) {
 					$parsed_url = AMP_WP_Utils::parse_url( $url );
-					// The JS AMP validator seems to consider 'relative' to mean
-					//	*protocol* relative, not *host* relative for this rule. So,
-					//	a url with an empty 'scheme' is considered "relative" by AMP.
-					// 	ie. '//domain.com/path' and '/path' should both be considered
-					//	relative for purposes of AMP validation.
+
+					/*
+					 *  The JS AMP validator seems to consider 'relative' to mean
+					 *  *protocol* relative, not *host* relative for this rule. So,
+					 *  a url with an empty 'scheme' is considered "relative" by AMP.
+					 *  ie. '//domain.com/path' and '/path' should both be considered
+					 *  relative for purposes of AMP validation.
+					 */
 					if ( empty( $parsed_url['scheme'] ) ) {
 						return AMP_Rule_Spec::FAIL;
 					}
 				}
 				return AMP_Rule_Spec::PASS;
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] ) ) {
-				foreach ( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] as $alternative_name ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] ) ) {
+				foreach ( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] as $alternative_name ) {
 					if ( $node->hasAttribute( $alternative_name ) ) {
-						$attr_value = $node->getAttribute( $alternative_name );
-						$attr_value = preg_replace( '/\s*,\s*/', ',', $attr_value );
+						$attr_value   = $node->getAttribute( $alternative_name );
+						$attr_value   = preg_replace( '/\s*,\s*/', ',', $attr_value );
 						$urls_to_test = explode( ',', $attr_value );
 						foreach ( $urls_to_test as $url ) {
 							$parsed_url = AMP_WP_Utils::parse_url( $url );
@@ -689,19 +900,20 @@ private function check_attr_spec_rule_disallowed_relative( $node, $attr_name, $a
 	}
 
 	/**
-	 * Checks to see if the given attribute exists, has a disallowed_relative rule,
-	 * and whether or not the value matches the rule.
-	 *
-	 * Returns:
-	 * 	- AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
-	 * 	- AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
-	 * 	- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
-	 * 		is no rule for this attribute.
+	 * Check if attribute has disallowed empty value rule determine if value is empty.
+	 *
+	 * @param DOMElement       $node           Node.
+	 * @param string           $attr_name      Attribute name.
+	 * @param array[]|string[] $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
+	 *      - AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
+	 *                                        is no rule for this attribute.
 	 */
 	private function check_attr_spec_rule_disallowed_empty( $node, $attr_name, $attr_spec_rule ) {
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::ALLOW_EMPTY] ) &&
-			( false == $attr_spec_rule[AMP_Rule_Spec::ALLOW_EMPTY] ) &&
-			 $node->hasAttribute( $attr_name ) ) {
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALLOW_EMPTY ] ) && ! ( $attr_spec_rule[ AMP_Rule_Spec::ALLOW_EMPTY ] ) && $node->hasAttribute( $attr_name ) ) {
 			$attr_value = $node->getAttribute( $attr_name );
 			if ( empty( $attr_value ) ) {
 				return AMP_Rule_Spec::FAIL;
@@ -712,23 +924,26 @@ private function check_attr_spec_rule_disallowed_empty( $node, $attr_name, $attr
 	}
 
 	/**
-	 * Checks to see if the given attribute exists, has a disallowed_domain rule,
-	 * and whether or not the value matches the rule.
-	 *
-	 * Returns:
-	 * 	- AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
-	 * 	- AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
-	 * 	- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
-	 * 		is no rule for this attribute.
+	 * Check if attribute has disallowed domain value rule determine if value matches.
+	 *
+	 * @param DOMElement       $node           Node.
+	 * @param string           $attr_name      Attribute name.
+	 * @param array[]|string[] $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
+	 *      - AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
+	 *                                        is no rule for this attribute.
 	 */
 	private function check_attr_spec_rule_disallowed_domain( $node, $attr_name, $attr_spec_rule ) {
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::DISALLOWED_DOMAIN] ) &&
-			$node->hasAttribute( $attr_name ) ) {
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::DISALLOWED_DOMAIN ] ) && $node->hasAttribute( $attr_name ) ) {
 			$attr_value = $node->getAttribute( $attr_name );
 			$url_domain = AMP_WP_Utils::parse_url( $attr_value, PHP_URL_HOST );
 			if ( ! empty( $url_domain ) ) {
-				foreach ( $attr_spec_rule[AMP_Rule_Spec::DISALLOWED_DOMAIN] as $disallowed_domain ) {
-					if ( strtolower( $url_domain ) == strtolower( $disallowed_domain ) ) {
+				foreach ( $attr_spec_rule[ AMP_Rule_Spec::DISALLOWED_DOMAIN ] as $disallowed_domain ) {
+					if ( strtolower( $url_domain ) === strtolower( $disallowed_domain ) ) {
+
 						// Found a disallowed domain, fail validation.
 						return AMP_Rule_Spec::FAIL;
 					}
@@ -740,18 +955,23 @@ private function check_attr_spec_rule_disallowed_domain( $node, $attr_name, $att
 	}
 
 	/**
-	 * Checks to see if the given attribute exists, has a blacklisted_value_regex rule,
-	 * and whether or not the value matches the rule.
-	 *
-	 * Returns:
-	 * 	- AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
-	 * 	- AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
-	 * 	- AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
-	 * 		is no rule for this attribute.
+	 * Check if attribute has blacklisted value via regex match determine if value matches.
+	 *
+	 * @since 0.5
+	 *
+	 * @param DOMElement       $node           Node.
+	 * @param string           $attr_name      Attribute name.
+	 * @param array[]|string[] $attr_spec_rule Attribute spec rule.
+	 *
+	 * @return string:
+	 *      - AMP_Rule_Spec::PASS - $attr_name has a value that matches the rule.
+	 *      - AMP_Rule_Spec::FAIL - $attr_name has a value that does *not* match rule.
+	 *      - AMP_Rule_Spec::NOT_APPLICABLE - $attr_name does not exist or there
+	 *                                        is no rule for this attribute.
 	 */
 	private function check_attr_spec_rule_blacklisted_value_regex( $node, $attr_name, $attr_spec_rule ) {
-		if ( isset( $attr_spec_rule[AMP_Rule_Spec::BLACKLISTED_VALUE_REGEX] ) ) {
-			$pattern = '/' . $attr_spec_rule[AMP_Rule_Spec::BLACKLISTED_VALUE_REGEX] . '/u';
+		if ( isset( $attr_spec_rule[ AMP_Rule_Spec::BLACKLISTED_VALUE_REGEX ] ) ) {
+			$pattern = '/' . $attr_spec_rule[ AMP_Rule_Spec::BLACKLISTED_VALUE_REGEX ] . '/u';
 			if ( $node->hasAttribute( $attr_name ) ) {
 				$attr_value = $node->getAttribute( $attr_name );
 				if ( preg_match( $pattern, $attr_value ) ) {
@@ -759,8 +979,8 @@ private function check_attr_spec_rule_blacklisted_value_regex( $node, $attr_name
 				} else {
 					return AMP_Rule_Spec::PASS;
 				}
-			} elseif ( isset( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] ) ) {
-				foreach( $attr_spec_rule[AMP_Rule_Spec::ALTERNATIVE_NAMES] as $alternative_name ) {
+			} elseif ( isset( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] ) ) {
+				foreach ( $attr_spec_rule[ AMP_Rule_Spec::ALTERNATIVE_NAMES ] as $alternative_name ) {
 					if ( $node->hasAttribute( $alternative_name ) ) {
 						$attr_value = $node->getAttribute( $alternative_name );
 						if ( preg_match( $pattern, $attr_value ) ) {
@@ -776,12 +996,16 @@ private function check_attr_spec_rule_blacklisted_value_regex( $node, $attr_name
 	}
 
 	/**
-	 * Return true if the attribute name is valid for this attr_spec, false otherwise.
+	 * Determine if the supplied attribute name is allowed for AMP.
+	 *
+	 * @since 0.5
+	 *
+	 * @param string           $attr_name      Attribute name.
+	 * @param array[]|string[] $attr_spec_list Attribute spec list.
+	 * @return bool Return true if attribute name is valid for this attr_spec_list, false otherwise.
 	 */
 	private function is_amp_allowed_attribute( $attr_name, $attr_spec_list ) {
-		if ( isset( $this->globally_allowed_attributes[ $attr_name ] ) ||
-			isset( $this->layout_allowed_attributes[ $attr_name ] ) ||
-			isset( $attr_spec_list[ $attr_name ] ) ) {
+		if ( isset( $this->globally_allowed_attributes[ $attr_name ] ) || isset( $this->layout_allowed_attributes[ $attr_name ] ) || isset( $attr_spec_list[ $attr_name ] ) ) {
 			return true;
 		} else {
 			foreach ( AMP_Rule_Spec::$whitelisted_attr_regex as $whitelisted_attr_regex ) {
@@ -794,31 +1018,52 @@ private function is_amp_allowed_attribute( $attr_name, $attr_spec_list ) {
 	}
 
