class-amp-theme-support.php

<?php
/**
 * Class AMP_Theme_Support
 *
 * @package AMP
 */

use AmpProject\Amp;
use AmpProject\AmpWP\Option;
use AmpProject\AmpWP\QueryVars;
use AmpProject\AmpWP\RemoteRequest\CachedRemoteGetRequest;
use AmpProject\AmpWP\ConfigurationArgument;
use AmpProject\AmpWP\Transformer;
use AmpProject\Attribute;
use AmpProject\Dom\Document;
use AmpProject\Extension;
use AmpProject\Optimizer;
use AmpProject\RemoteRequest\FallbackRemoteGetRequest;
use AmpProject\RemoteRequest\FilesystemRemoteGetRequest;
use AmpProject\AmpWP\RemoteRequest\WpHttpRemoteGetRequest;
use AmpProject\Tag;

/**
 * Class AMP_Theme_Support
 *
 * Callbacks for adding AMP-related things when theme support is added.
 */
class AMP_Theme_Support {

	/**
	 * Theme support slug.
	 *
	 * @var string
	 */
	const SLUG = 'amp';

	/**
	 * Slug identifying standard website mode.
	 *
	 * @since 1.2
	 * @var string
	 */
	const STANDARD_MODE_SLUG = 'standard';

	/**
	 * Slug identifying transitional website mode.
	 *
	 * @since 1.2
	 * @var string
	 */
	const TRANSITIONAL_MODE_SLUG = 'transitional';

	/**
	 * Slug identifying reader website mode.
	 *
	 * @since 1.2
	 * @var string
	 */
	const READER_MODE_SLUG = 'reader';

	/**
	 * Flag used in args passed to add_theme_support('amp') to indicate transitional mode supported.
	 *
	 * @since 1.2
	 * @var string
	 */
	const PAIRED_FLAG = 'paired';

	/**
	 * The directory name in a theme where Reader Mode templates can be.
	 *
	 * For example, this could be at your-theme-name/amp.
	 *
	 * @var string
	 */
	const READER_MODE_TEMPLATE_DIRECTORY = 'amp';

	/**
	 * Query var for requests to open the paired browsing interface.
	 *
	 * @var string
	 */
	const PAIRED_BROWSING_QUERY_VAR = 'amp-paired-browsing';

	/**
	 * Sanitizers, with keys as class names and values as arguments.
	 *
	 * @var array[]
	 */
	protected static $sanitizer_classes = [];

	/**
	 * Embed handlers.
	 *
	 * @var AMP_Base_Embed_Handler[]
	 */
	protected static $embed_handlers = [];

	/**
	 * Template types.
	 *
	 * @var array
	 */
	protected static $template_types = [
		'paged', // Deprecated.
		'index',
		'404',
		'archive',
		'author',
		'category',
		'tag',
		'taxonomy',
		'date',
		'home',
		'front_page',
		'page',
		'search',
		'single',
		'embed',
		'singular',
		'attachment',
	];

	/**
	 * Start time when init was called.
	 *
	 * @since 1.0
	 * @var float
	 */
	public static $init_start_time;

	/**
	 * Whether output buffering has started.
	 *
	 * @since 0.7
	 * @var bool
	 */
	protected static $is_output_buffering = false;

	/**
	 * Theme support mode that was added via option.
	 *
	 * This should be either null (reader), 'standard', or 'transitional'.
	 *
	 * @since 1.0
	 * @var null|string
	 */
	protected static $support_added_via_option;

	/**
	 * Theme support mode which was added via the theme.
	 *
	 * This should be either null (reader), 'standard', or 'transitional'.
	 *
	 * @var null|string
	 */
	protected static $support_added_via_theme;

	/**
	 * Initialize.
	 *
	 * @since 0.7
	 */
	public static function init() {
		self::read_theme_support();

		self::$init_start_time = microtime( true );

		if ( self::READER_MODE_SLUG !== self::get_support_mode() ) {
			// Ensure extra theme support for core themes is in place.
			AMP_Core_Theme_Sanitizer::extend_theme_support();
		}

		/*
		 * Note that wp action is use instead of template_redirect because some themes/plugins output
		 * the response at this action and then short-circuit with exit. So this is why the the preceding
		 * action to template_redirect--the wp action--is used instead.
		 */
		add_action( 'wp', [ __CLASS__, 'finish_init' ], PHP_INT_MAX );
	}

	/**
	 * Determine whether theme support was added via admin option.
	 *
	 * @since 1.0
	 * @see AMP_Theme_Support::read_theme_support()
	 * @see AMP_Theme_Support::get_support_mode()
	 * @codeCoverageIgnore
	 * @deprecated Use AMP_Theme_Support::get_support_mode_added_via_option().
	 *
	 * @return bool Support added via option.
	 */
	public static function is_support_added_via_option() {
		_deprecated_function( __METHOD__, '1.2', 'AMP_Theme_Support::get_support_mode_added_via_option' );
		return null !== self::$support_added_via_option;
	}

	/**
	 * Get the theme support mode added via admin option.
	 *
	 * @return null|string Support added via option, with null meaning Reader, and otherwise being 'standard' or 'transitional'.
	 * @see AMP_Theme_Support::read_theme_support()
	 * @see AMP_Theme_Support::TRANSITIONAL_MODE_SLUG
	 * @see AMP_Theme_Support::STANDARD_MODE_SLUG
	 *
	 * @since 1.2
	 */
	public static function get_support_mode_added_via_option() {
		return self::$support_added_via_option;
	}

	/**
	 * Get the theme support mode added via admin option.
	 *
	 * @return null|string Support added via option, with null meaning Reader, and otherwise being 'standard' or 'transitional'.
	 * @see AMP_Theme_Support::read_theme_support()
	 * @see AMP_Theme_Support::TRANSITIONAL_MODE_SLUG
	 * @see AMP_Theme_Support::STANDARD_MODE_SLUG
	 *
	 * @since 1.2
	 */
	public static function get_support_mode_added_via_theme() {
		return self::$support_added_via_theme;
	}

	/**
	 * Get theme support mode.
	 *
	 * @return string Theme support mode.
	 * @see AMP_Theme_Support::read_theme_support()
	 * @see AMP_Theme_Support::TRANSITIONAL_MODE_SLUG
	 * @see AMP_Theme_Support::STANDARD_MODE_SLUG
	 *
	 * @since 1.2
	 */
	public static function get_support_mode() {
		$theme_support = self::get_support_mode_added_via_option();
		if ( ! $theme_support ) {
			$theme_support = self::get_support_mode_added_via_theme();
		}
		if ( ! $theme_support ) {
			$theme_support = self::READER_MODE_SLUG;
		}
		return $theme_support;
	}

	/**
	 * Check theme support args or add theme support if option is set in the admin.
	 *
	 * The DB option is only considered if the theme does not already explicitly support AMP.
	 *
	 * @see AMP_Theme_Support::get_support_mode_added_via_theme()
	 * @see AMP_Theme_Support::get_support_mode_added_via_option()
	 * @see AMP_Post_Type_Support::add_post_type_support() For where post type support is added, since it is irrespective of theme support.
	 */
	public static function read_theme_support() {
		self::$support_added_via_theme  = null;
		self::$support_added_via_option = null;

		$theme_support_option = AMP_Options_Manager::get_option( Option::THEME_SUPPORT );
		if ( current_theme_supports( self::SLUG ) ) {
			$args = self::get_theme_support_args();

			// Validate theme support usage.
			$keys = [ 'template_dir', 'comments_live_list', self::PAIRED_FLAG, 'templates_supported', 'available_callback', 'service_worker', 'nav_menu_toggle', 'nav_menu_dropdown' ];

			if ( count( array_diff( array_keys( $args ), $keys ) ) !== 0 ) {
				_doing_it_wrong(
					'add_theme_support',
					esc_html(
						sprintf(  // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_trigger_error
							/* translators: 1: comma-separated list of expected keys, 2: comma-separated list of actual keys */
							__( 'Expected AMP theme support to keys (%1$s) but saw (%2$s)', 'amp' ),
							implode( ', ', $keys ),
							implode( ', ', array_keys( $args ) )
						)
					),
					'1.0'
				);
			}

			if ( isset( $args['available_callback'] ) ) {
				_doing_it_wrong(
					'add_theme_support',
					sprintf(
						/* translators: 1: available_callback. 2: supported_templates */
						esc_html__( 'The %1$s is deprecated when adding amp theme support in favor of declaratively setting the %2$s.', 'amp' ),
						'available_callback',
						Option::SUPPORTED_TEMPLATES // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
					),
					'1.0'
				);
			}

			// See amp_is_canonical().
			$is_paired = isset( $args[ self::PAIRED_FLAG ] ) ? $args[ self::PAIRED_FLAG ] : ! empty( $args['template_dir'] );

			self::$support_added_via_theme  = $is_paired ? self::TRANSITIONAL_MODE_SLUG : self::STANDARD_MODE_SLUG;
			self::$support_added_via_option = $theme_support_option;

			// Make sure the user option can override what the theme has specified.
			if ( $is_paired && self::STANDARD_MODE_SLUG === $theme_support_option ) {
				$args[ self::PAIRED_FLAG ] = false;
				add_theme_support( self::SLUG, $args );
			} elseif ( ! $is_paired && self::TRANSITIONAL_MODE_SLUG === $theme_support_option ) {
				$args[ self::PAIRED_FLAG ] = true;
				add_theme_support( self::SLUG, $args );
			} elseif ( self::READER_MODE_SLUG === $theme_support_option ) {
				remove_theme_support( self::SLUG );
			}
		} elseif ( self::READER_MODE_SLUG !== $theme_support_option ) {
			$is_paired = ( self::TRANSITIONAL_MODE_SLUG === $theme_support_option );
			add_theme_support(
				self::SLUG,
				[
					self::PAIRED_FLAG => $is_paired,
				]
			);
			self::$support_added_via_option = $is_paired ? self::TRANSITIONAL_MODE_SLUG : self::STANDARD_MODE_SLUG;
		}
	}

	/**
	 * Get the theme support args.
	 *
	 * This avoids having to repeatedly call `get_theme_support()`, check the args, shift an item off the array, and so on.
	 *
	 * @since 1.0
	 *
	 * @return array|false Theme support args, or false if theme support is not present.
	 */
	public static function get_theme_support_args() {
		if ( ! current_theme_supports( self::SLUG ) ) {
			return false;
		}
		$support = get_theme_support( self::SLUG );
		if ( true === $support ) {
			return [
				self::PAIRED_FLAG => false,
			];
		}
		if ( ! isset( $support[0] ) || ! is_array( $support[0] ) ) {
			return [];
		}
		return $support[0];
	}

	/**
	 * Gets whether the parent or child theme supports Reader Mode.
	 *
	 * True if the theme does not call add_theme_support( 'amp' ) at all,
	 * and it has an amp/ directory for templates.
	 *
	 * @return bool Whether the theme supports Reader Mode.
	 */
	public static function supports_reader_mode() {
		return (
			! self::get_support_mode_added_via_theme()
			&&
			(
				is_dir( trailingslashit( get_template_directory() ) . self::READER_MODE_TEMPLATE_DIRECTORY )
				||
				is_dir( trailingslashit( get_stylesheet_directory() ) . self::READER_MODE_TEMPLATE_DIRECTORY )
			)
		);
	}

