diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 00000000000..2911cedc5e1
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,2 @@
+lib/optimizer/resources/local_fallback/* linguist-generated=true
+lib/optimizer/tests/spec/*               linguist-generated=true
diff --git a/includes/embeds/class-amp-wordpress-tv-embed-handler.php b/includes/embeds/class-amp-wordpress-tv-embed-handler.php
index 512d38c7ef6..d02430ee427 100644
--- a/includes/embeds/class-amp-wordpress-tv-embed-handler.php
+++ b/includes/embeds/class-amp-wordpress-tv-embed-handler.php
@@ -9,17 +9,12 @@
 /**
  * Class AMP_WordPress_TV_Embed_Handler
  *
+ * This sanitizes embeds both for WordPress.tv and for VideoPress (which use the same underlying infrastructure).
+ *
  * @since 1.4
  */
 class AMP_WordPress_TV_Embed_Handler extends AMP_Base_Embed_Handler {
 
-	/**
-	 * The URL pattern to determine if an embed URL is for this type, copied from WP_oEmbed.
-	 *
-	 * @see https://github.com/WordPress/wordpress-develop/blob/e13480/src/wp-includes/class-wp-oembed.php#L64
-	 */
-	const URL_PATTERN = '#https?://wordpress\.tv/.*#i';
-
 	/**
 	 * Register embed.
 	 */
@@ -42,7 +37,8 @@ public function unregister_embed() {
 	 * @return string The filtered embed markup.
 	 */
 	public function filter_oembed_html( $cache, $url ) {
-		if ( ! preg_match( self::URL_PATTERN, $url ) ) {
+		$host = wp_parse_url( $url, PHP_URL_HOST );
+		if ( ! in_array( $host, [ 'wordpress.tv', 'videopress.com' ], true ) ) {
 			return $cache;
 		}
 
diff --git a/lib/common/src/RemoteRequest/FilesystemRemoteGetRequest.php b/lib/common/src/RemoteRequest/FilesystemRemoteGetRequest.php
index 00b0ef7e3ae..dc57c1c6f8b 100644
--- a/lib/common/src/RemoteRequest/FilesystemRemoteGetRequest.php
+++ b/lib/common/src/RemoteRequest/FilesystemRemoteGetRequest.php
@@ -20,16 +20,17 @@ final class FilesystemRemoteGetRequest implements RemoteGetRequest
 {
 
     /**
-     * Associative array of data for mapping between arguments and returned results.
+     * Associative array of data for mapping between arguments and filepaths pointing to the results to return.
      *
      * @var array
      */
     private $argumentMap;
 