 	/**
-	 * Return true if the specified node's name is an AMP allowed tag, false otherwise.
+	 * Determine if the supplied $node's HTML tag is allowed for AMP.
+	 *
+	 * @since 0.5
+	 *
+	 * @param DOMNode $node Node.
+	 * @return bool Return true if the specified node's name is an AMP allowed tag, false otherwise.
 	 */
 	private function is_amp_allowed_tag( $node ) {
-		// Return true if node is on the allowed tags list or if it is a text
-		// or comment node.
-		return ( ( XML_TEXT_NODE == $node->nodeType ) ||
+		if ( ! $node instanceof DOMElement ) {
+			return false;
+		}
+		/**
+		 * Return true if node is an allowed tags or is a text or comment node.
+		 */
+		return (
+			( XML_TEXT_NODE === $node->nodeType ) ||
 			isset( $this->allowed_tags[ $node->nodeName ] ) ||
-			( XML_COMMENT_NODE == $node->nodeType ) ||
-			( XML_CDATA_SECTION_NODE == $node->nodeType ) );
+			( XML_COMMENT_NODE === $node->nodeType ) ||
+			( XML_CDATA_SECTION_NODE === $node->nodeType )
+		);
 	}
 
 	/**
-	 * Return true if the given node has a direct parent with the given name,
-	 * otherwise return false.
+	 * Determine if the supplied $node has a parent with the specified tag name.
+	 *
+	 * @since 0.5
+	 *
+	 * @param DOMNode $node            Node.
+	 * @param string  $parent_tag_name Parent tag name.
+	 * @return bool Return true if given node has direct parent with the given name, false otherwise.
 	 */
 	private function has_parent( $node, $parent_tag_name ) {
-		if ( $node && $node->parentNode && ( $node->parentNode->nodeName == $parent_tag_name ) ) {
+		if ( $node && $node->parentNode && ( $node->parentNode->nodeName === $parent_tag_name ) ) {
 			return true;
 		}
 		return false;
 	}
 
 	/**
-	 * Return true if the given node has any ancestor with the give name,
-	 * otherwise return false.
+	 * Determine if the supplied $node has an ancestor with the specified tag name.
+	 *
+	 * @since 0.5
+	 *
+	 * @param DOMNode $node              Node.
+	 * @param string  $ancestor_tag_name Ancestor tag name.
+	 * @return bool Return true if given node has any ancestor with the give name, false otherwise.
 	 */
 	private function has_ancestor( $node, $ancestor_tag_name ) {
 		if ( $this->get_ancestor_with_tag_name( $node, $ancestor_tag_name ) ) {
@@ -828,12 +1073,18 @@ private function has_ancestor( $node, $ancestor_tag_name ) {
 	}
 
 	/**
-	 * Returns the first ancestor with the given tag name. If no ancestor
-	 * with that name is found, returns null.
+	 * Get the first ancestor node matching the specified tag name for the supplied $node.
+	 *
+	 * @since 0.5
+	 *
+	 * @param DOMNode $node              Node.
+	 * @param string  $ancestor_tag_name Ancestor tag name.
+	 * @return DOMNode|null Returns an ancestor node for the name specified, or null if not found.
 	 */
 	private function get_ancestor_with_tag_name( $node, $ancestor_tag_name ) {
-		while ( $node && $node = $node->parentNode ) {
-			if ( $node->nodeName == $ancestor_tag_name ) {
+		while ( $node && $node->parentNode ) {
+			$node = $node->parentNode;
+			if ( $node->nodeName === $ancestor_tag_name ) {
 				return $node;
 			}
 		}
@@ -841,44 +1092,64 @@ private function get_ancestor_with_tag_name( $node, $ancestor_tag_name ) {
 	}
 
 	/**
-	 * Replaces the given node with it's child nodes, if any, and adds them to
-	 * the stack for processing by the sanitize() function.
+	 * Replaces the given node with it's child nodes, if any
+	 *
+	 * Also adds them to the stack for processing by the sanitize() function.
+	 *
+	 * @since 0.3.3
+	 *
+	 * @param DOMNode $node Node.
 	 */
 	private function replace_node_with_children( $node ) {
-		// If node has children, replace it with them and push children onto stack
-		if ( $node->hasChildNodes() && $node->parentNode ) {
 
-			// create a DOM fragment to hold the children
+		if ( ! $node->hasChildNodes() || ! $node->parentNode ) {
+			// If node has no children or no parent, just remove the node.
+			$this->remove_node( $node );
+
+		} else {
+			/*
+			 * If node has children, replace it with them and push children onto stack
+			 *
+			 * Create a DOM fragment to hold the children
+			 */
 			$fragment = $this->dom->createDocumentFragment();
 
-			// Add all children to fragment/stack
+			// Add all children to fragment/stack.
 			$child = $node->firstChild;
-			while( $child ) {
+			while ( $child ) {
 				$fragment->appendChild( $child );
 				$this->stack[] = $child;
-				$child = $node->firstChild;
+				$child         = $node->firstChild;
 			}
 
-			// replace node with fragment
+			// Replace node with fragment.
 			$node->parentNode->replaceChild( $fragment, $node );
 
-		// If node has no children, just remove the node.
-		} else {
-			$this->remove_node( $node );
 		}
 	}
 
 	/**
-	 * Remove a node. If removing the node makes the parent node empty, then
-	 * remove the parent as well. Continue until a non-empty parent is reached
-	 * or the 'body' element is reached.
+	 * Removes a node from its parent node.
+	 *
+	 * If removing the node makes the parent node empty, then it will remove the parent
+	 * too. It will Continue until a non-empty parent or the 'body' element is reached.
+	 *
+	 * @since 0.5
+	 *
+	 * @param DOMNode $node Node.
 	 */
 	private function remove_node( $node ) {
-		if ( $node && $parent = $node->parentNode ) {
+		/**
+		 * Parent.
+		 *
+		 * @var DOMNode $parent
+		 */
+		$parent = $node->parentNode;
+		if ( $node && $parent ) {
 			$parent->removeChild( $node );
 		}
-		while( $parent && ( ! $parent->hasChildNodes() ) && ( 'body' != $parent->nodeName ) ) {
-			$node = $parent;
+		while ( $parent && ! $parent->hasChildNodes() && 'body' !== $parent->nodeName ) {
+			$node   = $parent;
 			$parent = $parent->parentNode;
 			if ( $parent ) {
 				$parent->removeChild( $node );
@@ -887,91 +1158,3 @@ private function remove_node( $node ) {
 	}
 }
 
-/**
- * This is a set of constants that are used throughout the sanitizer.
- * The rule name strings are listed here because it's easier to have the php
- * interpreter catch a typo than for me to catch mistyping a string.
- */
-abstract class AMP_Rule_Spec {
-
-	// AMP rule_spec types
-	const ATTR_SPEC_LIST = 'attr_spec_list';
-	const TAG_SPEC = 'tag_spec';
-
-	// AMP attr_spec value check results
-	const PASS = 'pass';
-	const FAIL = 'fail';
-	const NOT_APPLICABLE = 'not_applicable';
-
-	// tag rule names
-	const DISALLOWED_ANCESTOR = 'disallowed_ancestor';
-	const MANDATORY_ANCESTOR = 'mandatory_ancestor';
-	const MANDATORY_PARENT = 'mandatory_parent';
-
-	// attr rule names
-	const ALLOW_EMPTY = 'allow_empty';
-	const ALLOW_RELATIVE = 'allow_relative';
-	const ALLOWED_PROTOCOL = 'allowed_protocol';
-	const ALTERNATIVE_NAMES = 'alternative_names';
-	const BLACKLISTED_VALUE_REGEX = 'blacklisted_value_regex';
-	const DISALLOWED_DOMAIN = 'disallowed_domain';
-	const MANDATORY = 'mandatory';
-	const VALUE = 'value';
-	const VALUE_CASEI = 'value_casei';
-	const VALUE_REGEX = 'value_regex';
-	const VALUE_REGEX_CASEI = 'value_regex_casei';
-
-	// If a node type listed here is invalid, it and it's subtree will be
-	//	removed if it is invalid. This is mainly  because any children will be
-	//	non-functional without this parent.
-	//
-	// If a tag is not listed here, it will be replaced by its children if it
-	//	is invalid.
-	//
-	// TODO: There are other nodes that should probably be listed here as well.
-	static $node_types_to_remove_if_invalid = array(
-		'form',
-		'input',
-		'link',
-		'meta',
-		// 'script',
-		'style',
-	);
-
-	// It is mentioned in the documentation in several places that data-* is
-	// generally allowed, but there is no specific rule for it in the protoascii
-	// file, so I'm including it here.
-	static $whitelisted_attr_regex = array(
-		'@^data-[a-zA-Z][\\w:.-]*$@uis',
-		'(update|item|pagination)',	// allowed for live reference points
-	);
-
-	static $additional_allowed_tags = array(
-
-		// this is an experimental tag with no protoascii
-		'amp-share-tracking' => array(
-			'attr_spec_list' => array(),
-			'tag_spec' => array(),
-		),
-
-		// this is needed for some tags such as analytics
-		'script' => array(
-			'attr_spec_list' => array(
-				'type' => array(
-					'mandatory' => true,
-					'value_casei' => 'text/javascript',
-				),
-			),
-			'tag_spec' => array(),
-		),
-		'script' => array(
-			'attr_spec_list' => array(
-				'type' => array(
-					'mandatory' => true,
-					'value_casei' => 'application/json',
-				),
-			),
-			'tag_spec' => array(),
-		),
-	);
-}
diff --git a/includes/sanitizers/class-amp-video-sanitizer.php b/includes/sanitizers/class-amp-video-sanitizer.php
index c098ec8f6b1..aebc943596d 100644
--- a/includes/sanitizers/class-amp-video-sanitizer.php
+++ b/includes/sanitizers/class-amp-video-sanitizer.php
@@ -1,35 +1,87 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-base-sanitizer.php' );
+/**
+ * Class AMP_Video_Sanitizer.
+ *
+ * @package AMP
+ */
 