	/**
	 * Finish initialization once query vars are set.
	 *
	 * @since 0.7
	 */
	public static function finish_init() {
		if ( self::is_paired_available() ) {
			self::setup_paired_browsing_client();
			add_action( 'template_redirect', [ __CLASS__, 'sanitize_url_for_paired_browsing' ] );
			add_filter( 'template_include', [ __CLASS__, 'serve_paired_browsing_experience' ] );
		}

		$is_reader_mode = self::READER_MODE_SLUG === self::get_support_mode();
		$has_query_var  = (
			isset( $_GET[ amp_get_slug() ] ) // phpcs:ignore WordPress.Security.NonceVerification.Recommended
			||
			false !== get_query_var( amp_get_slug(), false )
		);

		if ( ! is_amp_endpoint() ) {
			/*
			 * Redirect to AMP-less URL if AMP is not available for this URL and yet the query var is present.
			 * Temporary redirect is used for admin users because implied transitional mode and template support can be
			 * enabled by user ay any time, so they will be able to make AMP available for this URL and see the change
			 * without wrestling with the redirect cache.
			 */
			if ( $has_query_var ) {
				self::redirect_non_amp_url( current_user_can( 'manage_options' ) ? 302 : 301 );
			}

			amp_add_frontend_actions();
			return;
		}

		self::ensure_proper_amp_location();

		$theme_support = self::get_theme_support_args();
		if ( ! empty( $theme_support['template_dir'] ) ) {
			self::add_amp_template_filters();
		} elseif ( $is_reader_mode ) {
			add_filter(
				'template_include',
				static function() {
					return AMP__DIR__ . '/includes/templates/reader-template-loader.php';
				},
				PHP_INT_MAX
			);
		}

		self::add_hooks();
		self::$sanitizer_classes = amp_get_content_sanitizers();
		self::$sanitizer_classes = AMP_Validation_Manager::filter_sanitizer_args( self::$sanitizer_classes );
		self::$embed_handlers    = self::register_content_embed_handlers();
		self::$sanitizer_classes['AMP_Embed_Sanitizer']['embed_handlers'] = self::$embed_handlers;

		foreach ( self::$sanitizer_classes as $sanitizer_class => $args ) {
			if ( method_exists( $sanitizer_class, 'add_buffering_hooks' ) ) {
				call_user_func( [ $sanitizer_class, 'add_buffering_hooks' ], $args );
			}
		}
	}

	/**
	 * Ensure that the current AMP location is correct.
	 *
	 * @since 1.0
	 * @since 1.6 Removed $exit param.
	 *
	 * @return bool Whether redirection should have been done.
	 */
	public static function ensure_proper_amp_location() {
		$has_query_var = false !== get_query_var( amp_get_slug(), false ); // May come from URL param or endpoint slug.
		$has_url_param = isset( $_GET[ amp_get_slug() ] ); // phpcs:ignore WordPress.Security.NonceVerification.Recommended

		if ( amp_is_canonical() ) {
			/*
			 * When AMP-first/canonical, then when there is an /amp/ endpoint or ?amp URL param,
			 * then a redirect needs to be done to the URL without any AMP indicator in the URL.
			 * Permanent redirect is used for unauthenticated users since switching between modes
			 * should happen infrequently. For admin users, this is kept temporary to allow them
			 * to not be hampered by browser remembering permanent redirects and preventing test.
			 */
			if ( $has_query_var || $has_url_param ) {
				return self::redirect_non_amp_url( current_user_can( 'manage_options' ) ? 302 : 301 );
			}
		} elseif ( self::READER_MODE_SLUG === self::get_support_mode() && is_singular() ) {
			// Prevent infinite URL space under /amp/ endpoint.
			global $wp;
			$path_args = [];
			wp_parse_str( $wp->matched_query, $path_args );
			if ( isset( $path_args[ amp_get_slug() ] ) && '' !== $path_args[ amp_get_slug() ] ) {
				if ( wp_safe_redirect( amp_get_permalink( get_queried_object_id() ), 301 ) ) {
					// @codeCoverageIgnoreStart
					exit;
					// @codeCoverageIgnoreEnd
				}
				return true;
			}
		} elseif ( $has_query_var && ! $has_url_param ) {
			/*
			 * When in AMP transitional mode *with* theme support, then the proper AMP URL has the 'amp' URL param
			 * and not the /amp/ endpoint. The URL param is now the exclusive way to mark AMP in transitional mode
			 * when amp theme support present. This is important for plugins to be able to reliably call
			 * is_amp_endpoint() before the parse_query action.
			 */
			$old_url = amp_get_current_url();
			$new_url = add_query_arg( amp_get_slug(), '', amp_remove_endpoint( $old_url ) );
			if ( $old_url !== $new_url ) {
				// A temporary redirect is used for admin users to allow them to see changes between reader mode and transitional modes.
				if ( wp_safe_redirect( $new_url, current_user_can( 'manage_options' ) ? 302 : 301 ) ) {
					// @codeCoverageIgnoreStart
					exit;
					// @codeCoverageIgnoreEnd
				}
				return true;
			}
		}
		return false;
	}

	/**
	 * Redirect to non-AMP version of the current URL, such as because AMP is canonical or there are unaccepted validation errors.
	 *
	 * If the current URL is already AMP-less then do nothing.
	 *
	 * @since 0.7
	 * @since 1.0 Added $exit param.
	 * @since 1.0 Renamed from redirect_canonical_amp().
	 * @since 1.6 Removed $exit param.
	 *
	 * @param int $status Status code (301 or 302).
	 * @return bool Whether redirection should have be done.
	 */
	public static function redirect_non_amp_url( $status = 302 ) {
		$current_url = amp_get_current_url();
		$non_amp_url = amp_remove_endpoint( $current_url );
		if ( $non_amp_url === $current_url ) {
			return false;
		}

		if ( wp_safe_redirect( $non_amp_url, $status ) ) {
			// @codeCoverageIgnoreStart
			exit;
			// @codeCoverageIgnoreEnd
		}
		return true;
	}

	/**
	 * Determines whether transitional mode is available.
	 *
	 * When 'amp' theme support has not been added or canonical mode is enabled, then this returns false.
	 *
	 * @since 0.7
	 *
	 * @see amp_is_canonical()
	 * @return bool Whether available.
	 */
	public static function is_paired_available() {
		if ( ! current_theme_supports( self::SLUG ) ) {
			return false;
		}

		if ( amp_is_canonical() ) {
			return false;
		}

		$availability = self::get_template_availability();
		return $availability['supported'];
	}

	/**
	 * Determine whether the user is in the Customizer preview iframe.
	 *
	 * @since 0.7
	 *
	 * @return bool Whether in Customizer preview iframe.
	 */
	public static function is_customize_preview_iframe() {
		global $wp_customize;
		return is_customize_preview() && $wp_customize->get_messenger_channel();
	}

	/**
	 * Register filters for loading AMP-specific templates.
	 */
	public static function add_amp_template_filters() {
		foreach ( self::$template_types as $template_type ) {
			// See get_query_template().
			$template_type = preg_replace( '|[^a-z0-9-]+|', '', $template_type );

			add_filter( "{$template_type}_template_hierarchy", [ __CLASS__, 'filter_amp_template_hierarchy' ] );
		}
	}

	/**
	 * Determine template availability of AMP for the given query.
	 *
	 * This is not intended to return whether AMP is available for a _specific_ post. For that, use `post_supports_amp()`.
	 *
	 * @since 1.0
	 * @global WP_Query $wp_query
	 * @see post_supports_amp()
	 *
	 * @param WP_Query|WP_Post|null $query Query or queried post. If null then the global query will be used.
	 * @return array {
	 *     Template availability.
	 *
	 *     @type bool        $supported Whether the template is supported in AMP.
	 *     @type bool|null   $immutable Whether the supported status is known to be unchangeable.
	 *     @type string|null $template  The ID of the matched template (conditional), such as 'is_singular', or null if nothing was matched.
	 *     @type string[]    $errors    List of the errors or reasons for why the template is not available.
	 * }
	 */
	public static function get_template_availability( $query = null ) {
		global $wp_query;
		if ( ! $query ) {
			$query = $wp_query;
		} elseif ( $query instanceof WP_Post ) {
			$post  = $query;
			$query = new WP_Query();
			if ( 'page' === $post->post_type ) {
				$query->set( 'page_id', $post->ID );
			} else {
				$query->set( 'p', $post->ID );
			}
			$query->queried_object    = $post;
			$query->queried_object_id = $post->ID;
			$query->parse_query_vars();
		}

		$default_response = [
			'errors'    => [],
			'supported' => false,
			'immutable' => null,
			'template'  => null,
		];

		if ( ! ( $query instanceof WP_Query ) ) {
			_doing_it_wrong( __METHOD__, esc_html__( 'No WP_Query available.', 'amp' ), '1.0' );
			return array_merge(
				$default_response,
				[ 'errors' => [ 'no_query_available' ] ]
			);
		}

		$theme_support_args = self::get_theme_support_args();
		if ( false === $theme_support_args ) {
			return array_merge(
				$default_response,
				[ 'errors' => [ 'no_theme_support' ] ]
			);
		}

		// Support available_callback from 0.7, though it is deprecated.
		if ( isset( $theme_support_args['available_callback'] ) && is_callable( $theme_support_args['available_callback'] ) ) {
			/**
			 * Queried object.
			 *
			 * @var WP_Post $queried_object
			 */
			$queried_object = $query->get_queried_object();
			if ( ( is_singular() || $query->is_posts_page ) && ! post_supports_amp( $queried_object ) ) {
				return array_merge(
					$default_response,
					[
						'errors'    => [ 'no-post-support' ],
						'supported' => false,
						'immutable' => true,
					]
				);
			}

			$response = array_merge(
				$default_response,
				[
					'supported' => call_user_func( $theme_support_args['available_callback'] ),
					'immutable' => true,
				]
			);
			if ( ! $response['supported'] ) {
				$response['errors'][] = 'available_callback';
			}
			return $response;
		}

		$all_templates_supported_by_theme_support = false;
		if ( isset( $theme_support_args['templates_supported'] ) ) {
			$all_templates_supported_by_theme_support = 'all' === $theme_support_args['templates_supported'];
		}
		$all_templates_supported = (
			$all_templates_supported_by_theme_support || AMP_Options_Manager::get_option( Option::ALL_TEMPLATES_SUPPORTED )
		);

		// Make sure global $wp_query is set in case of conditionals that unfortunately look at global scope.
		$prev_query = $wp_query;
		$wp_query   = $query; // phpcs:ignore WordPress.WP.GlobalVariablesOverride.Prohibited

		$matching_templates    = [];
		$supportable_templates = self::get_supportable_templates();
		foreach ( $supportable_templates as $id => $supportable_template ) {
			if ( empty( $supportable_template['callback'] ) ) {
				$callback = $id;
			} else {
				$callback = $supportable_template['callback'];
			}

			// If the callback is a method on the query, then call the method on the query itself.
			if ( is_string( $callback ) && 'is_' === substr( $callback, 0, 3 ) && method_exists( $query, $callback ) ) {
				$is_match = call_user_func( [ $query, $callback ] );
			} elseif ( is_callable( $callback ) ) {
				$is_match = $callback( $query );
			} else {
				/* translators: %s: the supportable template ID. */
				_doing_it_wrong( __FUNCTION__, esc_html( sprintf( __( 'Supportable template "%s" does not have a callable callback.', 'amp' ), $id ) ), '1.0' );
				$is_match = false;
			}

			if ( $is_match ) {
				$matching_templates[ $id ] = [
					'template'  => $id,
					'supported' => ! empty( $supportable_template['supported'] ),
					'immutable' => ! empty( $supportable_template['immutable'] ),
				];
			}
		}