     /**
-     * Instantiate a StubbedRemoteGetRequest object.
+     * Instantiate a FilesystemRemoteGetRequest object.
      *
-     * @param array $argumentMap Associative array of data for mapping between arguments and returned results.
+     * @param array $argumentMap Associative array of data for mapping between arguments and filepaths pointing to the
+     *                           results to return.
      */
     public function __construct($argumentMap)
     {
diff --git a/lib/optimizer/resources/local_fallback/rtv/metadata b/lib/optimizer/resources/local_fallback/rtv/metadata
index 075574e4725..031b783c4a1 100644
--- a/lib/optimizer/resources/local_fallback/rtv/metadata
+++ b/lib/optimizer/resources/local_fallback/rtv/metadata
@@ -1 +1 @@
-{"ampRuntimeVersion":"012004030010070","ampCssUrl":"https://cdn.ampproject.org/rtv/012004030010070/v0.css","canaryPercentage":"0.005","diversions":["002004012111560","002004012158290","022004030010070","032004012111560","032004012158290","042004030033570"],"ltsRuntimeVersion":"012002251816300","ltsCssUrl":"https://cdn.ampproject.org/rtv/012002251816300/v0.css"}
\ No newline at end of file
+{"ampRuntimeVersion":"012004252135000","ampCssUrl":"https://cdn.ampproject.org/rtv/012004252135000/v0.css","canaryPercentage":"0.005","diversions":["002004252135000","002005050322000","022004252135000","032004252135000","032005050322000","042005051629000","052004252135000","102004252135000"],"ltsRuntimeVersion":"012004030010070","ltsCssUrl":"https://cdn.ampproject.org/rtv/012004030010070/v0.css"}
\ No newline at end of file
diff --git a/lib/optimizer/resources/local_fallback/v0.css b/lib/optimizer/resources/local_fallback/v0.css
index 8bd733adce3..d3ec1fdd807 100644
--- a/lib/optimizer/resources/local_fallback/v0.css
+++ b/lib/optimizer/resources/local_fallback/v0.css
@@ -1,3 +1,3 @@
-html{overflow-x:hidden!important}html.i-amphtml-fie{height:100%!important;width:100%!important}html:not([amp4ads]),html:not([amp4ads]) body{height:auto!important}html:not([amp4ads]) body{margin:0!important}body{-webkit-text-size-adjust:100%;-moz-text-size-adjust:100%;-ms-text-size-adjust:100%;text-size-adjust:100%}html.i-amphtml-singledoc.i-amphtml-embedded{-ms-touch-action:pan-y;touch-action:pan-y}html.i-amphtml-fie>body,html.i-amphtml-singledoc>body{overflow:visible!important}html.i-amphtml-fie:not(.i-amphtml-inabox)>body,html.i-amphtml-singledoc:not(.i-amphtml-inabox)>body{position:relative!important}html.i-amphtml-webview>body{overflow-x:hidden!important;overflow-y:visible!important;min-height:100vh!important}html.i-amphtml-ios-embed-legacy>body{overflow-x:hidden!important;overflow-y:auto!important;position:absolute!important}html.i-amphtml-ios-embed{overflow-y:auto!important;position:static}#i-amphtml-wrapper{overflow-x:hidden!important;overflow-y:auto!important;position:absolute!important;top:0!important;left:0!important;right:0!important;bottom:0!important;margin:0!important;display:block!important}html.i-amphtml-ios-embed.i-amphtml-ios-overscroll,html.i-amphtml-ios-embed.i-amphtml-ios-overscroll>#i-amphtml-wrapper{-webkit-overflow-scrolling:touch!important}#i-amphtml-wrapper>body{position:relative!important;border-top:1px solid transparent!important}#i-amphtml-wrapper+body{visibility:visible}#i-amphtml-wrapper+body .i-amphtml-lightbox-element,#i-amphtml-wrapper+body[i-amphtml-lightbox]{visibility:hidden}#i-amphtml-wrapper+body[i-amphtml-lightbox] .i-amphtml-lightbox-element{visibility:visible}#i-amphtml-wrapper.i-amphtml-scroll-disabled,.i-amphtml-scroll-disabled{overflow-x:hidden!important;overflow-y:hidden!important}amp-instagram{padding:54px 0px 0px!important;background-color:#fff}amp-iframe iframe{box-sizing:border-box!important}[amp-access][amp-access-hide]{display:none}[subscriptions-dialog],body:not(.i-amphtml-subs-ready) [subscriptions-action],body:not(.i-amphtml-subs-ready) [subscriptions-section]{display:none!important}amp-experiment,amp-live-list>[update],amp-share-tracking{display:none}.i-amphtml-jank-meter{position:fixed;background-color:rgba(232,72,95,0.5);bottom:0;right:0;color:#fff;font-size:16px;z-index:1000;padding:5px}amp-list[resizable-children]>.i-amphtml-loading-container.amp-hidden{display:none!important}amp-list[load-more] [load-more-button],amp-list[load-more] [load-more-end],amp-list[load-more] [load-more-failed],amp-list[load-more] [load-more-loading]{display:none}amp-story-page,amp-story[standalone]{min-height:1px!important;display:block!important;height:100%!important;margin:0!important;padding:0!important;overflow:hidden!important;width:100%!important}amp-story[standalone]{background-color:#202125!important;position:relative!important}amp-story-page{background-color:#757575}amp-story .amp-active>div{display:none!important}amp-story-page:not(:first-of-type):not([distance]):not([active]){transform:translateY(1000vh)!important}amp-autocomplete{position:relative!important;display:inline-block!important}amp-autocomplete>input,amp-autocomplete>textarea{padding:0.5rem;border:1px solid rgba(0,0,0,0.33)}.i-amphtml-autocomplete-results,amp-autocomplete>input,amp-autocomplete>textarea{font-size:1rem;line-height:1.5rem}[amp-fx^=fly-in]{visibility:hidden}
-/*# sourceURL=/css/ampdoc.css*/[hidden]{display:none!important}.i-amphtml-element{display:inline-block}.i-amphtml-blurry-placeholder{transition:opacity 0.3s cubic-bezier(0.0,0.0,0.2,1)!important}[layout=nodisplay]:not(.i-amphtml-element){display:none!important}.i-amphtml-layout-fixed,[layout=fixed][width][height]:not(.i-amphtml-layout-fixed){display:inline-block;position:relative}.i-amphtml-layout-responsive,[layout=responsive][width][height]:not(.i-amphtml-layout-responsive),[width][height][sizes]:not(.i-amphtml-layout-responsive){display:block;position:relative}.i-amphtml-layout-intrinsic{display:inline-block;position:relative;max-width:100%}.i-amphtml-intrinsic-sizer{max-width:100%;display:block!important}.i-amphtml-layout-container,.i-amphtml-layout-fixed-height,[layout=container],[layout=fixed-height][height]{display:block;position:relative}.i-amphtml-layout-fill,[layout=fill]:not(.i-amphtml-layout-fill){display:block;overflow:hidden!important;position:absolute;top:0;left:0;bottom:0;right:0}.i-amphtml-layout-flex-item,[layout=flex-item]:not(.i-amphtml-layout-flex-item){display:block;position:relative;-ms-flex:1 1 auto;flex:1 1 auto}.i-amphtml-layout-fluid{position:relative}.i-amphtml-layout-size-defined{overflow:hidden!important}.i-amphtml-layout-awaiting-size{position:absolute!important;top:auto!important;bottom:auto!important}i-amphtml-sizer{display:block!important}.i-amphtml-blurry-placeholder,.i-amphtml-fill-content{display:block;height:0;max-height:100%;max-width:100%;min-height:100%;min-width:100%;width:0;margin:auto}.i-amphtml-layout-size-defined .i-amphtml-fill-content{position:absolute;top:0;left:0;bottom:0;right:0}.i-amphtml-layout-intrinsic .i-amphtml-sizer{max-width:100%}.i-amphtml-replaced-content,.i-amphtml-screen-reader{padding:0!important;border:none!important}.i-amphtml-screen-reader{position:fixed!important;top:0px!important;left:0px!important;width:4px!important;height:4px!important;opacity:0!important;overflow:hidden!important;margin:0!important;display:block!important;visibility:visible!important}.i-amphtml-screen-reader~.i-amphtml-screen-reader{left:8px!important}.i-amphtml-screen-reader~.i-amphtml-screen-reader~.i-amphtml-screen-reader{left:12px!important}.i-amphtml-screen-reader~.i-amphtml-screen-reader~.i-amphtml-screen-reader~.i-amphtml-screen-reader{left:16px!important}.i-amphtml-unresolved{position:relative;overflow:hidden!important}.i-amphtml-select-disabled{-webkit-user-select:none!important;-moz-user-select:none!important;-ms-user-select:none!important;user-select:none!important}.i-amphtml-notbuilt,[layout]:not(.i-amphtml-element){position:relative;overflow:hidden!important;color:transparent!important}.i-amphtml-notbuilt:not(.i-amphtml-layout-container)>*,[layout]:not([layout=container]):not(.i-amphtml-element)>*{display:none}.i-amphtml-ghost{visibility:hidden!important}.i-amphtml-element>[placeholder],[layout]:not(.i-amphtml-element)>[placeholder]{display:block}.i-amphtml-element>[placeholder].amp-hidden,.i-amphtml-element>[placeholder].hidden{visibility:hidden}.i-amphtml-element:not(.amp-notsupported)>[fallback],.i-amphtml-layout-container>[placeholder].amp-hidden,.i-amphtml-layout-container>[placeholder].hidden{display:none}.i-amphtml-layout-size-defined>[fallback],.i-amphtml-layout-size-defined>[placeholder]{position:absolute!important;top:0!important;left:0!important;right:0!important;bottom:0!important;z-index:1}.i-amphtml-notbuilt>[placeholder]{display:block!important}.i-amphtml-hidden-by-media-query{display:none!important}.i-amphtml-element-error{background:red!important;color:#fff!important;position:relative!important}.i-amphtml-element-error:before{content:attr(error-message)}i-amp-scroll-container,i-amphtml-scroll-container{position:absolute;top:0;left:0;right:0;bottom:0;display:block}i-amp-scroll-container.amp-active,i-amphtml-scroll-container.amp-active{overflow:auto;-webkit-overflow-scrolling:touch}.i-amphtml-loading-container{display:block!important;pointer-events:none;z-index:1}.i-amphtml-notbuilt>.i-amphtml-loading-container{display:block!important}.i-amphtml-loading-container.amp-hidden{visibility:hidden}.i-amphtml-element>[overflow]{cursor:pointer;position:relative;z-index:2;visibility:hidden}.i-amphtml-element>[overflow].amp-visible{visibility:visible}template{display:none!important}.amp-border-box,.amp-border-box *,.amp-border-box :after,.amp-border-box :before{box-sizing:border-box}amp-pixel{display:none!important}amp-analytics,amp-story-auto-ads{position:fixed!important;top:0!important;width:1px!important;height:1px!important;overflow:hidden!important;visibility:hidden}html.i-amphtml-fie>amp-analytics{position:initial!important}[visible-when-invalid]:not(.visible),amp-list [fetch-error],form [submit-error],form [submit-success],form [submitting]{display:none}amp-accordion{display:block!important}amp-accordion>section{float:none!important}amp-accordion>section>*{float:none!important;display:block!important;overflow:hidden!important;position:relative!important}amp-accordion,amp-accordion>section{margin:0}amp-accordion>section>:last-child{display:none!important}amp-accordion>section[expanded]>:last-child{display:block!important}
+html{overflow-x:hidden!important}html.i-amphtml-fie{height:100%!important;width:100%!important}html:not([amp4ads]),html:not([amp4ads]) body{height:auto!important}html:not([amp4ads]) body{margin:0!important}body{-webkit-text-size-adjust:100%;-moz-text-size-adjust:100%;-ms-text-size-adjust:100%;text-size-adjust:100%}html.i-amphtml-singledoc.i-amphtml-embedded{-ms-touch-action:pan-y;touch-action:pan-y}html.i-amphtml-fie>body,html.i-amphtml-singledoc>body{overflow:visible!important}html.i-amphtml-fie:not(.i-amphtml-inabox)>body,html.i-amphtml-singledoc:not(.i-amphtml-inabox)>body{position:relative!important}html.i-amphtml-webview>body{overflow-x:hidden!important;overflow-y:visible!important;min-height:100vh!important}html.i-amphtml-ios-embed-legacy>body{overflow-x:hidden!important;overflow-y:auto!important;position:absolute!important}html.i-amphtml-ios-embed{overflow-y:auto!important;position:static}#i-amphtml-wrapper{overflow-x:hidden!important;overflow-y:auto!important;position:absolute!important;top:0!important;left:0!important;right:0!important;bottom:0!important;margin:0!important;display:block!important}html.i-amphtml-ios-embed.i-amphtml-ios-overscroll,html.i-amphtml-ios-embed.i-amphtml-ios-overscroll>#i-amphtml-wrapper{-webkit-overflow-scrolling:touch!important}#i-amphtml-wrapper>body{position:relative!important;border-top:1px solid transparent!important}#i-amphtml-wrapper+body{visibility:visible}#i-amphtml-wrapper+body .i-amphtml-lightbox-element,#i-amphtml-wrapper+body[i-amphtml-lightbox]{visibility:hidden}#i-amphtml-wrapper+body[i-amphtml-lightbox] .i-amphtml-lightbox-element{visibility:visible}#i-amphtml-wrapper.i-amphtml-scroll-disabled,.i-amphtml-scroll-disabled{overflow-x:hidden!important;overflow-y:hidden!important}amp-instagram{padding:54px 0px 0px!important;background-color:#fff}amp-iframe iframe{box-sizing:border-box!important}[amp-access][amp-access-hide]{display:none}[subscriptions-dialog],body:not(.i-amphtml-subs-ready) [subscriptions-action],body:not(.i-amphtml-subs-ready) [subscriptions-section]{display:none!important}amp-experiment,amp-live-list>[update],amp-share-tracking{display:none}.i-amphtml-jank-meter{position:fixed;background-color:rgba(232,72,95,0.5);bottom:0;right:0;color:#fff;font-size:16px;z-index:1000;padding:5px}amp-list[resizable-children]>.i-amphtml-loading-container.amp-hidden{display:none!important}amp-list [fetch-error],amp-list[load-more] [load-more-button],amp-list[load-more] [load-more-end],amp-list[load-more] [load-more-failed],amp-list[load-more] [load-more-loading]{display:none}amp-list[diffable] div[role=list]{display:block}amp-story-page,amp-story[standalone]{min-height:1px!important;display:block!important;height:100%!important;margin:0!important;padding:0!important;overflow:hidden!important;width:100%!important}amp-story[standalone]{background-color:#202125!important;position:relative!important}amp-story-page{background-color:#757575}amp-story .amp-active>div{display:none!important}amp-story-page:not(:first-of-type):not([distance]):not([active]){transform:translateY(1000vh)!important}amp-autocomplete{position:relative!important;display:inline-block!important}amp-autocomplete>input,amp-autocomplete>textarea{padding:0.5rem;border:1px solid rgba(0,0,0,0.33)}.i-amphtml-autocomplete-results,amp-autocomplete>input,amp-autocomplete>textarea{font-size:1rem;line-height:1.5rem}[amp-fx^=fly-in]{visibility:hidden}