 /**
+ * Class AMP_Video_Sanitizer
+ *
+ * @since 0.2
+ *
  * Converts <video> tags to <amp-video>
  */
 class AMP_Video_Sanitizer extends AMP_Base_Sanitizer {
+
+	/**
+	 * Value used for height attribute when $attributes['height'] is empty.
+	 *
+	 * @since 0.2
+	 *
+	 * @const int
+	 */
 	const FALLBACK_HEIGHT = 400;
 
+	/**
+	 * Tag.
+	 *
+	 * @var string HTML <video> tag to identify and replace with AMP version.
+	 *
+	 * @since 0.2
+	 */
 	public static $tag = 'video';
 
+	/**
+	 * Script tag.
+	 *
+	 * @var string AMP HTML tag to use in place of HTML's <video> tag.
+	 *
+	 * @since 0.2
+	 */
 	private static $script_slug = 'amp-video';
+
+	/**
+	 * Script src.
+	 *
+	 * @var string URL to AMP Project's Video element's JavaScript file found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 */
 	private static $script_src = 'https://cdn.ampproject.org/v0/amp-video-0.1.js';
 
+	/**
+	 * Return one element array containing AMP HTML video tag and respective Javascript URL
+	 *
+	 * HTML tags and Javascript URLs found at cdn.ampproject.org
+	 *
+	 * @since 0.2
+	 *
+	 * @return string[] Returns AMP HTML video tag as array key and Javascript URL as array value,
+	 *                  respectively. Will return an empty array if sanitization has yet to be run
+	 *                  or if it did not find any HTML video elements to convert to AMP equivalents.
+	 */
 	public function get_scripts() {
 		if ( ! $this->did_convert_elements ) {
 			return array();
 		}
-
 		return array( self::$script_slug => self::$script_src );
 	}
 
+	/**
+	 * Sanitize the <video> elements from the HTML contained in this instance's DOMDocument.
+	 *
+	 * @since 0.2
+	 */
 	public function sanitize() {
-		$nodes = $this->dom->getElementsByTagName( self::$tag );
+		$nodes     = $this->dom->getElementsByTagName( self::$tag );
 		$num_nodes = $nodes->length;
 		if ( 0 === $num_nodes ) {
 			return;
 		}
 
 		for ( $i = $num_nodes - 1; $i >= 0; $i-- ) {
-			$node = $nodes->item( $i );
+			$node           = $nodes->item( $i );
 			$old_attributes = AMP_DOM_Utils::get_node_attributes_as_assoc_array( $node );
 
 			$new_attributes = $this->filter_attributes( $old_attributes );
@@ -39,23 +91,43 @@ public function sanitize() {
 
 			$new_node = AMP_DOM_Utils::create_node( $this->dom, 'amp-video', $new_attributes );
 
-			// TODO: `source` does not have closing tag, and DOMDocument doesn't handle it well.
 			foreach ( $node->childNodes as $child_node ) {
+				/**
+				 * Child node.
+				 *
+				 * @todo: Fix when `source` has no closing tag as DOMDocument does not handle well.
+				 *
+				 * @var DOMNode $child_node
+				 */
 				$new_child_node = $child_node->cloneNode( true );
+				if ( ! $new_child_node instanceof DOMElement ) {
+					continue;
+				}
+
 				$old_child_attributes = AMP_DOM_Utils::get_node_attributes_as_assoc_array( $new_child_node );
 				$new_child_attributes = $this->filter_attributes( $old_child_attributes );
 
-				// Only append source tags with a valid src attribute
-				if ( ! empty( $new_child_attributes['src'] ) && 'source' === $new_child_node->tagName ) {
-					$new_node->appendChild( $new_child_node );
+				if ( empty( $new_child_attributes['src'] ) ) {
+					continue;
+				}
+				if ( 'source' !== $new_child_node->tagName ) {
+					continue;
 				}
+
+				/**
+				 * Only append source tags with a valid src attribute
+				 */
+				$new_node->appendChild( $new_child_node );
+
 			}
 
-			// If the node has at least one valid source, replace the old node with it.
-			// Otherwise, just remove the node.
-			//
-			// TODO: Add a fallback handler.
-			// See: https://github.com/ampproject/amphtml/issues/2261
+			/*
+			 * If the node has at least one valid source, replace the old node with it.
+			 * Otherwise, just remove the node.
+			 *
+			 * TODO: Add a fallback handler.
+			 * See: https://github.com/ampproject/amphtml/issues/2261
+			 */
 			if ( 0 === $new_node->childNodes->length && empty( $new_attributes['src'] ) ) {
 				$node->parentNode->removeChild( $node );
 			} else {
@@ -63,9 +135,30 @@ public function sanitize() {
 			}
 
 			$this->did_convert_elements = true;
+
 		}
 	}
 
+	/**
+	 * "Filter" HTML attributes for <amp-audio> elements.
+	 *
+	 * @since 0.2
+	 *
+	 * @param string[] $attributes {
+	 *      Attributes.
+	 *
+	 *      @type string $src Video URL - Empty if HTTPS required per $this->args['require_https_src']
+	 *      @type int $width <video> attribute - Set to numeric value if px or %
+	 *      @type int $height <video> attribute - Set to numeric value if px or %
+	 *      @type string $poster <video> attribute - Pass along if found
+	 *      @type string $class <video> attribute - Pass along if found
+	 *      @type bool $controls <video> attribute - Convert 'false' to empty string ''
+	 *      @type bool $loop <video> attribute - Convert 'false' to empty string ''
+	 *      @type bool $muted <video> attribute - Convert 'false' to empty string ''
+	 *      @type bool $autoplay <video> attribute - Convert 'false' to empty string ''
+	 * }
+	 * @return array Returns HTML attributes; removes any not specifically declared above from input.
+	 */
 	private function filter_attributes( $attributes ) {
 		$out = array();
 
@@ -95,7 +188,7 @@ private function filter_attributes( $attributes ) {
 					}
 					break;
 