		// Restore previous $wp_query (if any).
		$wp_query = $prev_query; // phpcs:ignore WordPress.WP.GlobalVariablesOverride.Prohibited

		// Make sure children override their parents.
		$matching_template_ids = array_keys( $matching_templates );
		foreach ( array_diff( array_keys( $supportable_templates ), $matching_template_ids ) as $template_id ) {
			unset( $supportable_templates[ $template_id ] );
		}
		foreach ( $matching_template_ids as $id ) {
			$has_children = false;
			foreach ( $supportable_templates as $other_id => $supportable_template ) {
				if ( $other_id === $id ) {
					continue;
				}
				if ( isset( $supportable_template['parent'] ) && $id === $supportable_template['parent'] ) {
					$has_children = true;
					break;
				}
			}

			// Delete all matching parent templates since the child will override them.
			if ( ! $has_children ) {
				$supportable_template = $supportable_templates[ $id ];
				while ( ! empty( $supportable_template['parent'] ) ) {
					$parent = $supportable_template['parent'];

					/*
					 * If the parent is not amongst the supportable templates, then something is off in terms of hierarchy.
					 * Either the matching is off-track, or the template is badly configured.
					 */
					if ( ! array_key_exists( $parent, $supportable_templates ) ) {
						_doing_it_wrong(
							__METHOD__,
							esc_html(
								sprintf(
									/* translators: %s: amp_supportable_templates */
									__( 'An expected parent was not found. Did you filter %s to not honor the template hierarchy?', 'amp' ),
									'amp_supportable_templates'
								)
							),
							'1.4'
						);
						break;
					}

					$supportable_template = $supportable_templates[ $parent ];

					// Let the child supported status override the parent's supported status.
					unset( $matching_templates[ $parent ] );
				}
			}
		}

		// If there is more than 1 matching template, the is_home() condition is the default so discard it if there are other matching templates.
		if ( count( $matching_templates ) > 1 && isset( $matching_templates['is_home'] ) ) {
			unset( $matching_templates['is_home'] );
		}

		/*
		 * When there is still more than one matching template, account for ambiguous cases, informed by the order in template-loader.php.
		 * See <https://github.com/WordPress/wordpress-develop/blob/5.1.0/src/wp-includes/template-loader.php#L49-L68>.
		 */
		if ( count( $matching_templates ) > 1 ) {
			$template_conditional_priority_order = [
				'is_embed',
				'is_404',
				'is_search',
				'is_front_page',
				'is_home',
				'is_post_type_archive',
				'is_tax',
				'is_attachment',
				'is_single',
				'is_page',
				'is_singular',
				'is_category',
				'is_tag',
				'is_author',
				'is_date',
				'is_archive',
			];

			// Obtain the template conditionals for each matching template ID (e.g. 'is_post_type_archive[product]' => 'is_post_type_archive').
			$template_conditional_id_mapping = [];
			foreach ( array_keys( $matching_templates ) as $template_id ) {
				$template_conditional_id_mapping[ strtok( $template_id, '[' ) ] = $template_id;
			}

			// If there are any custom supportable templates, only consider them since they would override the conditional logic in core.
			$custom_template_conditions = array_diff(
				array_keys( $template_conditional_id_mapping ),
				$template_conditional_priority_order
			);
			if ( ! empty( $custom_template_conditions ) ) {
				$matching_templates = wp_array_slice_assoc(
					$matching_templates,
					array_values( wp_array_slice_assoc( $template_conditional_id_mapping, $custom_template_conditions ) )
				);
			} else {
				/*
				 * Otherwise, iterate over the template conditionals in the order they occur in the if/elseif/else conditional chain.
				 * to then populate $matching_templates with just this one entry.
				 */
				foreach ( $template_conditional_priority_order as $template_conditional ) {
					if ( isset( $template_conditional_id_mapping[ $template_conditional ] ) ) {
						$template_id        = $template_conditional_id_mapping[ $template_conditional ];
						$matching_templates = [
							$template_id => $matching_templates[ $template_id ],
						];
						break;
					}
				}
			}
		}

		/*
		 * If there are more than one matching templates, then something is probably not right.
		 * Template conditions need to be set up properly to prevent this from happening.
		 */
		if ( count( $matching_templates ) > 1 ) {
			_doing_it_wrong(
				__METHOD__,
				esc_html(
					sprintf(
						/* translators: %s: amp_supportable_templates */
						__( 'Did not expect there to be more than one matching template. Did you filter %s to not honor the template hierarchy?', 'amp' ),
						'amp_supportable_templates'
					)
				),
				'1.0'
			);
		}

		$matching_template = array_shift( $matching_templates );

		// If there aren't any matching templates left that are supported, then we consider it to not be available.
		if ( ! $matching_template ) {
			if ( $all_templates_supported ) {
				return array_merge(
					$default_response,
					[
						'supported' => true,
					]
				);
			}

			return array_merge(
				$default_response,
				[ 'errors' => [ 'no_matching_template' ] ]
			);
		}
		$matching_template = array_merge( $default_response, $matching_template );

		// If there aren't any matching templates left that are supported, then we consider it to not be available.
		if ( empty( $matching_template['supported'] ) ) {
			$matching_template['errors'][] = 'template_unsupported';
		}

		// For singular queries, post_supports_amp() is given the final say.
		if ( $query->is_singular() || $query->is_posts_page ) {
			/**
			 * Queried object.
			 *
			 * @var WP_Post $queried_object
			 */
			$queried_object = $query->get_queried_object();
			if ( $queried_object instanceof WP_Post ) {
				$support_errors = AMP_Post_Type_Support::get_support_errors( $queried_object );
				if ( ! empty( $support_errors ) ) {
					$matching_template['errors']    = array_merge( $matching_template['errors'], $support_errors );
					$matching_template['supported'] = false;
				}
			}
		}

		return $matching_template;
	}

	/**
	 * Get the templates which can be supported.
	 *
	 * @return array Supportable templates.
	 */
	public static function get_supportable_templates() {
		$templates = [
			'is_singular' => [
				'label'       => __( 'Singular', 'amp' ),
				'description' => __( 'Required for the above content types.', 'amp' ),
			],
		];
		if ( 'page' === get_option( 'show_on_front' ) ) {
			$templates['is_front_page'] = [
				'label'  => __( 'Homepage', 'amp' ),
				'parent' => 'is_singular',
			];
			if ( AMP_Post_Meta_Box::DISABLED_STATUS === get_post_meta( get_option( 'page_on_front' ), AMP_Post_Meta_Box::STATUS_POST_META_KEY, true ) ) {
				/* translators: %s: the URL to the edit post screen. */
				$templates['is_front_page']['description'] = sprintf( __( 'Currently disabled at the <a href="%s">page level</a>.', 'amp' ), esc_url( get_edit_post_link( get_option( 'page_on_front' ) ) ) );
			}

			// In other words, same as is_posts_page, *but* it not is_singular.
			$templates['is_home'] = [
				'label' => __( 'Blog', 'amp' ),
			];
			if ( AMP_Post_Meta_Box::DISABLED_STATUS === get_post_meta( get_option( 'page_for_posts' ), AMP_Post_Meta_Box::STATUS_POST_META_KEY, true ) ) {
				/* translators: %s: the URL to the edit post screen. */
				$templates['is_home']['description'] = sprintf( __( 'Currently disabled at the <a href="%s">page level</a>.', 'amp' ), esc_url( get_edit_post_link( get_option( 'page_for_posts' ) ) ) );
			}
		} else {
			$templates['is_home'] = [
				'label' => __( 'Homepage', 'amp' ),
			];
		}

		$templates = array_merge(
			$templates,
			[
				'is_archive' => [
					'label' => __( 'Archives', 'amp' ),
				],
				'is_author'  => [
					'label'  => __( 'Author', 'amp' ),
					'parent' => 'is_archive',
				],
				'is_date'    => [
					'label'  => __( 'Date', 'amp' ),
					'parent' => 'is_archive',
				],
				'is_search'  => [
					'label' => __( 'Search', 'amp' ),
				],
				'is_404'     => [
					'label' => __( 'Not Found (404)', 'amp' ),
				],
			]
		);

		if ( taxonomy_exists( 'category' ) ) {
			$templates['is_category'] = [
				'label'  => get_taxonomy( 'category' )->labels->name,
				'parent' => 'is_archive',
			];
		}
		if ( taxonomy_exists( 'post_tag' ) ) {
			$templates['is_tag'] = [
				'label'  => get_taxonomy( 'post_tag' )->labels->name,
				'parent' => 'is_archive',
			];
		}

		$taxonomy_args = [
			'_builtin' => false,
			'public'   => true,
		];
		foreach ( get_taxonomies( $taxonomy_args, 'objects' ) as $taxonomy ) {
			$templates[ sprintf( 'is_tax[%s]', $taxonomy->name ) ] = [
				'label'    => $taxonomy->labels->name,
				'parent'   => 'is_archive',
				'callback' => static function ( WP_Query $query ) use ( $taxonomy ) {
					return $query->is_tax( $taxonomy->name );
				},
			];
		}

		$post_type_args = [
			'has_archive' => true,
			'public'      => true,
		];
		foreach ( get_post_types( $post_type_args, 'objects' ) as $post_type ) {
			$templates[ sprintf( 'is_post_type_archive[%s]', $post_type->name ) ] = [
				'label'    => $post_type->labels->archives,
				'parent'   => 'is_archive',
				'callback' => static function ( WP_Query $query ) use ( $post_type ) {
					return $query->is_post_type_archive( $post_type->name );
				},
			];
		}

		/**
		 * Filters list of supportable templates.
		 *
		 * A theme or plugin can force a given template to be supported or not by preemptively
		 * setting the 'supported' flag for a given template. Otherwise, if the flag is undefined
		 * then the user will be able to toggle it themselves in the admin. Each array item should
		 * have a key that corresponds to a template conditional function. If the key is such a
		 * function, then the key is used to evaluate whether the given template entry is a match.
		 * Otherwise, a supportable template item can include a callback value which is used instead.
		 * Each item needs a 'label' value. Additionally, if the supportable template is a subset of
		 * another condition (e.g. is_singular > is_single) then this relationship needs to be
		 * indicated via the 'parent' value.
		 *
		 * @since 1.0
		 *
		 * @param array $templates Supportable templates.
		 */
		$templates = apply_filters( 'amp_supportable_templates', $templates );

		$theme_support_args        = self::get_theme_support_args();
		$theme_supported_templates = [];
		if ( isset( $theme_support_args['templates_supported'] ) ) {
			$theme_supported_templates = $theme_support_args['templates_supported'];
		}

		$supported_templates = AMP_Options_Manager::get_option( Option::SUPPORTED_TEMPLATES );
		foreach ( $templates as $id => &$template ) {

			// Capture user-elected support from options. This allows us to preserve the original user selection through programmatic overrides.
			$template['user_supported'] = in_array( $id, $supported_templates, true );