+/*# sourceURL=/css/ampdoc.css*/[hidden]{display:none!important}.i-amphtml-element{display:inline-block}.i-amphtml-blurry-placeholder{transition:opacity 0.3s cubic-bezier(0.0,0.0,0.2,1)!important}[layout=nodisplay]:not(.i-amphtml-element){display:none!important}.i-amphtml-layout-fixed,[layout=fixed][width][height]:not(.i-amphtml-layout-fixed){display:inline-block;position:relative}.i-amphtml-layout-responsive,[layout=responsive][width][height]:not(.i-amphtml-layout-responsive),[width][height][sizes]:not(.i-amphtml-layout-responsive){display:block;position:relative}.i-amphtml-layout-intrinsic{display:inline-block;position:relative;max-width:100%}.i-amphtml-intrinsic-sizer{max-width:100%;display:block!important}.i-amphtml-layout-container,.i-amphtml-layout-fixed-height,[layout=container],[layout=fixed-height][height]{display:block;position:relative}.i-amphtml-layout-fill,[layout=fill]:not(.i-amphtml-layout-fill){display:block;overflow:hidden!important;position:absolute;top:0;left:0;bottom:0;right:0}.i-amphtml-layout-flex-item,[layout=flex-item]:not(.i-amphtml-layout-flex-item){display:block;position:relative;-ms-flex:1 1 auto;flex:1 1 auto}.i-amphtml-layout-fluid{position:relative}.i-amphtml-layout-size-defined{overflow:hidden!important}.i-amphtml-layout-awaiting-size{position:absolute!important;top:auto!important;bottom:auto!important}i-amphtml-sizer{display:block!important}.i-amphtml-blurry-placeholder,.i-amphtml-fill-content{display:block;height:0;max-height:100%;max-width:100%;min-height:100%;min-width:100%;width:0;margin:auto}.i-amphtml-layout-size-defined .i-amphtml-fill-content{position:absolute;top:0;left:0;bottom:0;right:0}.i-amphtml-layout-intrinsic .i-amphtml-sizer{max-width:100%}.i-amphtml-replaced-content,.i-amphtml-screen-reader{padding:0!important;border:none!important}.i-amphtml-screen-reader{position:fixed!important;top:0px!important;left:0px!important;width:4px!important;height:4px!important;opacity:0!important;overflow:hidden!important;margin:0!important;display:block!important;visibility:visible!important}.i-amphtml-screen-reader~.i-amphtml-screen-reader{left:8px!important}.i-amphtml-screen-reader~.i-amphtml-screen-reader~.i-amphtml-screen-reader{left:12px!important}.i-amphtml-screen-reader~.i-amphtml-screen-reader~.i-amphtml-screen-reader~.i-amphtml-screen-reader{left:16px!important}.i-amphtml-unresolved{position:relative;overflow:hidden!important}.i-amphtml-select-disabled{-webkit-user-select:none!important;-moz-user-select:none!important;-ms-user-select:none!important;user-select:none!important}.i-amphtml-notbuilt,[layout]:not(.i-amphtml-element){position:relative;overflow:hidden!important;color:transparent!important}.i-amphtml-notbuilt:not(.i-amphtml-layout-container)>*,[layout]:not([layout=container]):not(.i-amphtml-element)>*{display:none}.i-amphtml-ghost{visibility:hidden!important}.i-amphtml-element>[placeholder],[layout]:not(.i-amphtml-element)>[placeholder]{display:block}.i-amphtml-element>[placeholder].amp-hidden,.i-amphtml-element>[placeholder].hidden{visibility:hidden}.i-amphtml-element:not(.amp-notsupported)>[fallback],.i-amphtml-layout-container>[placeholder].amp-hidden,.i-amphtml-layout-container>[placeholder].hidden{display:none}.i-amphtml-layout-size-defined>[fallback],.i-amphtml-layout-size-defined>[placeholder]{position:absolute!important;top:0!important;left:0!important;right:0!important;bottom:0!important;z-index:1}.i-amphtml-notbuilt>[placeholder]{display:block!important}.i-amphtml-hidden-by-media-query{display:none!important}.i-amphtml-element-error{background:red!important;color:#fff!important;position:relative!important}.i-amphtml-element-error:before{content:attr(error-message)}i-amp-scroll-container,i-amphtml-scroll-container{position:absolute;top:0;left:0;right:0;bottom:0;display:block}i-amp-scroll-container.amp-active,i-amphtml-scroll-container.amp-active{overflow:auto;-webkit-overflow-scrolling:touch}.i-amphtml-loading-container{display:block!important;pointer-events:none;z-index:1}.i-amphtml-notbuilt>.i-amphtml-loading-container{display:block!important}.i-amphtml-loading-container.amp-hidden{visibility:hidden}.i-amphtml-element>[overflow]{cursor:pointer;position:relative;z-index:2;visibility:hidden;display:initial}.i-amphtml-element>[overflow].amp-visible{visibility:visible}template{display:none!important}.amp-border-box,.amp-border-box *,.amp-border-box :after,.amp-border-box :before{box-sizing:border-box}amp-pixel{display:none!important}amp-analytics,amp-story-auto-ads{position:fixed!important;top:0!important;width:1px!important;height:1px!important;overflow:hidden!important;visibility:hidden}html.i-amphtml-fie>amp-analytics{position:initial!important}[visible-when-invalid]:not(.visible),form [submit-error],form [submit-success],form [submitting]{display:none}amp-accordion{display:block!important}amp-accordion>section{float:none!important}amp-accordion>section>*{float:none!important;display:block!important;overflow:hidden!important;position:relative!important}amp-accordion,amp-accordion>section{margin:0}amp-accordion>section>:last-child{display:none!important}amp-accordion>section[expanded]>:last-child{display:block!important}
 /*# sourceURL=/css/ampshared.css*/
\ No newline at end of file
diff --git a/lib/optimizer/src/Configurable.php b/lib/optimizer/src/Configurable.php
deleted file mode 100644
index 592cd3d5b59..00000000000
--- a/lib/optimizer/src/Configurable.php
+++ /dev/null
@@ -1,13 +0,0 @@
-<?php
-
-namespace AmpProject\Optimizer;
-
-/**
- * Marker interface to denote a transformer that accepts configuration settings.
- *
- * @package ampproject/optimizer
- */
-interface Configurable
-{
-
-}
diff --git a/lib/optimizer/src/Configuration/AmpRuntimeCssConfiguration.php b/lib/optimizer/src/Configuration/AmpRuntimeCssConfiguration.php
index 82185b5b20b..926b6e66948 100644
--- a/lib/optimizer/src/Configuration/AmpRuntimeCssConfiguration.php
+++ b/lib/optimizer/src/Configuration/AmpRuntimeCssConfiguration.php
@@ -20,10 +20,21 @@ final class AmpRuntimeCssConfiguration extends BaseTransformerConfiguration
     /**
      * Configuration key that holds the version number to use.
      *
+     * If the version is not provided, the latest runtime version is fetched from cdn.ampproject.org.
+     *
      * @var string
      */
     const VERSION = 'version';
 