-				default;
+				default:
 					break;
 			}
 		}
diff --git a/includes/templates/class-amp-content-sanitizer.php b/includes/templates/class-amp-content-sanitizer.php
new file mode 100644
index 00000000000..dcbd62097ef
--- /dev/null
+++ b/includes/templates/class-amp-content-sanitizer.php
@@ -0,0 +1,53 @@
+<?php
+/**
+ * Class AMP_Content_Sanitizer
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Content_Sanitizer
+ */
+class AMP_Content_Sanitizer {
+
+	/**
+	 * Sanitize.
+	 *
+	 * @param string   $content           Content.
+	 * @param string[] $sanitizer_classes Sanitizer classes.
+	 * @param array    $global_args       Global args.
+	 *
+	 * @return array
+	 */
+	public static function sanitize( $content, array $sanitizer_classes, $global_args = array() ) {
+		$scripts = array();
+		$styles  = array();
+		$dom     = AMP_DOM_Utils::get_dom_from_content( $content );
+
+		foreach ( $sanitizer_classes as $sanitizer_class => $args ) {
+			if ( ! class_exists( $sanitizer_class ) ) {
+				/* translators: %s is sanitizer class */
+				_doing_it_wrong( __METHOD__, sprintf( esc_html__( 'Sanitizer (%s) class does not exist', 'amp' ), esc_html( $sanitizer_class ) ), '0.4.1' );
+				continue;
+			}
+
+			$sanitizer = new $sanitizer_class( $dom, array_merge( $global_args, $args ) );
+
+			if ( ! is_subclass_of( $sanitizer, 'AMP_Base_Sanitizer' ) ) {
+				/* translators: %s is sanitizer class */
+				_doing_it_wrong( __METHOD__, sprintf( esc_html__( 'Sanitizer (%s) must extend `AMP_Base_Sanitizer`', 'amp' ), esc_html( $sanitizer_class ) ), '0.1' );
+				continue;
+			}
+
+			$sanitizer->sanitize();
+
+			$scripts = array_merge( $scripts, $sanitizer->get_scripts() );
+			$styles  = array_merge( $styles, $sanitizer->get_styles() );
+		}
+
+		$sanitized_content = AMP_DOM_Utils::get_content_from_dom( $dom );
+
+		return array( $sanitized_content, $scripts, $styles );
+	}
+}
+
diff --git a/includes/class-amp-content.php b/includes/templates/class-amp-content.php
similarity index 50%
rename from includes/class-amp-content.php
rename to includes/templates/class-amp-content.php
index 37ff1db939c..06897b8edf1 100644
--- a/includes/class-amp-content.php
+++ b/includes/templates/class-amp-content.php
@@ -1,61 +1,148 @@
 <?php
-
-require_once( AMP__DIR__ . '/includes/utils/class-amp-dom-utils.php' );
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-base-sanitizer.php' );
-require_once( AMP__DIR__ . '/includes/embeds/class-amp-base-embed-handler.php' );
-
+/**
+ * Class AMP_Content
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Content
+ */
 class AMP_Content {
+
+	/**
+	 * Content.
+	 *
+	 * @var string
+	 */
 	private $content;
+
+	/**
+	 * AMP content.
+	 *
+	 * @var string
+	 */
 	private $amp_content = '';
+
+	/**
+	 * AMP scripts.
+	 *
+	 * @var array
+	 */
 	private $amp_scripts = array();
+
+	/**
+	 * AMP styles.
+	 *
+	 * @var array
+	 */
 	private $amp_styles = array();
+
+	/**
+	 * Args.
+	 *
+	 * @var array
+	 */
 	private $args = array();
+
+	/**
+	 * Embed handler class names.
+	 *
+	 * @var string[]
+	 */
 	private $embed_handler_classes = array();
+
+	/**
+	 * Sanitizer class names.
+	 *
+	 * @var string[]
+	 */
 	private $sanitizer_classes = array();
 
+	/**
+	 * AMP_Content constructor.
+	 *
+	 * @param string   $content               Content.
+	 * @param string[] $embed_handler_classes Embed handler class names.
+	 * @param string[] $sanitizer_classes     Sanitizer class names.
+	 * @param array    $args                  Args.
+	 */
 	public function __construct( $content, $embed_handler_classes, $sanitizer_classes, $args = array() ) {
-		$this->content = $content;
-		$this->args = $args;
+		$this->content               = $content;
+		$this->args                  = $args;
 		$this->embed_handler_classes = $embed_handler_classes;
-		$this->sanitizer_classes = $sanitizer_classes;
+		$this->sanitizer_classes     = $sanitizer_classes;
 
 		$this->transform();
 	}
 
+	/**
+	 * Get AMP content.
+	 *
+	 * @return string
+	 */
 	public function get_amp_content() {
 		return $this->amp_content;
 	}
 
+	/**
+	 * Get AMP scripts.
+	 *
+	 * @return array
+	 */
 	public function get_amp_scripts() {
 		return $this->amp_scripts;
 	}
 
+	/**
+	 * Get AMP styles.
+	 *
+	 * @return array
+	 */
 	public function get_amp_styles() {
 		return $this->amp_styles;
 	}
 
+	/**
+	 * Transform.
+	 */
 	private function transform() {
 		$content = $this->content;
 
-		// First, embeds + the_content filter
+		// First, embeds + the_content filter.
 		$embed_handlers = $this->register_embed_handlers();
-		$content = apply_filters( 'the_content', $content );
+		$content        = apply_filters( 'the_content', $content );
 		$this->unregister_embed_handlers( $embed_handlers );
 
-		// Then, sanitize to strip and/or convert non-amp content
+		// Then, sanitize to strip and/or convert non-amp content.
 		$content = $this->sanitize( $content );
 
 		$this->amp_content = $content;
 	}
 
+	/**
+	 * Add scripts.
+	 *
+	 * @param array $scripts Scripts.
+	 */
 	private function add_scripts( $scripts ) {
 		$this->amp_scripts = array_merge( $this->amp_scripts, $scripts );
 	}
 
+	/**
+	 * Add styles.
+	 *
+	 * @param array $styles Styles.
+	 */
 	private function add_styles( $styles ) {
 		$this->amp_styles = array_merge( $this->amp_styles, $styles );
 	}
 
+	/**
+	 * Register embed handlers.
+	 *
+	 * @return array
+	 */
 	private function register_embed_handlers() {
 		$embed_handlers = array();
 
@@ -63,7 +150,8 @@ private function register_embed_handlers() {
 			$embed_handler = new $embed_handler_class( array_merge( $this->args, $args ) );
 
 			if ( ! is_subclass_of( $embed_handler, 'AMP_Base_Embed_Handler' ) ) {
-				_doing_it_wrong( __METHOD__, sprintf( esc_html__( 'Embed Handler (%s) must extend `AMP_Embed_Handler`', 'amp' ), $embed_handler_class ), '0.1' );
+				/* translators: %s is embed handler */
+				_doing_it_wrong( __METHOD__, esc_html( sprintf( __( 'Embed Handler (%s) must extend `AMP_Embed_Handler`', 'amp' ), $embed_handler_class ) ), '0.1' );
 				continue;
 			}
 
@@ -74,13 +162,25 @@ private function register_embed_handlers() {
 		return $embed_handlers;
 	}
 
+	/**
+	 * Unregister embed handlers.
+	 *
+	 * @param array $embed_handlers Embed handlers.
+	 */
 	private function unregister_embed_handlers( $embed_handlers ) {
 		foreach ( $embed_handlers as $embed_handler ) {
-			 $this->add_scripts( $embed_handler->get_scripts() );
-			 $embed_handler->unregister_embed();
+			$this->add_scripts( $embed_handler->get_scripts() );
+			$embed_handler->unregister_embed();
 		}
 	}
 
+	/**
+	 * Sanitize.
+	 *
+	 * @see AMP_Content_Sanitizer::sanitize()
+	 * @param string $content Content.
+	 * @return array Sanitized content.
+	 */
 	private function sanitize( $content ) {
 		list( $sanitized_content, $scripts, $styles ) = AMP_Content_Sanitizer::sanitize( $content, $this->sanitizer_classes, $this->args );
 
@@ -90,35 +190,3 @@ private function sanitize( $content ) {
 		return $sanitized_content;
 	}
 }
-
-// @todo Split this class out into a separate file.
-class AMP_Content_Sanitizer {
-	public static function sanitize( $content, array $sanitizer_classes, $global_args = array() ) {
-		$scripts = array();
-		$styles = array();
-		$dom = AMP_DOM_Utils::get_dom_from_content( $content );
-
-		foreach ( $sanitizer_classes as $sanitizer_class => $args ) {
-			if ( ! class_exists( $sanitizer_class ) ) {
-				_doing_it_wrong( __METHOD__, sprintf( esc_html__( 'Sanitizer (%s) class does not exist', 'amp' ), esc_html( $sanitizer_class ) ), '0.4.1' );
-				continue;
-			}
-
-			$sanitizer = new $sanitizer_class( $dom, array_merge( $global_args, $args ) );
-
-			if ( ! is_subclass_of( $sanitizer, 'AMP_Base_Sanitizer' ) ) {
-				_doing_it_wrong( __METHOD__, sprintf( esc_html__( 'Sanitizer (%s) must extend `AMP_Base_Sanitizer`', 'amp' ), esc_html( $sanitizer_class ) ), '0.1' );
-				continue;
-			}
-
-			$sanitizer->sanitize();
-
-			$scripts = array_merge( $scripts, $sanitizer->get_scripts() );
-			$styles = array_merge( $styles, $sanitizer->get_styles() );
-		}
-
-		$sanitized_content = AMP_DOM_Utils::get_content_from_dom( $dom );
-
-		return array( $sanitized_content, $scripts, $styles );
-	}
-}
diff --git a/includes/class-amp-post-template.php b/includes/templates/class-amp-post-template.php
similarity index 83%
rename from includes/class-amp-post-template.php
rename to includes/templates/class-amp-post-template.php
index 2dc75018468..e1c5cd3ea82 100644
--- a/includes/class-amp-post-template.php
+++ b/includes/templates/class-amp-post-template.php
@@ -5,33 +5,6 @@
  * @package AMP
  */
 