			// Consider supported templates from theme support args.
			if ( ! isset( $template['supported'] ) ) {
				if ( 'all' === $theme_supported_templates ) {
					$template['supported'] = true;
				} elseif ( is_array( $theme_supported_templates ) && isset( $theme_supported_templates[ $id ] ) ) {
					$template['supported'] = $theme_supported_templates[ $id ];
				}
			}

			// Make supported state immutable if it was programmatically set.
			$template['immutable'] = isset( $template['supported'] );

			// Set supported state from user preference.
			if ( ! $template['immutable'] ) {
				$template['supported'] = AMP_Options_Manager::get_option( Option::ALL_TEMPLATES_SUPPORTED ) || $template['user_supported'];
			}
		}

		return $templates;
	}

	/**
	 * Register hooks.
	 */
	public static function add_hooks() {

		// Remove core actions which are invalid AMP.
		remove_action( 'wp_head', 'wp_post_preview_js', 1 );
		remove_action( 'wp_head', 'wp_oembed_add_host_js' );

		// Replace JS-based emoji with PHP-based, if the JS-based emoji replacement was not already removed.
		if ( has_action( 'wp_head', 'print_emoji_detection_script' ) ) {
			remove_action( 'wp_head', 'print_emoji_detection_script', 7 );
			remove_action( 'wp_print_styles', 'print_emoji_styles' );
			add_action( 'wp_print_styles', [ __CLASS__, 'print_emoji_styles' ] );
			add_filter( 'the_title', 'wp_staticize_emoji' );
			add_filter( 'the_excerpt', 'wp_staticize_emoji' );
			add_filter( 'the_content', 'wp_staticize_emoji' );
			add_filter( 'comment_text', 'wp_staticize_emoji' );
			add_filter( 'widget_text', 'wp_staticize_emoji' );
		}

		// @todo The wp_mediaelement_fallback() should still run to be injected inside of the audio/video generated by wp_audio_shortcode()/wp_video_shortcode() respectively.
		// Prevent MediaElement.js scripts/styles from being enqueued.
		add_filter(
			'wp_video_shortcode_library',
			static function() {
				return 'amp';
			}
		);
		add_filter(
			'wp_audio_shortcode_library',
			static function() {
				return 'amp';
			}
		);

		// Don't show loading indicator on custom logo since it makes most sense for larger images.
		add_filter(
			'get_custom_logo',
			static function( $html ) {
				return preg_replace( '/(?<=<img\s)/', ' data-amp-noloading="" ', $html );
			},
			1
		);

		add_action( 'admin_bar_init', [ __CLASS__, 'init_admin_bar' ] );
		add_action( 'wp_head', 'amp_add_generator_metadata', 20 );
		add_action( 'wp_enqueue_scripts', [ __CLASS__, 'enqueue_assets' ], 0 ); // Enqueue before theme's styles.
		add_action( 'wp_enqueue_scripts', [ __CLASS__, 'dequeue_customize_preview_scripts' ], 1000 );
		add_filter( 'customize_partial_render', [ __CLASS__, 'filter_customize_partial_render' ] );

		add_action( 'wp_footer', 'amp_print_analytics' );

		/*
		 * Start output buffering at very low priority for sake of plugins and themes that use template_redirect
		 * instead of template_include.
		 */
		$priority = defined( 'PHP_INT_MIN' ) ? PHP_INT_MIN : ~PHP_INT_MAX; // phpcs:ignore PHPCompatibility.Constants.NewConstants.php_int_minFound
		add_action( 'template_redirect', [ __CLASS__, 'start_output_buffering' ], $priority );

		// Commenting hooks.
		add_filter( 'comment_form_defaults', [ __CLASS__, 'filter_comment_form_defaults' ], PHP_INT_MAX );
		add_filter( 'comment_reply_link', [ __CLASS__, 'filter_comment_reply_link' ], 10, 4 );
		add_filter( 'cancel_comment_reply_link', [ __CLASS__, 'filter_cancel_comment_reply_link' ], 10, 3 );
		add_action( 'comment_form', [ __CLASS__, 'amend_comment_form' ], 100 );
		remove_action( 'comment_form', 'wp_comment_form_unfiltered_html_nonce' );
		add_filter( 'wp_kses_allowed_html', [ __CLASS__, 'include_layout_in_wp_kses_allowed_html' ], 10 );
		add_filter( 'get_header_image_tag', [ __CLASS__, 'amend_header_image_with_video_header' ], PHP_INT_MAX );
		add_action(
			'wp_print_footer_scripts',
			static function() {
				wp_dequeue_script( 'wp-custom-header' );
			},
			0
		);
		add_action(
			'wp_enqueue_scripts',
			static function() {
				wp_dequeue_script( 'comment-reply' ); // Handled largely by AMP_Comments_Sanitizer and *reply* methods in this class.
			}
		);
	}

	/**
	 * Register/override widgets.
	 *
	 * @deprecated As of 1.6 the AMP_Core_Block_Handler will sanitize the core widgets instead.
	 * @global WP_Widget_Factory
	 * @return void
	 */
	public static function register_widgets() {
		_deprecated_function( __METHOD__, '1.6' );
		global $wp_widget_factory;
		foreach ( $wp_widget_factory->widgets as $registered_widget ) {
			$registered_widget_class_name = get_class( $registered_widget );
			if ( ! preg_match( '/^WP_Widget_(.+)$/', $registered_widget_class_name, $matches ) ) {
				continue;
			}
			$amp_class_name = 'AMP_Widget_' . $matches[1];
			if ( ! class_exists( $amp_class_name ) || is_a( $amp_class_name, $registered_widget_class_name ) ) {
				continue;
			}

			unregister_widget( $registered_widget_class_name );
			register_widget( $amp_class_name );
		}
	}

	/**
	 * Register content embed handlers.
	 *
	 * This was copied from `AMP_Content::register_embed_handlers()` due to being a private method
	 * and due to `AMP_Content` not being well suited for use in AMP canonical.
	 *
	 * @see AMP_Content::register_embed_handlers()
	 * @global int $content_width
	 * @return AMP_Base_Embed_Handler[] Handlers.
	 */
	public static function register_content_embed_handlers() {
		global $content_width;

		$embed_handlers = [];
		foreach ( amp_get_content_embed_handlers() as $embed_handler_class => $args ) {

			/**
			 * Embed handler.
			 *
			 * @type AMP_Base_Embed_Handler $embed_handler
			 */
			$embed_handler = new $embed_handler_class(
				array_merge(
					[
						'content_max_width' => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
					],
					$args
				)
			);

			if ( ! $embed_handler instanceof AMP_Base_Embed_Handler ) {
				_doing_it_wrong(
					__METHOD__,
					esc_html(
						sprintf(
							/* translators: 1: embed handler. 2: AMP_Embed_Handler */
							__( 'Embed Handler (%1$s) must extend `%2$s`', 'amp' ),
							esc_html( $embed_handler_class ),
							'AMP_Embed_Handler'
						)
					),
					'0.1'
				);
				continue;
			}

			$embed_handler->register_embed();
			$embed_handlers[] = $embed_handler;
		}

		return $embed_handlers;
	}

	/**
	 * Add the comments template placeholder marker
	 *
	 * @codeCoverageIgnore
	 * @deprecated 1.1.0 This functionality was moved to AMP_Comments_Sanitizer
	 *
	 * @param array $args the args for the comments list.
	 * @return array Args to return.
	 */
	public static function set_comments_walker( $args ) {
		_deprecated_function( __METHOD__, '1.1' );
		$amp_walker     = new AMP_Comment_Walker();
		$args['walker'] = $amp_walker;
		return $args;
	}

	/**
	 * Amend the comment form with the redirect_to field to persist the AMP page after submission.
	 */
	public static function amend_comment_form() {
		?>
		<?php if ( is_singular() && ! amp_is_canonical() ) : ?>
			<input type="hidden" name="redirect_to" value="<?php echo esc_url( amp_get_permalink( get_the_ID() ) ); ?>">
		<?php endif; ?>
		<?php
	}

	/**
	 * Prepends template hierarchy with template_dir for AMP transitional mode templates.
	 *
	 * @param array $templates Template hierarchy.
	 * @return array Templates.
	 */
	public static function filter_amp_template_hierarchy( $templates ) {
		$args = self::get_theme_support_args();
		if ( isset( $args['template_dir'] ) ) {
			$amp_templates = [];
			foreach ( $templates as $template ) {
				$amp_templates[] = $args['template_dir'] . '/' . $template; // Let template_dir have precedence.
				$amp_templates[] = $template;
			}
			$templates = $amp_templates;
		}
		return $templates;
	}

	/**
	 * Get canonical URL for current request.
	 *
	 * @see rel_canonical()
	 * @global WP $wp
	 * @global WP_Rewrite $wp_rewrite
	 * @link https://www.ampproject.org/docs/reference/spec#canon.
	 * @link https://core.trac.wordpress.org/ticket/18660
	 *
	 * @return string Canonical non-AMP URL.
	 */
	public static function get_current_canonical_url() {
		global $wp, $wp_rewrite;

		$url = null;
		if ( is_singular() ) {
			$url = wp_get_canonical_url();
		}

		// For non-singular queries, make use of the request URI and public query vars to determine canonical URL.
		if ( empty( $url ) && $wp instanceof WP && $wp_rewrite instanceof WP_Rewrite ) {
			$added_query_vars = $wp->query_vars;
			if ( ! $wp_rewrite->permalink_structure || empty( $wp->request ) ) {
				$url = home_url( '/' );
			} else {
				$url = home_url( user_trailingslashit( $wp->request ) );
				parse_str( $wp->matched_query, $matched_query_vars );
				foreach ( $wp->query_vars as $key => $value ) {

					// Remove query vars that were matched in the rewrite rules for the request.
					if ( isset( $matched_query_vars[ $key ] ) ) {
						unset( $added_query_vars[ $key ] );
					}
				}
			}
		}

		if ( ! empty( $added_query_vars ) ) {
			$url = add_query_arg( $added_query_vars, $url );
		}

		return amp_remove_endpoint( $url );
	}

	/**
	 * Get the ID for the amp-state.
	 *
	 * @since 0.7
	 *
	 * @param int $post_id Post ID.
	 * @return string ID for amp-state.
	 */
	public static function get_comment_form_state_id( $post_id ) {
		return sprintf( 'commentform_post_%d', $post_id );
	}

	/**
	 * Filter comment form args to an element with [text] AMP binding wrap the title reply.
	 *
	 * @since 0.7
	 * @see comment_form()
	 *
	 * @param array $default_args Comment form arg defaults.
	 * @return array Filtered comment form args.
	 */
	public static function filter_comment_form_defaults( $default_args ) {

		// Obtain the actual args provided to the comment_form() function since it is not available in the filter.
		$args      = [];
		$backtrace = debug_backtrace(); // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_debug_backtrace -- Due to limitation in WordPress core.
		foreach ( $backtrace as $call ) {
			if ( 'comment_form' === $call['function'] ) {
				$args = isset( $call['args'][0] ) ? $call['args'][0] : [];
				break;
			}
		}

		// Abort if the comment_form() was called with arguments which we cannot override the defaults for.
		// @todo This and the debug_backtrace() call above would be unnecessary if WordPress had a comment_form_args filter.
		$overridden_keys = [ 'cancel_reply_before', 'title_reply', 'title_reply_before', 'title_reply_to' ];
		foreach ( $overridden_keys as $key ) {
			if ( array_key_exists( $key, $args ) && array_key_exists( $key, $default_args ) && $default_args[ $key ] !== $args[ $key ] ) {
				return $default_args;
			}
		}