+    /**
+     * Configuration key that holds the actual runtime CSS styles to use.
+     *
+     * If the styles are not provided, the latest runtime styles are fetched from cdn.ampproject.org.
+     *
+     * @var string
+     */
+    const STYLES = 'styles';
+
     /**
      * Configuration key that holds the flag for the canary version of the runtime style.
      *
@@ -42,6 +53,7 @@ protected function getAllowedKeys()
     {
         return [
             self::VERSION => '',
+            self::STYLES  => '',
             self::CANARY  => false,
         ];
     }
@@ -63,6 +75,13 @@ protected function validate($key, $value)
                 $value = trim($value);
                 break;
 
+            case self::STYLES:
+                if (! is_string($value)) {
+                    throw InvalidConfigurationValue::forInvalidSubValueType(self::class, self::STYLES, 'string', gettype($value));
+                }
+                $value = trim($value);
+                break;
+
             case self::CANARY:
                 if (! is_bool($value)) {
                     throw InvalidConfigurationValue::forInvalidSubValueType(self::class, self::CANARY, 'boolean', gettype($value));
diff --git a/lib/optimizer/src/MakesRemoteRequests.php b/lib/optimizer/src/MakesRemoteRequests.php
deleted file mode 100644
index 2b8526cdfdf..00000000000
--- a/lib/optimizer/src/MakesRemoteRequests.php
+++ /dev/null
@@ -1,13 +0,0 @@
-<?php
-
-namespace AmpProject\Optimizer;
-
-/**
- * Marker interface to denote a transformer that makes remote requests.
- *
- * @package ampproject/optimizer
- */
-interface MakesRemoteRequests
-{
-
-}
diff --git a/lib/optimizer/src/TransformationEngine.php b/lib/optimizer/src/TransformationEngine.php
index 279063ecbb7..45470898e68 100644
--- a/lib/optimizer/src/TransformationEngine.php
+++ b/lib/optimizer/src/TransformationEngine.php
@@ -5,6 +5,8 @@
 use AmpProject\Dom\Document;
 use AmpProject\RemoteGetRequest;
 use AmpProject\RemoteRequest\CurlRemoteGetRequest;
+use ReflectionClass;
+use ReflectionException;
 