-require_once AMP__DIR__ . '/includes/utils/class-amp-dom-utils.php';
-require_once AMP__DIR__ . '/includes/utils/class-amp-html-utils.php';
-require_once AMP__DIR__ . '/includes/utils/class-amp-string-utils.php';
-require_once AMP__DIR__ . '/includes/utils/class-amp-wp-utils.php';
-
-require_once AMP__DIR__ . '/includes/class-amp-content.php';
-
-require_once AMP__DIR__ . '/includes/sanitizers/class-amp-style-sanitizer.php';
-require_once AMP__DIR__ . '/includes/sanitizers/class-amp-blacklist-sanitizer.php';
-require_once AMP__DIR__ . '/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php';
-require_once AMP__DIR__ . '/includes/sanitizers/class-amp-img-sanitizer.php';
-require_once AMP__DIR__ . '/includes/sanitizers/class-amp-video-sanitizer.php';
-require_once AMP__DIR__ . '/includes/sanitizers/class-amp-iframe-sanitizer.php';
-require_once AMP__DIR__ . '/includes/sanitizers/class-amp-audio-sanitizer.php';
-require_once AMP__DIR__ . '/includes/sanitizers/class-amp-playbuzz-sanitizer.php';
-
-require_once AMP__DIR__ . '/includes/embeds/class-amp-twitter-embed.php';
-require_once AMP__DIR__ . '/includes/embeds/class-amp-youtube-embed.php';
-require_once AMP__DIR__ . '/includes/embeds/class-amp-dailymotion-embed.php';
-require_once AMP__DIR__ . '/includes/embeds/class-amp-vimeo-embed.php';
-require_once AMP__DIR__ . '/includes/embeds/class-amp-soundcloud-embed.php';
-require_once AMP__DIR__ . '/includes/embeds/class-amp-gallery-embed.php';
-require_once AMP__DIR__ . '/includes/embeds/class-amp-instagram-embed.php';
-require_once AMP__DIR__ . '/includes/embeds/class-amp-vine-embed.php';
-require_once AMP__DIR__ . '/includes/embeds/class-amp-facebook-embed.php';
-require_once AMP__DIR__ . '/includes/embeds/class-amp-pinterest-embed.php';
-
 /**
  * Class AMP_Post_Template
  *
@@ -188,16 +161,32 @@ public function __construct( $post ) {
 		$this->data = apply_filters( 'amp_post_template_data', $this->data, $this->post );
 	}
 
+	/**
+	 * Getter.
+	 *
+	 * @param string $property Property name.
+	 * @param mixed  $default  Default value.
+	 *
+	 * @return mixed Value.
+	 */
 	public function get( $property, $default = null ) {
 		if ( isset( $this->data[ $property ] ) ) {
 			return $this->data[ $property ];
 		} else {
-			_doing_it_wrong( __METHOD__, sprintf( esc_html__( 'Called for non-existant key ("%s").', 'amp' ), esc_html( $property ) ), '0.1' );
+			/* translators: %s is key name */
+			_doing_it_wrong( __METHOD__, esc_html( sprintf( __( 'Called for non-existent key ("%s").', 'amp' ), $property ) ), '0.1' );
 		}
 
 		return $default;
 	}
 
+	/**
+	 * Get customizer setting.
+	 *
+	 * @param string $name    Name.
+	 * @param mixed  $default Default value.
+	 * @return mixed value.
+	 */
 	public function get_customizer_setting( $name, $default = null ) {
 		$settings = $this->get( 'customizer_settings' );
 		if ( ! empty( $settings[ $name ] ) ) {
@@ -207,11 +196,19 @@ public function get_customizer_setting( $name, $default = null ) {
 		return $default;
 	}
 
+	/**
+	 * Load and print the template parts for the given post.
+	 */
 	public function load() {
 		$template = is_page() ? 'page' : 'single';
 		$this->load_parts( array( $template ) );
 	}
 
+	/**
+	 * Load template parts.
+	 *
+	 * @param string[] $templates Templates.
+	 */
 	public function load_parts( $templates ) {
 		foreach ( $templates as $template ) {
 			$file = $this->get_template_path( $template );
@@ -219,18 +216,41 @@ public function load_parts( $templates ) {
 		}
 	}
 
+	/**
+	 * Get template path.
+	 *
+	 * @param string $template Template name.
+	 * @return string Template path.
+	 */
 	private function get_template_path( $template ) {
 		return sprintf( '%s/%s.php', $this->template_dir, $template );
 	}
 
+	/**
+	 * Add data.
+	 *
+	 * @param array $data Data.
+	 */
 	private function add_data( $data ) {
 		$this->data = array_merge( $this->data, $data );
 	}
 
+	/**
+	 * Add data by key.
+	 *
+	 * @param string $key   Key.
+	 * @param mixed  $value Value.
+	 */
 	private function add_data_by_key( $key, $value ) {
 		$this->data[ $key ] = $value;
 	}
 
+	/**
+	 * Merge data for key.
+	 *
+	 * @param string $key   Key.
+	 * @param mixed  $value Value.
+	 */
 	private function merge_data_for_key( $key, $value ) {
 		if ( is_array( $this->data[ $key ] ) ) {
 			$this->data[ $key ] = array_merge( $this->data[ $key ], $value );
@@ -245,10 +265,10 @@ private function merge_data_for_key( $key, $value ) {
 	 * @since 0.2
 	 */
 	private function build_post_data() {
-		$post_title = get_the_title( $this->ID );
-		$post_publish_timestamp = get_the_date( 'U', $this->ID );
+		$post_title              = get_the_title( $this->ID );
+		$post_publish_timestamp  = get_the_date( 'U', $this->ID );
 		$post_modified_timestamp = get_post_modified_time( 'U', false, $this->post );
-		$post_author = get_userdata( $this->post->post_author );
+		$post_author             = get_userdata( $this->post->post_author );
 
 		$this->add_data( array(
 			'post' => $this->post,
@@ -299,6 +319,9 @@ private function build_post_data() {
 		$this->build_post_commments_data();
 	}
 
+	/**
+	 * Buuild post comments data.
+	 */
 	private function build_post_commments_data() {
 		if ( ! post_type_supports( $this->post->post_type, 'comments' ) ) {
 			return;
@@ -306,13 +329,13 @@ private function build_post_commments_data() {
 
 		$comments_open = comments_open( $this->ID );
 
-		// Don't show link if close and no comments
+		// Don't show link if close and no comments.
 		if ( ! $comments_open
 			&& ! $this->post->comment_count ) {
 			return;
 		}
 
-		$comments_link_url = get_comments_link( $this->ID );
+		$comments_link_url  = get_comments_link( $this->ID );
 		$comments_link_text = $comments_open
 			? __( 'Leave a Comment', 'amp' )
 			: __( 'View Comments', 'amp' );
@@ -323,6 +346,9 @@ private function build_post_commments_data() {
 		) );
 	}
 
+	/**
+	 * Build post content.
+	 */
 	private function build_post_content() {
 		$amp_content = new AMP_Content( $this->post->post_content,
 			apply_filters( 'amp_content_embed_handlers', array(
@@ -338,16 +364,15 @@ private function build_post_content() {
 				'AMP_Gallery_Embed_Handler' => array(),
 			), $this->post ),
 			apply_filters( 'amp_content_sanitizers', array(
-				 'AMP_Style_Sanitizer' => array(),
-				 // 'AMP_Blacklist_Sanitizer' => array(),
-				 'AMP_Img_Sanitizer' => array(),
-				 'AMP_Video_Sanitizer' => array(),
-				 'AMP_Audio_Sanitizer' => array(),
-				 'AMP_Playbuzz_Sanitizer' => array(),
-				 'AMP_Iframe_Sanitizer' => array(
-					 'add_placeholder' => true,
-				 ),
-				 'AMP_Tag_And_Attribute_Sanitizer' => array(),
+				'AMP_Style_Sanitizer'             => array(),
+				'AMP_Img_Sanitizer'               => array(),
+				'AMP_Video_Sanitizer'             => array(),
+				'AMP_Audio_Sanitizer'             => array(),
+				'AMP_Playbuzz_Sanitizer'          => array(),
+				'AMP_Tag_And_Attribute_Sanitizer' => array(),
+				'AMP_Iframe_Sanitizer'            => array(
+					'add_placeholder' => true,
+				),
 			), $this->post ),
 			array(
 				'content_max_width' => $this->get( 'content_max_width' ),
@@ -359,8 +384,11 @@ private function build_post_content() {
 		$this->merge_data_for_key( 'post_amp_styles', $amp_content->get_amp_styles() );
 	}
 