		$state_id     = self::get_comment_form_state_id( get_the_ID() );
		$text_binding = sprintf(
			'%s.replyToName ? %s : %s',
			$state_id,
			str_replace(
				'%s',
				sprintf( '" + %s.replyToName + "', $state_id ),
				wp_json_encode( $default_args['title_reply_to'], JSON_UNESCAPED_UNICODE )
			),
			wp_json_encode( $default_args['title_reply'], JSON_UNESCAPED_UNICODE )
		);

		$default_args['title_reply_before'] .= sprintf(
			'<span [text]="%s">',
			esc_attr( $text_binding )
		);
		$default_args['cancel_reply_before'] = '</span>' . $default_args['cancel_reply_before'];
		return $default_args;
	}

	/**
	 * Modify the comment reply link for AMP.
	 *
	 * @since 0.7
	 * @see get_comment_reply_link()
	 *
	 * @param string     $link    The HTML markup for the comment reply link.
	 * @param array      $args    An array of arguments overriding the defaults.
	 * @param WP_Comment $comment The object of the comment being replied.
	 * @return string Comment reply link.
	 */
	public static function filter_comment_reply_link( $link, $args, $comment ) {

		// Continue to show default link to wp-login when user is not logged-in.
		if ( get_option( 'comment_registration' ) && ! is_user_logged_in() ) {
			return $args['before'] . $link . $args['after'];
		}

		$state_id  = self::get_comment_form_state_id( get_the_ID() );
		$tap_state = [
			$state_id => [
				'replyToName' => $comment->comment_author,
				'values'      => [
					'comment_parent' => (string) $comment->comment_ID,
				],
			],
		];

		// @todo Figure out how to support add_below. Instead of moving the form, what about letting the form get a fixed position?
		$link = sprintf(
			'<a rel="nofollow" class="comment-reply-link" href="%s" on="%s" aria-label="%s">%s</a>',
			esc_attr( '#' . $args['respond_id'] ),
			esc_attr( sprintf( 'tap:AMP.setState( %s )', wp_json_encode( $tap_state, JSON_UNESCAPED_UNICODE ) ) ),
			esc_attr( sprintf( $args['reply_to_text'], $comment->comment_author ) ),
			$args['reply_text']
		);
		return $args['before'] . $link . $args['after'];
	}

	/**
	 * Filters the cancel comment reply link HTML.
	 *
	 * @since 0.7
	 * @see get_cancel_comment_reply_link()
	 *
	 * @param string $formatted_link The HTML-formatted cancel comment reply link.
	 * @param string $link           Cancel comment reply link URL.
	 * @param string $text           Cancel comment reply link text.
	 * @return string Cancel reply link.
	 */
	public static function filter_cancel_comment_reply_link( $formatted_link, $link, $text ) {
		if ( empty( $text ) ) {
			$text = __( 'Click here to cancel reply.', 'default' );
		}

		$state_id  = self::get_comment_form_state_id( get_the_ID() );
		$tap_state = [
			$state_id => [
				'replyToName' => '',
				'values'      => [
					'comment_parent' => '0',
				],
			],
		];

		$respond_id = 'respond'; // Hard-coded in comment_form() and default value in get_comment_reply_link().
		return sprintf(
			'<a id="cancel-comment-reply-link" href="%s" %s [hidden]="%s" on="%s">%s</a>',
			esc_url( remove_query_arg( 'replytocom' ) . '#' . $respond_id ),
			isset( $_GET['replytocom'] ) ? '' : ' hidden', // phpcs:ignore
			esc_attr( sprintf( '%s.values.comment_parent == "0"', self::get_comment_form_state_id( get_the_ID() ) ) ),
			esc_attr( sprintf( 'tap:AMP.setState( %s )', wp_json_encode( $tap_state, JSON_UNESCAPED_UNICODE ) ) ),
			esc_html( $text )
		);
	}

	/**
	 * Configure the admin bar for AMP.
	 *
	 * @since 1.0
	 */
	public static function init_admin_bar() {
		add_filter( 'style_loader_tag', [ __CLASS__, 'filter_admin_bar_style_loader_tag' ], 10, 2 );
		add_filter( 'script_loader_tag', [ __CLASS__, 'filter_admin_bar_script_loader_tag' ], 10, 2 );

		// Inject the data-ampdevmode attribute into the admin bar bump style. See \WP_Admin_Bar::initialize().
		if ( current_theme_supports( 'admin-bar' ) ) {
			$admin_bar_args  = get_theme_support( 'admin-bar' );
			$header_callback = $admin_bar_args[0]['callback'];
		} else {
			$header_callback = '_admin_bar_bump_cb';
		}
		remove_action( 'wp_head', $header_callback );
		if ( '__return_false' !== $header_callback ) {
			ob_start();
			$header_callback();
			$style = ob_get_clean();
			$data  = trim( preg_replace( '#<style[^>]*>(.*)</style>#is', '$1', $style ) ); // See wp_add_inline_style().

			// Override AMP's position:relative on the body for the sake of the AMP viewer, which is not relevant an an Admin Bar context.
			if ( amp_is_dev_mode() ) {
				$data .= 'html:not(#_) > body { position:unset !important; }';
			}

			wp_add_inline_style( 'admin-bar', $data );
		}

		// Emulate customize support script in PHP, to assume Customizer.
		add_action(
			'admin_bar_menu',
			static function() {
				remove_action( 'wp_before_admin_bar_render', 'wp_customize_support_script' );
			},
			41
		);
		add_filter(
			'body_class',
			static function( $body_classes ) {
				return array_merge(
					array_diff(
						$body_classes,
						[ 'no-customize-support' ]
					),
					[ 'customize-support' ]
				);
			}
		);
	}

	/**
	 * Recursively determine if a given dependency depends on another.
	 *
	 * @since 1.3
	 *
	 * @param WP_Dependencies $dependencies      Dependencies.
	 * @param string          $current_handle    Current handle.
	 * @param string          $dependency_handle Dependency handle.
	 * @return bool Whether the current handle is a dependency of the dependency handle.
	 */
	protected static function has_dependency( WP_Dependencies $dependencies, $current_handle, $dependency_handle ) {
		if ( $current_handle === $dependency_handle ) {
			return true;
		}
		if ( ! isset( $dependencies->registered[ $current_handle ] ) ) {
			return false;
		}
		foreach ( $dependencies->registered[ $current_handle ]->deps as $handle ) {
			if ( self::has_dependency( $dependencies, $handle, $dependency_handle ) ) {
				return true;
			}
		}
		return false;
	}

	/**
	 * Check if a handle is exclusively a dependency of another handle.
	 *
	 * For example, check if dashicons is being added exclusively because it is a dependency of admin-bar, as opposed
	 * to being added because it was directly enqueued by a theme or a dependency of some other style.
	 *
	 * @since 1.4.2
	 *
	 * @param WP_Dependencies $dependencies      Dependencies.
	 * @param string          $dependency_handle Dependency handle.
	 * @param string          $dependent_handle  Dependent handle.
	 * @return bool Whether the $handle is exclusively a handle of the $exclusive_dependency handle.
	 */
	protected static function is_exclusively_dependent( WP_Dependencies $dependencies, $dependency_handle, $dependent_handle ) {

		// If a dependency handle is the same as the dependent handle, then this self-referential relationship is exclusive.
		if ( $dependency_handle === $dependent_handle ) {
			return true;
		}

		// Short-circuit if there is no dependency relationship up front.
		if ( ! self::has_dependency( $dependencies, $dependent_handle, $dependency_handle ) ) {
			return false;
		}

		// Check whether any enqueued handle depends on the dependency.
		foreach ( $dependencies->queue as $queued_handle ) {
			// Skip considering the dependent handle.
			if ( $dependent_handle === $queued_handle ) {
				continue;
			}

			// If the dependency handle was directly enqueued, then it is not exclusively dependent.
			if ( $dependency_handle === $queued_handle ) {
				return false;
			}

			// Otherwise, if the dependency handle is depended on by the queued handle while at the same time the queued
			// handle _does_ have a dependency on the supplied dependent handle, then the dependency handle is not
			// exclusively dependent on the dependent handle.
			if (
				self::has_dependency( $dependencies, $queued_handle, $dependency_handle )
				&&
				! self::has_dependency( $dependencies, $queued_handle, $dependent_handle )
			) {
				return false;
			}
		}
		return true;
	}

	/**
	 * Add data-ampdevmode attribute to any enqueued style that depends on the admin-bar.
	 *
	 * @since 1.3
	 *
	 * @param string $tag    The link tag for the enqueued style.
	 * @param string $handle The style's registered handle.
	 * @return string Tag.
	 */
	public static function filter_admin_bar_style_loader_tag( $tag, $handle ) {
		if (
			is_array( wp_styles()->registered['admin-bar']->deps ) && in_array( $handle, wp_styles()->registered['admin-bar']->deps, true ) ?
				self::is_exclusively_dependent( wp_styles(), $handle, 'admin-bar' ) :
				self::has_dependency( wp_styles(), $handle, 'admin-bar' )
		) {
			$tag = preg_replace( '/(?<=<link)(?=\s|>)/i', ' ' . AMP_Rule_Spec::DEV_MODE_ATTRIBUTE, $tag );
		}
		return $tag;
	}

	/**
	 * Add data-ampdevmode attribute to any enqueued script that depends on the admin-bar.
	 *
	 * @since 1.3
	 *
	 * @param string $tag    The `<script>` tag for the enqueued script.
	 * @param string $handle The script's registered handle.
	 * @return string Tag.
	 */
	public static function filter_admin_bar_script_loader_tag( $tag, $handle ) {
		if (
			is_array( wp_scripts()->registered['admin-bar']->deps ) && in_array( $handle, wp_scripts()->registered['admin-bar']->deps, true ) ?
				self::is_exclusively_dependent( wp_scripts(), $handle, 'admin-bar' ) :
				self::has_dependency( wp_scripts(), $handle, 'admin-bar' )
		) {
			$tag = preg_replace( '/(?<=<script)(?=\s|>)/i', ' ' . AMP_Rule_Spec::DEV_MODE_ATTRIBUTE, $tag );
		}
		return $tag;
	}

	/**
	 * Ensure the markup exists as required by AMP and elements are in the optimal loading order.
	 *
	 * Ensure meta[charset], meta[name=viewport], and link[rel=canonical] exist, as the validating sanitizer
	 * may have removed an illegal meta[http-equiv] or meta[name=viewport]. For a singular post, core only outputs a
	 * canonical URL by default. Adds the preload links.
	 *
	 * @since 0.7
	 * @link https://www.ampproject.org/docs/reference/spec#required-markup
	 * @link https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/
	 * @todo All of this might be better placed inside of a sanitizer.
	 *
	 * @param Document $dom            Document.
	 * @param string[] $script_handles AMP script handles for components identified during output buffering.
	 */
	public static function ensure_required_markup( Document $dom, $script_handles = [] ) {
		// Gather all links.
		$links         = [
			Attribute::REL_PRECONNECT => [
				// Include preconnect link for AMP CDN for browsers that don't support preload.
				AMP_DOM_Utils::create_node(
					$dom,
					Tag::LINK,
					[
						Attribute::REL  => Attribute::REL_PRECONNECT,
						Attribute::HREF => 'https://cdn.ampproject.org',
					]
				),
			],
		];
		$link_elements = $dom->head->getElementsByTagName( Tag::LINK );
		/**
		 * Link element.
		 *
		 * @var DOMElement $link
		 */
		foreach ( $link_elements as $link ) {
			if ( $link->hasAttribute( Attribute::REL ) ) {
				$links[ $link->getAttribute( Attribute::REL ) ][] = $link;
			}
		}