 /**
  * Transformation engine that accepts HTML and returns optimized HTML.
@@ -28,17 +30,26 @@ final class TransformationEngine
      */
     private $remoteRequest;
 
+    /**
+     * Collection of transformers that were initialized.
+     *
+     * @var Transformer[]
+     */
+    private $transformers;
+
     /**
      * Instantiate a TransformationEngine object.
      *
-     * @param Configuration    $configuration Configuration data to use for setting up the transformers.
-     * @param RemoteGetRequest $remoteRequest Optional. Transport to use for remote requests. Defaults to the
+     * @param Configuration|null    $configuration Optional. Configuration data to use for setting up the transformers.
+     * @param RemoteGetRequest|null $remoteRequest Optional. Transport to use for remote requests. Defaults to the
      *                                     CurlRemoteGetRequest implementation shipped with the library.
      */
-    public function __construct(Configuration $configuration, RemoteGetRequest $remoteRequest = null)
+    public function __construct(Configuration $configuration = null, RemoteGetRequest $remoteRequest = null)
     {
-        $this->configuration = $configuration;
+        $this->configuration = isset($configuration) ? $configuration : new Configuration();
         $this->remoteRequest = isset($remoteRequest) ? $remoteRequest : new CurlRemoteGetRequest();
+
+        $this->initializeTransformers();
     }
 