+	/**
+	 * Build post featured image.
+	 */
 	private function build_post_featured_image() {
-		$post_id = $this->ID;
+		$post_id       = $this->ID;
 		$featured_html = get_the_post_thumbnail( $post_id, 'large' );
 
 		// Skip featured image if no featured image is available.
@@ -403,6 +431,9 @@ private function build_post_featured_image() {
 		}
 	}
 
+	/**
+	 * Build customizer settings.
+	 */
 	private function build_customizer_settings() {
 		$settings = AMP_Customizer_Settings::get_settings();
 
@@ -430,7 +461,7 @@ private function build_customizer_settings() {
 	 */
 	private function get_post_image_metadata() {
 		$post_image_meta = null;
-		$post_image_id = false;
+		$post_image_id   = false;
 
 		if ( has_post_thumbnail( $this->ID ) ) {
 			$post_image_id = get_post_thumbnail_id( $this->ID );
@@ -469,6 +500,9 @@ private function get_post_image_metadata() {
 		return $post_image_meta;
 	}
 
+	/**
+	 * Build HTML tag attributes.
+	 */
 	private function build_html_tag_attributes() {
 		$attributes = array();
 
@@ -484,6 +518,12 @@ private function build_html_tag_attributes() {
 		$this->add_data_by_key( 'html_tag_attributes', $attributes );
 	}
 
+	/**
+	 * Verify and include.
+	 *
+	 * @param string $file          File.
+	 * @param string $template_type Template type.
+	 */
 	private function verify_and_include( $file, $template_type ) {
 		$located_file = $this->locate_template( $file );
 		if ( $located_file ) {
@@ -498,15 +538,26 @@ private function verify_and_include( $file, $template_type ) {
 		}
 
 		do_action( 'amp_post_template_include_' . $template_type, $this );
-		include( $file );
+		include $file;
 	}
 
-
+	/**
+	 * Locate template.
+	 *
+	 * @param string $file File.
+	 * @return string The template filename if one is located.
+	 */
 	private function locate_template( $file ) {
 		$search_file = sprintf( 'amp/%s', basename( $file ) );
 		return locate_template( array( $search_file ), false );
 	}
 
+	/**
+	 * Is valid template.
+	 *
+	 * @param string $template Template name.
+	 * @return bool Whether valid.
+	 */
 	private function is_valid_template( $template ) {
 		if ( false !== strpos( $template, '..' ) ) {
 			return false;
diff --git a/includes/utils/class-amp-dom-utils.php b/includes/utils/class-amp-dom-utils.php
index 2b112ff9d70..1af767828ca 100644
--- a/includes/utils/class-amp-dom-utils.php
+++ b/includes/utils/class-amp-dom-utils.php
@@ -1,14 +1,40 @@
 <?php
+/**
+ * Class AMP_DOM_Utils.
+ *
+ * @package AMP
+ */
 
+/**
+ * Class AMP_DOM_Utils
+ *
+ * Functionality to simplify working with DOMDocuments and DOMElements.
+ */
 class AMP_DOM_Utils {
+
+	/**
+	 * Return a valid DOMDocument representing arbitrary HTML content passed as a parameter.
+	 *
+	 * @see Reciprocal function get_content_from_dom()
+	 *
+	 * @since 0.2
+	 *
+	 * @param string $content Valid HTML content to be represented by a DOMDocument.
+	 *
+	 * @return DOMDocument|false Returns DOMDocument, or false if conversion failed.
+	 */
 	public static function get_dom_from_content( $content ) {
 		$libxml_previous_state = libxml_use_internal_errors( true );
 
-		$dom = new DOMDocument;
-		// Wrap in dummy tags, since XML needs one parent node.
-		// It also makes it easier to loop through nodes.
-		// We can later use this to extract our nodes.
-		// Add utf-8 charset so loadHTML does not have problems parsing it. See: http://php.net/manual/en/domdocument.loadhtml.php#78243
+		$dom = new DOMDocument();
+
+		/*
+		 * Wrap in dummy tags, since XML needs one parent node.
+		 * It also makes it easier to loop through nodes.
+		 * We can later use this to extract our nodes.
+		 * Add utf-8 charset so loadHTML does not have problems parsing it.
+		 * See: http://php.net/manual/en/domdocument.loadhtml.php#78243
+		 */
 		$result = $dom->loadHTML( '<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body>' . $content . '</body></html>' );
 
 		libxml_clear_errors();
@@ -21,28 +47,124 @@ public static function get_dom_from_content( $content ) {
 		return $dom;
 	}
 
+	/**
+	 * Return valid HTML content extracted from the DOMDocument passed as a parameter.
+	 *
+	 * @see Reciprocal function get_dom_from_content()
+	 *
+	 * @since 0.2
+	 *
+	 * @param DOMDocument $dom Represents an HTML document from which to extract HTML content.
+	 *
+	 * @return string Returns the HTML content represented in the DOMDocument
+	 */
 	public static function get_content_from_dom( $dom ) {
-		// Only want children of the body tag, since we have a subset of HTML.
-		$out = '';
+
+		/**
+		 * We only want children of the body tag, since we have a subset of HTML.
+		 */
 		$body = $dom->getElementsByTagName( 'body' )->item( 0 );
 
-		// AMP elements always need closing tags.
-		// To force them, we can't use saveHTML (node support is 5.3+) and LIBXML_NOEMPTYTAG results in issues with self-closing tags like `br` and `hr`.
-		// So, we're manually forcing closing tags.
-		self::recursive_force_closing_tags( $dom, $body );
+		/**
+		 * The DOMDocument may contain no body. In which case return nothing.
+		 */
+		if ( is_null( $body ) ) {
+			return '';
+		}
+
+		$out = '';
 
-		foreach ( $body->childNodes as $node ) {
-			$out .= $dom->saveXML( $node );
+		foreach ( $body->childNodes as $child_node ) {
+			$out .= self::get_content_from_dom_node( $dom, $child_node );
 		}
+
 		return $out;
 	}
 
+
+	/**
+	 * Return valid HTML content extracted from the DOMNode passed as a parameter.
+	 *
+	 * @see Called by function get_content_from_dom()
+	 *
+	 * @since 0.6.0
+	 *
+	 * @param DOMDocument $dom  Represents an HTML document.
+	 * @param DOMNode     $node Represents an HTML element of the $dom from which to extract HTML content.
+	 * @return string Returns the HTML content represented in the DOMNode
+	 */
+	public static function get_content_from_dom_node( $dom, $node ) {
+		/**
+		 * Self closing tags regex.
+		 *
+		 * @var string Regular expression to match self-closing tags
+		 *      that saveXML() has generated a closing tag for.
+		 */
+		static $self_closing_tags_regex;
+
+		/*
+		 * Most AMP elements need closing tags. To force them, we cannot use
+		 * saveHTML (node support is 5.3+) and LIBXML_NOEMPTYTAG results in
+		 * issues with self-closing tags like `br` and `hr`. So, we're manually
+		 * forcing closing tags.
+		 */
+		self::recursive_force_closing_tags( $dom, $node );
+
+		/*
+		 * Cache this regex so we don't have to recreate it every call.
+		 */
+		if ( ! isset( $self_closing_tags_regex ) ) {
+			$self_closing_tags       = implode( '|', self::get_self_closing_tags() );
+			$self_closing_tags_regex = "#></({$self_closing_tags})>#i";
+		}
+
+		$html = $dom->saveXML( $node );
+
+		// Whitespace just causes unit tests to fail... so whitespace begone.
+		if ( '' === trim( $html ) ) {
+			return '';
+		}
+
+		/*
+		 * Travis w/PHP 7.1 generates <br></br> and <hr></hr> vs. <br/> and <hr/>, respectively.
+		 * Travis w/PHP 7.x generates <source ...></source> vs. <source ... />.  Etc.
+		 * Seems like LIBXML_NOEMPTYTAG was passed, but as you can see it was not.
+		 * This does not happen in my (@mikeschinkel) local testing, btw.
+		 */
+		$html = preg_replace( $self_closing_tags_regex, '/>', $html );
+
+		return $html;
+
+	}
+
+	/**
+	 * Create a new node w/attributes (a DOMElement) and add to the passed DOMDocument.
+	 *
+	 * @since 0.2
+	 *
+	 * @param DOMDocument $dom        A representation of an HTML document to add the new node to.
+	 * @param string      $tag        A valid HTML element tag for the element to be added.
+	 * @param string[]    $attributes One of more valid attributes for the new node.
+	 *
+	 * @return DOMElement|false The DOMElement for the given $tag, or false on failure
+	 */
 	public static function create_node( $dom, $tag, $attributes ) {
 		$node = $dom->createElement( $tag );
 		self::add_attributes_to_node( $node, $attributes );
+
 		return $node;
 	}
 