		// Ensure rel=canonical link.
		$rel_canonical = null;
		if ( empty( $links['canonical'] ) ) {
			$rel_canonical = AMP_DOM_Utils::create_node(
				$dom,
				Tag::LINK,
				[
					Attribute::REL  => Attribute::REL_CANONICAL,
					Attribute::HREF => self::get_current_canonical_url(),
				]
			);
			$dom->head->appendChild( $rel_canonical );
		}

		// Store the last meta tag as the previous node to append to.
		$meta_tags     = $dom->head->getElementsByTagName( Tag::META );
		$previous_node = $meta_tags->length > 0 ? $meta_tags->item( $meta_tags->length - 1 ) : $dom->head->firstChild;

		// Handle the title.
		$title = $dom->head->getElementsByTagName( Tag::TITLE )->item( 0 );
		if ( $title ) {
			$title->parentNode->removeChild( $title ); // So we can move it.
			$dom->head->insertBefore( $title, $previous_node->nextSibling );
			$previous_node = $title;
		}

		// Obtain the existing AMP scripts.
		$amp_scripts     = [];
		$ordered_scripts = [];
		$head_scripts    = [];
		$runtime_src     = wp_scripts()->registered[ Amp::RUNTIME ]->src;

		/**
		 * Script element.
		 *
		 * @var DOMElement $script
		 */
		foreach ( $dom->head->getElementsByTagName( Tag::SCRIPT ) as $script ) { // Note that prepare_response() already moved body scripts to head.
			$head_scripts[] = $script;
		}
		foreach ( $head_scripts as $script ) {
			$src = $script->getAttribute( Attribute::SRC );
			if ( ! $src || 'https://cdn.ampproject.org/' !== substr( $src, 0, 27 ) ) {
				continue;
			}
			if ( $runtime_src === $src ) {
				$amp_scripts[ Amp::RUNTIME ] = $script;
			} elseif ( $script->hasAttribute( Attribute::CUSTOM_ELEMENT ) ) {
				$amp_scripts[ $script->getAttribute( Attribute::CUSTOM_ELEMENT ) ] = $script;
			} elseif ( $script->hasAttribute( Attribute::CUSTOM_TEMPLATE ) ) {
				$amp_scripts[ $script->getAttribute( Attribute::CUSTOM_TEMPLATE ) ] = $script;
			} else {
				continue;
			}
			$script->parentNode->removeChild( $script ); // So we can move it.
		}

		// Create scripts for any components discovered from output buffering that are missing.
		foreach ( array_diff( $script_handles, array_keys( $amp_scripts ) ) as $missing_script_handle ) {
			if ( ! wp_script_is( $missing_script_handle, 'registered' ) ) {
				continue;
			}
			$attrs = [
				Attribute::SRC   => wp_scripts()->registered[ $missing_script_handle ]->src,
				Attribute::ASYNC => '',
			];
			if ( Extension::MUSTACHE === $missing_script_handle ) {
				$attrs[ Attribute::CUSTOM_TEMPLATE ] = $missing_script_handle;
			} else {
				$attrs[ Attribute::CUSTOM_ELEMENT ] = $missing_script_handle;
			}

			$amp_scripts[ $missing_script_handle ] = AMP_DOM_Utils::create_node( $dom, Tag::SCRIPT, $attrs );
		}

		// Remove scripts that had already been added but couldn't be detected from output buffering.
		$extension_specs            = AMP_Allowed_Tags_Generated::get_extension_specs();
		$superfluous_script_handles = array_diff(
			array_keys( $amp_scripts ),
			array_merge( $script_handles, [ Amp::RUNTIME ] )
		);
		foreach ( $superfluous_script_handles as $superfluous_script_handle ) {
			if ( ! empty( $extension_specs[ $superfluous_script_handle ]['requires_usage'] ) ) {
				unset( $amp_scripts[ $superfluous_script_handle ] );
			}
		}

		/* phpcs:ignore Squiz.PHP.CommentedOutCode.Found
		 *
		 * "2. Next, preload the AMP runtime v0.js <script> tag with <link as=script href=https://cdn.ampproject.org/v0.js rel=preload>.
		 * The AMP runtime should start downloading as soon as possible because the AMP boilerplate hides the document via body { visibility:hidden }
		 * until the AMP runtime has loaded. Preloading the AMP runtime tells the browser to download the script with a higher priority."
		 * {@link https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/ Optimize the AMP Runtime loading}
		 */
		$prioritized_preloads = [];
		if ( ! isset( $links[ Attribute::REL_PRELOAD ] ) ) {
			$links[ Attribute::REL_PRELOAD ] = [];
		}

		$prioritized_preloads[] = AMP_DOM_Utils::create_node(
			$dom,
			Tag::LINK,
			[
				Attribute::REL  => Attribute::REL_PRELOAD,
				'as'            => Tag::SCRIPT,
				Attribute::HREF => $runtime_src,
			]
		);

		/*
		 * "3. If your page includes render-delaying extensions (e.g., amp-experiment, amp-dynamic-css-classes, amp-story),
		 * preload those extensions as they're required by the AMP runtime for rendering the page."
		 */
		$amp_script_handles = array_keys( $amp_scripts );
		foreach ( array_intersect( Amp::RENDER_DELAYING_EXTENSIONS, $amp_script_handles ) as $script_handle ) {
			if ( ! in_array( $script_handle, Amp::RENDER_DELAYING_EXTENSIONS, true ) ) {
				continue;
			}
			$prioritized_preloads[] = AMP_DOM_Utils::create_node(
				$dom,
				Tag::LINK,
				[
					Attribute::REL  => Attribute::REL_PRELOAD,
					'as'            => Tag::SCRIPT,
					Attribute::HREF => $amp_scripts[ $script_handle ]->getAttribute( Attribute::SRC ),
				]
			);
		}
		$links[ Attribute::REL_PRELOAD ] = array_merge( $prioritized_preloads, $links[ Attribute::REL_PRELOAD ] );

		/*
		 * "4. Use preconnect to speedup the connection to other origin where the full resource URL is not known ahead of time,
		 * for example, when using Google Fonts."
		 *
		 * Note that \AMP_Style_Sanitizer::process_link_element() will ensure preconnect links for Google Fonts are present.
		 */
		$link_relations = [ Attribute::REL_PRECONNECT, Attribute::REL_DNS_PREFETCH, Attribute::REL_PRELOAD, Attribute::REL_PRERENDER, Attribute::REL_PREFETCH ];
		foreach ( $link_relations as $rel ) {
			if ( ! isset( $links[ $rel ] ) ) {
				continue;
			}
			foreach ( $links[ $rel ] as $link ) {
				if ( $link->parentNode ) {
					$link->parentNode->removeChild( $link ); // So we can move it.
				}
				$dom->head->insertBefore( $link, $previous_node->nextSibling );
				$previous_node = $link;
			}
		}

		// "5. Load the AMP runtime."
		if ( isset( $amp_scripts[ Amp::RUNTIME ] ) ) {
			$ordered_scripts[ Amp::RUNTIME ] = $amp_scripts[ Amp::RUNTIME ];
			unset( $amp_scripts[ Amp::RUNTIME ] );
		} else {
			$script = $dom->createElement( Tag::SCRIPT );
			$script->setAttribute( Attribute::ASYNC, '' );
			$script->setAttribute( Attribute::SRC, $runtime_src );
			$ordered_scripts[ Amp::RUNTIME ] = $script;
		}

		/*
		 * "6. Specify the <script> tags for render-delaying extensions (e.g., amp-experiment amp-dynamic-css-classes and amp-story"
		 *
		 * {@link https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/ AMP Hosting Guide}
		 */
		foreach ( Amp::RENDER_DELAYING_EXTENSIONS as $extension ) {
			if ( isset( $amp_scripts[ $extension ] ) ) {
				$ordered_scripts[ $extension ] = $amp_scripts[ $extension ];
				unset( $amp_scripts[ $extension ] );
			}
		}

		/*
		 * "7. Specify the <script> tags for remaining extensions (e.g., amp-bind ...). These extensions are not render-delaying
		 * and therefore should not be preloaded as they might take away important bandwidth for the initial render."
		 */
		ksort( $amp_scripts );
		$ordered_scripts = array_merge( $ordered_scripts, $amp_scripts );
		foreach ( $ordered_scripts as $ordered_script ) {
			$dom->head->insertBefore( $ordered_script, $previous_node->nextSibling );
			$previous_node = $ordered_script;
		}

		unset( $previous_node );
	}

	/**
	 * Dequeue Customizer assets which are not necessary outside the preview iframe.
	 *
	 * Prevent enqueueing customize-preview styles if not in customizer preview iframe.
	 * These are only needed for when there is live editing of content, such as selective refresh.
	 *
	 * @since 0.7
	 */
	public static function dequeue_customize_preview_scripts() {

		// Dequeue styles unnecessary unless in customizer preview iframe when editing (such as for edit shortcuts).
		if ( ! self::is_customize_preview_iframe() ) {
			wp_dequeue_style( 'customize-preview' );
			foreach ( wp_styles()->registered as $handle => $dependency ) {
				if ( in_array( 'customize-preview', $dependency->deps, true ) ) {
					wp_dequeue_style( $handle );
				}
			}
		}
	}

	/**
	 * Start output buffering.
	 *
	 * @since 0.7
	 * @see AMP_Theme_Support::finish_output_buffering()
	 */
	public static function start_output_buffering() {
		/*
		 * Disable the New Relic Browser agent on AMP responses.
		 * This prevents the New Relic from causing invalid AMP responses due the NREUM script it injects after the meta charset:
		 * https://docs.newrelic.com/docs/browser/new-relic-browser/troubleshooting/google-amp-validator-fails-due-3rd-party-script
		 * Sites with New Relic will need to specially configure New Relic for AMP:
		 * https://docs.newrelic.com/docs/browser/new-relic-browser/installation/monitor-amp-pages-new-relic-browser
		 */
		if ( function_exists( 'newrelic_disable_autorum' ) ) {
			newrelic_disable_autorum();
		}

		ob_start( [ __CLASS__, 'finish_output_buffering' ] );
		self::$is_output_buffering = true;
	}

	/**
	 * Determine whether output buffering has started.
	 *
	 * @since 0.7
	 * @see AMP_Theme_Support::start_output_buffering()
	 * @see AMP_Theme_Support::finish_output_buffering()
	 *
	 * @return bool Whether output buffering has started.
	 */
	public static function is_output_buffering() {
		return self::$is_output_buffering;
	}

	/**
	 * Finish output buffering.
	 *
	 * @since 0.7
	 * @see AMP_Theme_Support::start_output_buffering()
	 *
	 * @param string $response Buffered Response.
	 * @return string Processed Response.
	 */
	public static function finish_output_buffering( $response ) {
		self::$is_output_buffering = false;
		return self::prepare_response( $response );
	}