     /**
@@ -50,7 +61,7 @@ public function __construct(Configuration $configuration, RemoteGetRequest $remo
      */
     public function optimizeDom(Document $document, ErrorCollection $errors)
     {
-        foreach ($this->getTransformers() as $transformer) {
+        foreach ($this->transformers as $transformer) {
             $transformer->transform($document, $errors);
         }
     }
@@ -71,31 +82,58 @@ public function optimizeHtml($html, ErrorCollection $errors)
     }
 
     /**
-     * Get the array of transformers to use.
+     * Initialize the array of transformers to use.
+     */
+    private function initializeTransformers()
+    {
+        $this->transformers = [];
+
+        foreach ($this->configuration->get(Configuration::KEY_TRANSFORMERS) as $transformerClass) {
+            $this->transformers[$transformerClass] = new $transformerClass(
+                ...$this->getTransformerDependencies($transformerClass)
+            );
+        }
+    }
+
+    /**
+     * Get the dependencies of a transformer and put them in the correct order.
      *
-     * @return Transformer[] Array of transformers to use.
+     * @param string $transformerClass Class of the transformer to get the dependencies for.
+     * @return array Array of dependencies in the order as they appear in the transformer's constructor.
+     * @throws ReflectionException If the transformer could not be reflected upon.
      */
-    private function getTransformers()
+    private function getTransformerDependencies($transformerClass)
     {
-        static $transformers = null;
+        $constructor = (new ReflectionClass($transformerClass))->getConstructor();
+
+        if ($constructor === null) {
+            return [];
+        }
 
-        if (null === $transformers) {
-            $transformers = [];
-            foreach ($this->configuration->get(Configuration::KEY_TRANSFORMERS) as $transformerClass) {
-                $arguments = [];
+        $dependencies = [];
+        foreach ($constructor->getParameters() as $parameter) {
+            $dependencyType = $parameter->getClass();
 
-                if (is_a($transformerClass, MakesRemoteRequests::class, true)) {
-                    $arguments[] = $this->remoteRequest;
-                }
+            if ($dependencyType === null) {
+                // No type provided, so we pass `null` in the hopes that the argument is optional.
+                $dependencies[] = null;
+                continue;
+            }
 
-                if (is_a($transformerClass, Configurable::class, true)) {
-                    $arguments[] = $this->configuration->getTransformerConfiguration($transformerClass);
-                }
+            if (is_a($dependencyType->name, TransformerConfiguration::class, true)) {
+                $dependencies[] = $this->configuration->getTransformerConfiguration($transformerClass);
+                continue;
+            }
 
-                $transformers[$transformerClass] = new $transformerClass(...$arguments);
+            if (is_a($dependencyType->name, RemoteGetRequest::class, true)) {
+                $dependencies[] = $this->remoteRequest;
+                continue;
             }
+
+            // Unknown dependency type, so we pass `null` in the hopes that the argument is optional.
+            $dependencies[] = null;
         }
 
-        return $transformers;
+        return $dependencies;
     }
 }
diff --git a/lib/optimizer/src/Transformer/AmpRuntimeCss.php b/lib/optimizer/src/Transformer/AmpRuntimeCss.php
index c0927e11d7f..949c83347a3 100644
--- a/lib/optimizer/src/Transformer/AmpRuntimeCss.php
+++ b/lib/optimizer/src/Transformer/AmpRuntimeCss.php
@@ -5,11 +5,9 @@
 use AmpProject\Amp;
 use AmpProject\Attribute;
 use AmpProject\Dom\Document;
-use AmpProject\Optimizer\Configurable;
 use AmpProject\Optimizer\Configuration\AmpRuntimeCssConfiguration;
 use AmpProject\Optimizer\Error;
 use AmpProject\Optimizer\ErrorCollection;
-use AmpProject\Optimizer\MakesRemoteRequests;
 use AmpProject\Optimizer\TransformerConfiguration;
 use AmpProject\RemoteGetRequest;
 use AmpProject\Optimizer\Transformer;
@@ -36,7 +34,7 @@
  *
  * @package ampproject/optimizer
  */
-final class AmpRuntimeCss implements Transformer, Configurable, MakesRemoteRequests
+final class AmpRuntimeCss implements Transformer
 {
 