+	/**
+	 * Extract a DOMElement node's HTML element attributes and return as an array.
+	 *
+	 * @since 0.2
+	 *
+	 * @param DOMNode $node Represents an HTML element for which to extract attributes.
+	 *
+	 * @return string[] The attributes for the passed node, or an
+	 *                  empty array if it has no attributes.
+	 */
 	public static function get_node_attributes_as_assoc_array( $node ) {
 		$attributes = array();
 		if ( ! $node->hasAttributes() ) {
@@ -52,48 +174,110 @@ public static function get_node_attributes_as_assoc_array( $node ) {
 		foreach ( $node->attributes as $attribute ) {
 			$attributes[ $attribute->nodeName ] = $attribute->nodeValue;
 		}
+
 		return $attributes;
 	}
 
+	/**
+	 * Add one or more HTML element attributes to a node's DOMElement.
+	 *
+	 * @since 0.2
+	 *
+	 * @param DOMElement $node       Represents an HTML element.
+	 * @param string[]   $attributes One or more attributes for the node's HTML element.
+	 */
 	public static function add_attributes_to_node( $node, $attributes ) {
 		foreach ( $attributes as $name => $value ) {
 			$node->setAttribute( $name, $value );
 		}
 	}
 
+	/**
+	 * Determines if a DOMElement's node is empty or not..
+	 *
+	 * @since 0.2
+	 *
+	 * @param DOMElement $node Represents an HTML element.
+	 * @return bool Returns true if the DOMElement has no child nodes and
+	 *              the textContent property of the DOMElement is empty;
+	 *              Otherwise it returns false.
+	 */
 	public static function is_node_empty( $node ) {
-		return false === $node->hasChildNodes()
-			&& empty( $node->textContent );
+		return false === $node->hasChildNodes() && empty( $node->textContent );
 	}
 
-	public static function recursive_force_closing_tags( $dom, $node ) {
+	/**
+	 * Forces HTML element closing tags given a DOMDocument and optional DOMElement
+	 *
+	 * @since 0.2
+	 *
+	 * @param DOMDocument $dom  Represents HTML document on which to force closing tags.
+	 * @param DOMElement  $node Represents HTML element to start closing tags on.
+	 *                          If not passed, defaults to first child of body.
+	 */
+	public static function recursive_force_closing_tags( $dom, $node = null ) {
+
+		if ( is_null( $node ) ) {
+			$node = $dom->getElementsByTagName( 'body' )->item( 0 );
+		}
+
 		if ( XML_ELEMENT_NODE !== $node->nodeType ) {
 			return;
 		}
 
 		if ( self::is_self_closing_tag( $node->nodeName ) ) {
+			/*
+			 * Ensure there is no text content to accidentally force a child
+			 */
+			$node->textContent = null;
 			return;
 		}
 
 		if ( self::is_node_empty( $node ) ) {
 			$text_node = $dom->createTextNode( '' );
 			$node->appendChild( $text_node );
+
 			return;
 		}
 
 		$num_children = $node->childNodes->length;
-		for ( $i = $num_children - 1; $i >= 0; $i-- ) {
+		for ( $i = $num_children - 1; $i >= 0; $i -- ) {
 			$child = $node->childNodes->item( $i );
 			self::recursive_force_closing_tags( $dom, $child );
 		}
+
 	}
 
+	/**
+	 * Determines if an HTML element tag is validly a self-closing tag per W3C HTML5 specs.
+	 *
+	 * @since 0.2
+	 *
+	 * @param string $tag Tag.
+	 * @return bool Returns true if a valid self-closing tag, false if not.
+	 */
 	private static function is_self_closing_tag( $tag ) {
-		// This function is called a lot; the static var prevents having to re-create the array every time.
+		return in_array( $tag, self::get_self_closing_tags(), true );
+	}
+
+	/**
+	 * Returns array of self closing tags
+	 *
+	 * @since 0.6.0
+	 *
+	 * @return string[]
+	 */
+	private static function get_self_closing_tags() {
+		/*
+		 * As this function is called a lot the static var
+		 * prevents having to re-create the array every time.
+		 */
 		static $self_closing_tags;
 		if ( ! isset( $self_closing_tags ) ) {
-			// https://www.w3.org/TR/html5/syntax.html#serializing-html-fragments
-			// Not all are valid AMP, but we include them for completeness.
+			/*
+			 * https://www.w3.org/TR/html5/syntax.html#serializing-html-fragments
+			 * Not all are valid AMP, but we include them for completeness.
+			 */
 			$self_closing_tags = array(
 				'area',
 				'base',
@@ -115,7 +299,6 @@ private static function is_self_closing_tag( $tag ) {
 				'wbr',
 			);
 		}
-
-		return in_array( $tag, $self_closing_tags, true );
+		return $self_closing_tags;
 	}
 }
diff --git a/includes/utils/class-amp-image-dimension-extractor.php b/includes/utils/class-amp-image-dimension-extractor.php
index 47a0b923c25..e659b4fc860 100644
--- a/includes/utils/class-amp-image-dimension-extractor.php
+++ b/includes/utils/class-amp-image-dimension-extractor.php
@@ -172,12 +172,9 @@ private static function fetch_images( $urls_to_fetch, &$images, $mode ) {
 	 * @param array $images Array to populate with results of image/dimension inspection.
 	 */
 	private static function fetch_images_via_fast_image( $urls_to_fetch, &$images ) {
-		if ( ! class_exists( 'FastImage' ) ) {
-			require_once( AMP__DIR__ . '/includes/lib/fastimage/class-fastimage.php' );
-		}
 
 		$image = new FastImage();
-		$urls = array_keys( $urls_to_fetch );
+		$urls  = array_keys( $urls_to_fetch );
 
 		foreach ( $urls as $url ) {
 			$result = $image->load( $url );
@@ -185,6 +182,7 @@ private static function fetch_images_via_fast_image( $urls_to_fetch, &$images )
 				$images[ $url ]['size'] = self::STATUS_IMAGE_EXTRACTION_FAILED;
 			} else {
 				$size = $image->getSize();
+
 				$images[ $url ]['size'] = $size;
 			}
 		}
diff --git a/phpcs.xml b/phpcs.xml
index 10f6b6b7a6b..289569d8ff4 100644
--- a/phpcs.xml
+++ b/phpcs.xml
@@ -11,6 +11,18 @@
 		</properties>
 	</rule>
 
+	<rule ref="WordPress.Arrays.MultipleStatementAlignment.DoubleArrowNotAligned">
+		<severity>0</severity>
+	</rule>
+	<rule ref="WordPress.WP.EnqueuedResources">
+		<exclude-pattern>includes/actions/class-amp-paired-post-actions.php</exclude-pattern>
+	</rule>
+	<rule ref="WordPress.NamingConventions.ValidVariableName">
+		<properties>
+			<property name="customPropertiesWhitelist" value="childNodes,tagName,textContent,parentNode,nodeType,nodeName,nextSibling,firstChild,nodeValue,DEFAULT_ARGS" />
+		</properties>
+	</rule>
+
 	<arg value="s"/>
 	<arg name="extensions" value="php"/>
 	<file>.</file>
diff --git a/tests/amp-tag-and-attribute-sanitizer-private-methods-tests.php b/tests/amp-tag-and-attribute-sanitizer-private-methods-tests.php
index 0fea4c87c83..111c439148b 100644
--- a/tests/amp-tag-and-attribute-sanitizer-private-methods-tests.php
+++ b/tests/amp-tag-and-attribute-sanitizer-private-methods-tests.php
@@ -1,7 +1,5 @@
 <?php
 
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-allowed-tags-generated.php' );
-
 class AMP_Tag_And_Attribute_Sanitizer_Attr_Spec_Rules_Test extends WP_UnitTestCase {
 
 	protected $allowed_tags;
@@ -1943,15 +1941,21 @@ public function test_check_attr_spec_rule_disallowed_relative( $data, $expected
 		$this->assertEquals( $expected, $got );
 	}
 
-	// Use this to call private methods
-	public function invoke_method(&$object, $methodName, array $parameters = array()) {
-	    $reflection = new \ReflectionClass(get_class($object));
-	    $method = $reflection->getMethod($methodName);
-	    $method->setAccessible(true);
+	/**
+	 * Use this to call private methods.
+	 *
+	 * @param object $object      Object.
+	 * @param string $method_name Method name.
+	 * @param array  $parameters  Parameters.
+	 * @return mixed Result.
+	 */
+	public function invoke_method( &$object, $method_name, array $parameters = array() ) {
+		$reflection = new ReflectionClass( get_class( $object ) );
 
-	    return $method->invokeArgs($object, $parameters);
+		$method = $reflection->getMethod( $method_name );
+		$method->setAccessible( true );
+
+		return $method->invokeArgs( $object, $parameters );
 	}
 