	/**
	 * Filter rendered partial to convert to AMP.
	 *
	 * @see WP_Customize_Partial::render()
	 *
	 * @param string|mixed $partial Rendered partial.
	 * @return string|mixed Filtered partial.
	 * @global int $content_width
	 */
	public static function filter_customize_partial_render( $partial ) {
		global $content_width;
		if ( is_string( $partial ) && preg_match( '/<\w/', $partial ) ) {
			$dom  = AMP_DOM_Utils::get_dom_from_content( $partial );
			$args = [
				'content_max_width'    => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
				'use_document_element' => false,
				'allow_dirty_styles'   => true,
				'allow_dirty_scripts'  => false,
			];
			AMP_Content_Sanitizer::sanitize_document( $dom, self::$sanitizer_classes, $args ); // @todo Include script assets in response?
			$partial = AMP_DOM_Utils::get_content_from_dom( $dom );
		}
		return $partial;
	}

	/**
	 * Process response to ensure AMP validity.
	 *
	 * @since 0.7
	 *
	 * @param string $response HTML document response. By default it expects a complete document.
	 * @param array  $args     Args to send to the preprocessor/sanitizer/optimizer.
	 * @return string AMP document response.
	 * @global int $content_width
	 */
	public static function prepare_response( $response, $args = [] ) {
		global $content_width;

		if ( isset( $args['validation_error_callback'] ) ) {
			_doing_it_wrong( __METHOD__, 'Do not supply validation_error_callback arg.', '1.0' );
			unset( $args['validation_error_callback'] );
		}

		$status_code = http_response_code();

		/*
		 * Send a JSON response when the site is failing to handle AMP form submissions with a JSON response as required
		 * or an AMP-Redirect-To response header was not sent. This is a common scenario for plugins that handle form
		 * submissions and show the success page via the POST request's response body instead of invoking wp_redirect(),
		 * in which case AMP_HTTP::intercept_post_request_redirect() will automatically send the AMP-Redirect-To header.
		 * If the POST response is an HTML document then the form submission will appear to not have worked since there
		 * is no success or failure message shown. By catching the case where HTML is sent in the response, we can
		 * automatically send a generic success message when a 200 status is returned or a failure message when a 400+
		 * response code is sent.
		 */
		$is_form_submission = (
			isset( AMP_HTTP::$purged_amp_query_vars[ AMP_HTTP::ACTION_XHR_CONVERTED_QUERY_VAR ] ) // phpcs:ignore WordPress.Security.NonceVerification.Recommended
			&&
			isset( $_SERVER['REQUEST_METHOD'] )
			&&
			'POST' === $_SERVER['REQUEST_METHOD']
		);
		if ( $is_form_submission && null === json_decode( $response ) && json_last_error() && ( is_bool( $status_code ) || ( $status_code >= 200 && $status_code < 300 ) || $status_code >= 400 ) ) {
			if ( is_bool( $status_code ) ) {
				$status_code = 200; // Not a web server environment.
			}
			return wp_json_encode(
				[
					'status_code' => $status_code,
					'status_text' => get_status_header_desc( $status_code ),
				]
			);
		}

		/*
		 * Abort if the response was not HTML. To be post-processed as an AMP page, the output-buffered document must
		 * have the HTML mime type and it must start with <html> followed by <head> tag (with whitespace, doctype, and comments optionally interspersed).
		 */
		if ( Attribute::TYPE_HTML !== substr( AMP_HTTP::get_response_content_type(), 0, 9 ) || ! preg_match( '#^(?:<!.*?>|\s+)*<html.*?>(?:<!.*?>|\s+)*<head\b(.*?)>#is', $response ) ) {
			return $response;
		}

		// Enforce UTF-8 encoding as it is a requirement for AMP.
		if ( ! headers_sent() ) {
			header( 'Content-Type: text/html; charset=utf-8' );
		}

		// @todo Both allow_dirty_styles and allow_dirty_scripts should eventually use AMP dev mode instead.
		$args = array_merge(
			[
				'content_max_width'    => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
				'use_document_element' => true,
				'allow_dirty_styles'   => self::is_customize_preview_iframe(), // Dirty styles only needed when editing (e.g. for edit shortcuts).
				'allow_dirty_scripts'  => is_customize_preview(), // Scripts are always needed to inject changeset UUID.
				'user_can_validate'    => AMP_Validation_Manager::has_cap(),
			],
			$args
		);

		AMP_HTTP::send_server_timing( 'amp_output_buffer', -self::$init_start_time, 'AMP Output Buffer' );

		$dom_parse_start = microtime( true );

		$dom = Document::fromHtml( $response );

		if ( AMP_Validation_Manager::$is_validate_request ) {
			AMP_Validation_Manager::remove_illegal_source_stack_comments( $dom );
		}

		AMP_HTTP::send_server_timing( 'amp_dom_parse', -$dom_parse_start, 'AMP DOM Parse' );

		// Make sure scripts from the body get moved to the head.
		foreach ( $dom->xpath->query( '//body//script[ @custom-element or @custom-template or @src = "https://cdn.ampproject.org/v0.js" ]' ) as $script ) {
			$dom->head->appendChild( $script->parentNode->removeChild( $script ) );
		}

		// Ensure the mandatory amp attribute is present on the html element.
		if ( ! $dom->documentElement->hasAttribute( Attribute::AMP )
			&& ! $dom->documentElement->hasAttribute( Attribute::AMP_EMOJI )
			&& ! $dom->documentElement->hasAttribute( Attribute::AMP_EMOJI_ALT ) ) {
			$dom->documentElement->setAttribute( Attribute::AMP, '' );
		}

		$sanitization_results = AMP_Content_Sanitizer::sanitize_document( $dom, self::$sanitizer_classes, $args );

		// Respond early with results if performing a validate request.
		if ( AMP_Validation_Manager::$is_validate_request ) {
			status_header( 200 );
			header( 'Content-Type: application/json; charset=utf-8' );
			$data       = [
				'http_status_code' => $status_code,
				'php_fatal_error'  => false,
			];
			$last_error = error_get_last();
			if ( $last_error && in_array( $last_error['type'], [ E_ERROR, E_RECOVERABLE_ERROR, E_CORE_ERROR, E_COMPILE_ERROR, E_USER_ERROR, E_PARSE ], true ) ) {
				$data['php_fatal_error'] = $last_error;
			}
			$data = array_merge( $data, AMP_Validation_Manager::get_validate_response_data( $sanitization_results ) );
			return wp_json_encode( $data, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES );
		}

		$dom_serialize_start = microtime( true );

		// Gather all component scripts that are used in the document and then render any not already printed.
		$amp_scripts = $sanitization_results['scripts'];
		foreach ( self::$embed_handlers as $embed_handler ) {
			$amp_scripts = array_merge(
				$amp_scripts,
				$embed_handler->get_scripts()
			);
		}
		foreach ( $amp_scripts as $handle => $src ) {
			/*
			 * Make sure the src is up-to-date. This allows for embed handlers to override the
			 * default extension version by defining a different URL.
			 */
			if ( is_string( $src ) && wp_script_is( $handle, 'registered' ) ) {
				wp_scripts()->registered[ $handle ]->src = $src;
			}
		}

		$enable_optimizer = array_key_exists( ConfigurationArgument::ENABLE_OPTIMIZER, $args )
			? $args[ ConfigurationArgument::ENABLE_OPTIMIZER ]
			: true;

		/**
		 * Filter whether the generated HTML output should be run through the AMP Optimizer or not.
		 *
		 * @since 1.5.0
		 *
		 * @param bool $enable_optimizer Whether the generated HTML output should be run through the AMP Optimizer or not.
		 * @return bool Filtered value of whether the generated HTML output should be run through the AMP Optimizer or not.
		 */
		$enable_optimizer = apply_filters( 'amp_enable_optimizer', $enable_optimizer );

		if ( $enable_optimizer ) {
			$errors = new Optimizer\ErrorCollection();
			self::get_optimizer( $args )->optimizeDom( $dom, $errors );

			if ( count( $errors ) > 0 ) {
				$error_messages = array_map(
					static function( Optimizer\Error $error ) {
						return ' - ' . $error->getCode() . ': ' . $error->getMessage();
					},
					iterator_to_array( $errors )
				);
				$dom->head->appendChild(
					$dom->createComment( "\n" . __( 'AMP optimization could not be completed due to the following:', 'amp' ) . "\n" . implode( "\n", $error_messages ) . "\n" )
				);
				// @todo Include errors elsewhere than HTML comment?
			}
		}

		self::ensure_required_markup( $dom, array_keys( $amp_scripts ) );

		$can_serve = AMP_Validation_Manager::finalize_validation( $dom );

		// Redirect to the non-AMP version if not on an AMP-first site.
		if ( ! $can_serve && ! amp_is_canonical() ) {
			$non_amp_url = amp_remove_endpoint( amp_get_current_url() );

			// Redirect to include query var to preventing AMP from even being considered available.
			$non_amp_url = add_query_arg( QueryVars::NOAMP, QueryVars::NOAMP_AVAILABLE, $non_amp_url );

			wp_safe_redirect( $non_amp_url, 302 );
			return esc_html__( 'Redirecting since AMP version not available.', 'amp' );
		}

		$response = $dom->saveHTML();

		AMP_HTTP::send_server_timing( 'amp_dom_serialize', -$dom_serialize_start, 'AMP DOM Serialize' );

		return $response;
	}

	/**
	 * Optimizer instance to use.
	 *
	 * @param array $args Associative array of arguments to pass into the transformation engine.
	 * @return Optimizer\TransformationEngine Optimizer transformation engine to use.
	 */
	private static function get_optimizer( $args ) {
		$configuration = self::get_optimizer_configuration( $args );

		$fallback_remote_request_pipeline = new FallbackRemoteGetRequest(
			new WpHttpRemoteGetRequest(),
			new FilesystemRemoteGetRequest( Optimizer\LocalFallback::getMappings() )
		);

		$cached_remote_request = new CachedRemoteGetRequest( $fallback_remote_request_pipeline, WEEK_IN_SECONDS );

		return new Optimizer\TransformationEngine(
			$configuration,
			$cached_remote_request
		);
	}

	/**
	 * Get the AmpProject\Optimizer configuration object to use.
	 *
	 * @param array $args Associative array of arguments to pass into the transformation engine.
	 * @return Optimizer\Configuration Optimizer configuration to use.
	 */
	private static function get_optimizer_configuration( $args ) {
		$transformers = Optimizer\Configuration::DEFAULT_TRANSFORMERS;

		$enable_ssr = array_key_exists( ConfigurationArgument::ENABLE_SSR, $args )
			? $args[ ConfigurationArgument::ENABLE_SSR ]
			: true;

		/**
		 * Filter whether the AMP Optimizer should use server-side rendering or not.
		 *
		 * @since 1.5.0
		 *
		 * @param bool $enable_ssr Whether the AMP Optimizer should use server-side rendering or not.
		 * @return bool Filtered value of whether the AMP Optimizer should use server-side rendering or not.
		 */
		$enable_ssr = apply_filters( 'amp_enable_ssr', $enable_ssr );