 }
-
-?>
diff --git a/tests/test-amp-audio-converter.php b/tests/test-amp-audio-converter.php
index 986929918ec..b5e4ef2498e 100644
--- a/tests/test-amp-audio-converter.php
+++ b/tests/test-amp-audio-converter.php
@@ -1,5 +1,17 @@
 <?php
-
+/**
+ * Class AMP_Audio_Converter_Test.
+ *
+ * @package AMP
+ */
+
+/**
+ * Class AMP_Audio_Converter_Test
+ *
+ * This is here because PhpStorm cannot find them because of phpunit6-compat.php
+ *
+ * @method void assertEquals( mixed $expected, mixed $actual, string $errorMessage=null )
+ */
 class AMP_Audio_Converter_Test extends WP_UnitTestCase {
 	public function get_data() {
 		return array(
diff --git a/tests/test-amp-iframe-sanitizer.php b/tests/test-amp-iframe-sanitizer.php
index ae2e535a47e..fd082914382 100644
--- a/tests/test-amp-iframe-sanitizer.php
+++ b/tests/test-amp-iframe-sanitizer.php
@@ -69,11 +69,7 @@ public function get_data() {
 <iframe src="https://example.com/embed/132886713" width="500" height="281"></iframe>
 <iframe src="https://example.com/embed/132886713" width="500" height="281"></iframe>
 				',
-				'
-<amp-iframe src="https://example.com/embed/132886713" width="500" height="281" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 500px) 500px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe>
-<amp-iframe src="https://example.com/embed/132886713" width="500" height="281" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 500px) 500px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe>
-<amp-iframe src="https://example.com/embed/132886713" width="500" height="281" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 500px) 500px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe>
-				',
+				'<amp-iframe src="https://example.com/embed/132886713" width="500" height="281" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 500px) 500px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe><amp-iframe src="https://example.com/embed/132886713" width="500" height="281" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 500px) 500px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe><amp-iframe src="https://example.com/embed/132886713" width="500" height="281" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 500px) 500px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe>',
 			),
 
 			'multiple_different_iframes' => array(
@@ -82,11 +78,7 @@ public function get_data() {
 <iframe src="https://example.com/embed/67890" width="280" height="501"></iframe>
 <iframe src="https://example.com/embed/11111" width="700" height="601"></iframe>
 				',
-				'
-<amp-iframe src="https://example.com/embed/12345" width="500" height="281" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 500px) 500px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe>
-<amp-iframe src="https://example.com/embed/67890" width="280" height="501" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 280px) 280px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe>
-<amp-iframe src="https://example.com/embed/11111" width="700" height="601" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 700px) 700px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe>
-				',
+				'<amp-iframe src="https://example.com/embed/12345" width="500" height="281" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 500px) 500px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe><amp-iframe src="https://example.com/embed/67890" width="280" height="501" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 280px) 280px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe><amp-iframe src="https://example.com/embed/11111" width="700" height="601" sandbox="allow-scripts allow-same-origin" sizes="(min-width: 700px) 700px, 100vw" class="amp-wp-enforced-sizes"></amp-iframe>',
 			),
 			'iframe_in_p_tag' => array(
 				'<p><iframe src="https://example.com/video/132886713" width="500" height="281"></iframe></p>',
diff --git a/tests/test-class-amp-dom-utils.php b/tests/test-class-amp-dom-utils.php
index 188100ac957..afc838f80a7 100644
--- a/tests/test-class-amp-dom-utils.php
+++ b/tests/test-class-amp-dom-utils.php
@@ -1,5 +1,14 @@
 <?php
 
+/**
+ * Class AMP_DOM_Utils_Test
+ *
+ * These are here because PhpStorm cannot find them because of phpunit6-compat.php
+ *
+ * @method void assertEquals( mixed $expected, mixed $actual, string $errorMessage=null )
+ * @method void assertTrue( bool $expectsTrue, string $errorMessage=null )
+ * @method void assertFalse( bool $expectsFalse, string $errorMessage=null )
+ */
 class AMP_DOM_Utils_Test extends WP_UnitTestCase {
 	public function test_utf8_content() {
 		$source = '<p>Iñtërnâtiônàlizætiøn</p>';
@@ -87,9 +96,8 @@ public function test__recursive_force_closing_tags__ignore_non_elements() {
 		$node = $dom->createAttribute( 'src' );
 		$expected = ' src=""';
 
-		$actual = AMP_DOM_Utils::recursive_force_closing_tags( $dom, $node );
-
-		$this->assertEquals( $expected, $dom->saveXML( $node ) );
+		$actual = AMP_DOM_Utils::get_content_from_dom_node( $dom, $node );
+		$this->assertEquals( $expected, $actual );
 	}
 
 	public function test__recursive_force_closing_tags__ignore_self_closing() {
@@ -97,9 +105,9 @@ public function test__recursive_force_closing_tags__ignore_self_closing() {
 		$node = $dom->createElement( 'br' );
 		$expected = '<br/>';
 
-		$actual = AMP_DOM_Utils::recursive_force_closing_tags( $dom, $node );
+		$actual = AMP_DOM_Utils::get_content_from_dom_node( $dom, $node );
 
-		$this->assertEquals( $expected, $dom->saveXML( $node ) );
+		$this->assertEquals( $expected, $actual );
 	}
 
 	public function test__recursive_force_closing_tags__ignore_non_empty() {
@@ -109,9 +117,10 @@ public function test__recursive_force_closing_tags__ignore_non_empty() {
 		$node->appendChild( $text );
 		$expected = '<p>Hello</p>';
 
-		$actual = AMP_DOM_Utils::recursive_force_closing_tags( $dom, $node );
+		$actual = AMP_DOM_Utils::get_content_from_dom_node( $dom, $node );
+
+		$this->assertEquals( $expected, $actual );
 
-		$this->assertEquals( $expected, $dom->saveXML( $node ) );
 	}
 
 	public function test__recursive_force_closing_tags__force_close() {
@@ -136,8 +145,8 @@ public function test__recursive_force_closing_tags__force_close_with_children()
 		$node->appendChild( $child_self_closing );
 		$expected = '<div><amp-img></amp-img><br/></div>';
 
-		AMP_DOM_Utils::recursive_force_closing_tags( $dom, $node );
+		$actual = AMP_DOM_Utils::get_content_from_dom_node( $dom, $node );
 
-		$this->assertEquals( $expected, $dom->saveXML( $node ) );
+		$this->assertEquals( $expected, $actual );
 	}
 }
diff --git a/tests/test-tag-and-attribute-sanitizer.php b/tests/test-tag-and-attribute-sanitizer.php
index d934de59dee..ab7468ca1d4 100644
--- a/tests/test-tag-and-attribute-sanitizer.php
+++ b/tests/test-tag-and-attribute-sanitizer.php
@@ -1,7 +1,5 @@
 <?php
 
-require_once( AMP__DIR__ . '/includes/sanitizers/class-amp-allowed-tags-generated.php' );
-
 class AMP_Tag_And_Attribute_Sanitizer_Test extends WP_UnitTestCase {
 
 	protected $allowed_tags;
diff --git a/wpcom/class-amp-polldaddy-embed.php b/wpcom/class-amp-polldaddy-embed.php
index 3810a4a60cd..f48c4e8da5b 100644
--- a/wpcom/class-amp-polldaddy-embed.php
+++ b/wpcom/class-amp-polldaddy-embed.php
@@ -6,7 +6,7 @@ public function register_embed() {
 	}
 
 	public function unregister_embed() {
-		remove_shortcode( 'polldaddy', array( $this, 'shortcode' ) );
+		remove_shortcode( 'polldaddy' );
 	}
 
 	public function shortcode( $attr ) {
diff --git a/wpcom/shortcodes.php b/wpcom/shortcodes.php
index 9355c956313..e50402b4c24 100644
--- a/wpcom/shortcodes.php
+++ b/wpcom/shortcodes.php
@@ -1,10 +1,20 @@
 <?php
+/**
+ * Shortcode functions for WordPress.com.
+ *
+ * @package AMP
+ */
 
 add_filter( 'amp_content_embed_handlers', 'wpcom_amp_add_custom_embeds', 10, 2 );
 
+/**
+ * Add custom embeds for WordPress.com.
+ *
+ * @param array   $embed_handler_classes Embed handler classes.
+ * @param WP_Post $post                  Post.
+ * @return mixed
+ */
 function wpcom_amp_add_custom_embeds( $embed_handler_classes, $post ) {
-	require_once( dirname( __FILE__ ) . '/class-amp-polldaddy-embed.php' );
 	$embed_handler_classes['WPCOM_AMP_Polldaddy_Embed'] = array();
-
 	return $embed_handler_classes;
 }