		// In debugging mode, we don't use server-side rendering, as it further obfuscates the HTML markup.
		if ( ! $enable_ssr ) {
			$transformers = array_diff(
				$transformers,
				[
					Optimizer\Transformer\AmpRuntimeCss::class,
					Optimizer\Transformer\ServerSideRendering::class,
					Optimizer\Transformer\TransformedIdentifier::class,
				]
			);
		}

		array_unshift( $transformers, Transformer\AmpSchemaOrgMetadata::class );

		/**
		 * Filter the configuration to be used for the AMP Optimizer.
		 *
		 * @since 1.5.0
		 *
		 * @param array $configuration Associative array of configuration data.
		 * @return array Filtered associative array of configuration data.
		 */
		$configuration = apply_filters(
			'amp_optimizer_config',
			array_merge(
				[ Optimizer\Configuration::KEY_TRANSFORMERS => $transformers ],
				$args
			)
		);

		$config = new Optimizer\Configuration( $configuration );
		$config->registerConfigurationClass(
			Transformer\AmpSchemaOrgMetadata::class,
			Transformer\AmpSchemaOrgMetadataConfiguration::class
		);

		return $config;
	}

	/**
	 * Adds 'data-amp-layout' to the allowed <img> attributes for wp_kses().
	 *
	 * @since 0.7
	 *
	 * @param array $context Allowed tags and their allowed attributes.
	 * @return array $context Filtered allowed tags and attributes.
	 */
	public static function include_layout_in_wp_kses_allowed_html( $context ) {
		if ( ! empty( $context[ Tag::IMG ][ Attribute::WIDTH ] ) && ! empty( $context[ Tag::IMG ][ Attribute::HEIGHT ] ) ) {
			$context[ Tag::IMG ]['data-amp-layout'] = true;
		}

		return $context;
	}

	/**
	 * Enqueue AMP assets if this is an AMP endpoint.
	 *
	 * @since 0.7
	 *
	 * @return void
	 */
	public static function enqueue_assets() {
		// Enqueue default styles expected by sanitizer.
		wp_enqueue_style( 'amp-default', amp_get_asset_url( 'css/amp-default.css' ), [], AMP__VERSION );
		wp_styles()->add_data( 'amp-default', 'rtl', 'replace' );
	}

	/**
	 * Setup pages to have the paired browsing client script so that the app can interact with it.
	 *
	 * @since 1.5.0
	 *
	 * @return void
	 */
	public static function setup_paired_browsing_client() {
		// phpcs:ignore WordPress.Security.NonceVerification.Recommended
		if ( isset( $_GET[ self::PAIRED_BROWSING_QUERY_VAR ] ) ) {
			return;
		}

		// Paired browsing requires a custom script which in turn requires dev mode.
		if ( ! amp_is_dev_mode() ) {
			return;
		}

		$asset_file   = AMP__DIR__ . '/assets/js/amp-paired-browsing-client.asset.php';
		$asset        = require $asset_file;
		$dependencies = $asset['dependencies'];
		$version      = $asset['version'];

		wp_enqueue_script(
			'amp-paired-browsing-client',
			amp_get_asset_url( '/js/amp-paired-browsing-client.js' ),
			$dependencies,
			$version,
			true
		);

		// Mark enqueued script for AMP dev mode so that it is not removed.
		// @todo Revisit with <https://github.com/google/site-kit-wp/pull/505#discussion_r348683617>.
		add_filter(
			'script_loader_tag',
			static function( $tag, $handle ) {
				if ( is_amp_endpoint() && self::has_dependency( wp_scripts(), 'amp-paired-browsing-client', $handle ) ) {
					$tag = preg_replace( '/(?<=<script)(?=\s|>)/i', ' ' . AMP_Rule_Spec::DEV_MODE_ATTRIBUTE, $tag );
				}
				return $tag;
			},
			10,
			2
		);
	}

	/**
	 * Get paired browsing URL for a given URL.
	 *
	 * @since 1.5.0
	 *
	 * @param string $url URL.
	 * @return string Paired browsing URL.
	 */
	public static function get_paired_browsing_url( $url = null ) {
		if ( ! $url ) {
			$url = wp_unslash( $_SERVER['REQUEST_URI'] );
		}
		$url = remove_query_arg(
			[ amp_get_slug(), QueryVars::NOAMP, AMP_Validated_URL_Post_Type::VALIDATE_ACTION, AMP_Validation_Manager::VALIDATION_ERROR_TERM_STATUS_QUERY_VAR ],
			$url
		);
		$url = add_query_arg( self::PAIRED_BROWSING_QUERY_VAR, '1', $url );
		return $url;
	}

	/**
	 * Remove any unnecessary query vars that could hamper the paired browsing experience.
	 *
	 * @since 1.5.0
	 */
	public static function sanitize_url_for_paired_browsing() {
		if ( isset( $_GET[ self::PAIRED_BROWSING_QUERY_VAR ] ) ) { // phpcs:ignore WordPress.Security.NonceVerification.Recommended
			$original_url = wp_unslash( $_SERVER['REQUEST_URI'] );
			$updated_url  = self::get_paired_browsing_url( $original_url );
			if ( $updated_url !== $original_url ) {
				wp_safe_redirect( $updated_url );
				exit;
			}
		}
	}

	/**
	 * Serve paired browsing experience if it is being requested.
	 *
	 * Includes a custom template that acts as an interface to facilitate a side-by-side comparison of a
	 * non-AMP page and its AMP version to review any discrepancies.
	 *
	 * @since 1.5.0
	 *
	 * @param string $template Path of the template to include.
	 * @return string Custom template if in paired browsing mode, else the supplied template.
	 */
	public static function serve_paired_browsing_experience( $template ) {
		// phpcs:ignore WordPress.Security.NonceVerification.Recommended
		if ( ! isset( $_GET[ self::PAIRED_BROWSING_QUERY_VAR ] ) ) {
			return $template;
		}

		if ( ! amp_is_dev_mode() ) {
			wp_die(
				esc_html__( 'Paired browsing is only available when AMP dev mode is enabled (e.g. when logged-in and admin bar is showing).', 'amp' ),
				esc_html__( 'AMP Paired Browsing Unavailable', 'amp' ),
				[ 'response' => 403 ]
			);
		}

		wp_enqueue_style(
			'amp-paired-browsing-app',
			amp_get_asset_url( '/css/amp-paired-browsing-app.css' ),
			[ 'dashicons' ],
			AMP__VERSION
		);

		wp_styles()->add_data( 'amp-paired-browsing-app', 'rtl', 'replace' );

		$asset_file   = AMP__DIR__ . '/assets/js/amp-paired-browsing-app.asset.php';
		$asset        = require $asset_file;
		$dependencies = $asset['dependencies'];
		$version      = $asset['version'];

		wp_enqueue_script(
			'amp-paired-browsing-app',
			amp_get_asset_url( '/js/amp-paired-browsing-app.js' ),
			$dependencies,
			$version,
			true
		);

		wp_localize_script(
			'amp-paired-browsing-app',
			'app',
			[
				'ampSlug'                   => amp_get_slug(),
				'ampPairedBrowsingQueryVar' => self::PAIRED_BROWSING_QUERY_VAR,
				'noampQueryVar'             => QueryVars::NOAMP,
				'documentTitlePrefix'       => __( 'AMP Paired Browsing:', 'amp' ),
			]
		);

		return AMP__DIR__ . '/includes/templates/amp-paired-browsing.php';
	}

	/**
	 * Print the important emoji-related styles.
	 *
	 * @see print_emoji_styles()
	 * @staticvar bool $printed
	 */
	public static function print_emoji_styles() {
		static $printed = false;

		if ( $printed ) {
			return;
		}

		$printed = true;
		?>
		<style type="text/css">
			img.wp-smiley,
			img.emoji {
				display: inline-block !important; /* Patched from core, which had display:inline */
				border: none !important;
				box-shadow: none !important;
				height: 1em !important;
				width: 1em !important;
				margin: 0 .07em !important;
				vertical-align: -0.1em !important;
				background: none !important;
				padding: 0 !important;
			}
		</style>
		<?php
	}

	/**
	 * Conditionally replace the header image markup with a header video or image.
	 *
	 * This is JS-driven in Core themes like Twenty Sixteen and Twenty Seventeen.
	 * So in order for the header video to display, this replaces the markup of the header image.
	 *
	 * @since 1.0
	 * @link https://github.com/WordPress/wordpress-develop/blob/d002fde80e5e3a083e5f950313163f566561517f/src/wp-includes/js/wp-custom-header.js#L54
	 * @link https://github.com/WordPress/wordpress-develop/blob/d002fde80e5e3a083e5f950313163f566561517f/src/wp-includes/js/wp-custom-header.js#L78
	 *
	 * @param string $image_markup The image markup to filter.
	 * @return string $html Filtered markup.
	 */
	public static function amend_header_image_with_video_header( $image_markup ) {

		// If there is no video, just pass the image through.
		if ( ! has_header_video() || ! is_header_video_active() ) {
			return $image_markup;
		}

		$video_settings   = get_header_video_settings();
		$parsed_url       = wp_parse_url( $video_settings['videoUrl'] );
		$query            = isset( $parsed_url['query'] ) ? wp_parse_args( $parsed_url['query'] ) : [];
		$video_attributes = [
			Attribute::MEDIA    => '(min-width: ' . $video_settings['minWidth'] . 'px)',
			Attribute::WIDTH    => $video_settings[ Attribute::WIDTH ],
			Attribute::HEIGHT   => $video_settings[ Attribute::HEIGHT ],
			Attribute::LAYOUT   => 'responsive',
			Attribute::AUTOPLAY => '',
			Attribute::LOOP     => '',
			Attribute::ID       => 'wp-custom-header-video',
		];

		$youtube_id = null;
		if ( isset( $parsed_url['host'] ) && preg_match( '/(^|\.)(youtube\.com|youtu\.be)$/', $parsed_url['host'] ) ) {
			if ( 'youtu.be' === $parsed_url['host'] && ! empty( $parsed_url['path'] ) ) {
				$youtube_id = trim( $parsed_url['path'], '/' );
			} elseif ( isset( $query['v'] ) ) {
				$youtube_id = $query['v'];
			}
		}

		// If the video URL is for YouTube, return an <amp-youtube> element.
		if ( ! empty( $youtube_id ) ) {
			$video_markup = AMP_HTML_Utils::build_tag(
				Extension::YOUTUBE,
				array_merge(
					$video_attributes,
					[
						'data-videoid'              => $youtube_id,

						// For documentation on the params, see <https://developers.google.com/youtube/player_parameters>.
						'data-param-rel'            => '0', // Don't show related videos.
						'data-param-showinfo'       => '0', // Don't show video title at the top.
						'data-param-controls'       => '0', // Don't show video controls.
						'data-param-iv_load_policy' => '3', // Suppress annotations.
						'data-param-modestbranding' => '1', // Show modest branding.
						'data-param-playsinline'    => '1', // Prevent fullscreen playback on iOS.
						'data-param-disablekb'      => '1', // Disable keyboard conttrols.
						'data-param-fs'             => '0', // Suppress full screen button.
					]
				)
			);

			// Hide equalizer video animation.
			$video_markup .= '<style>#wp-custom-header-video .amp-video-eq { display:none; }</style>';
		} else {
			$video_markup = AMP_HTML_Utils::build_tag(
				Extension::VIDEO,
				array_merge(
					$video_attributes,
					[
						Attribute::SRC => $video_settings['videoUrl'],
					]
				)
			);
		}

		return $image_markup . $video_markup;
	}
}