     /**
@@ -61,29 +59,29 @@ final class AmpRuntimeCss implements Transformer, Configurable, MakesRemoteReque
     const V0_CSS_URL = Amp::CACHE_HOST . '/' . self::V0_CSS;
 
     /**
-     * Transport to use for remote requests.
+     * Configuration store to use.
      *
-     * @var RemoteGetRequest
+     * @var TransformerConfiguration
      */
-    private $remoteRequest;
+    private $configuration;
 
     /**
-     * Configuration store to use.
+     * Transport to use for remote requests.
      *
-     * @var TransformerConfiguration
+     * @var RemoteGetRequest
      */
-    private $configuration;
+    private $remoteRequest;
 
     /**
      * Instantiate an AmpRuntimeCss object.
      *
-     * @param RemoteGetRequest         $remoteRequest Transport to use for remote requests.
      * @param TransformerConfiguration $configuration Configuration store to use.
+     * @param RemoteGetRequest         $remoteRequest Transport to use for remote requests.
      */
-    public function __construct(RemoteGetRequest $remoteRequest, TransformerConfiguration $configuration)
+    public function __construct(TransformerConfiguration $configuration, RemoteGetRequest $remoteRequest)
     {
-        $this->remoteRequest = $remoteRequest;
         $this->configuration = $configuration;
+        $this->remoteRequest = $remoteRequest;
     }
 
     /**
@@ -143,6 +141,7 @@ private function addStaticCss(Document $document, DOMElement $ampRuntimeStyle, E
         } catch (Exception $exception) {
             $errors->add(Error\CannotInlineRuntimeCss::fromException($exception, $ampRuntimeStyle, $version));
             $this->linkCss($document, $ampRuntimeStyle);
+            $ampRuntimeStyle->parentNode->removeChild($ampRuntimeStyle);
         }
     }
 
@@ -164,14 +163,21 @@ private function inlineCss(DOMElement $ampRuntimeStyle, $version)
         }
 
         $ampRuntimeStyle->setAttribute(Attribute::I_AMPHTML_VERSION, $version);
-        $response   = $this->remoteRequest->get($v0CssUrl);
-        $statusCode = $response->getStatusCode();
 
-        if ($statusCode < 200 || $statusCode >= 300) {
-            return;
+        $styles = $this->configuration->get(AmpRuntimeCssConfiguration::STYLES);
+
+        if (empty($styles)) {
+            $response   = $this->remoteRequest->get($v0CssUrl);
+            $statusCode = $response->getStatusCode();
+
+            if (200 < $statusCode || $statusCode >= 300) {
+                return;
+            }
+
+            $styles = $response->getBody();
         }
 
-        $ampRuntimeStyle->textContent = $response->getBody();
+        $ampRuntimeStyle->textContent = $styles;
     }
 
     /**
diff --git a/lib/optimizer/src/Transformer/ReorderHead.php b/lib/optimizer/src/Transformer/ReorderHead.php
index 3d7b20e1692..c5280d4acd8 100644
--- a/lib/optimizer/src/Transformer/ReorderHead.php
+++ b/lib/optimizer/src/Transformer/ReorderHead.php
@@ -17,7 +17,7 @@
  * ReorderHead reorders the children of <head>. Specifically, it
  * orders the <head> like so:
  * (0) <meta charset> tag
- * (1) <style amp-runtime> (inserted by ampruntimecss.go)
+ * (1) <style amp-runtime> (inserted by AmpRuntimeCss transformer)
  * (2) remaining <meta> tags (those other than <meta charset>)
  * (3) AMP runtime .js <script> tag
  * (4) AMP viewer runtime .js <script>
diff --git a/lib/optimizer/src/Transformer/TransformedIdentifier.php b/lib/optimizer/src/Transformer/TransformedIdentifier.php
index f6d4deb3c3b..701c9c2d36f 100644
--- a/lib/optimizer/src/Transformer/TransformedIdentifier.php
+++ b/lib/optimizer/src/Transformer/TransformedIdentifier.php
@@ -3,7 +3,6 @@
 namespace AmpProject\Optimizer\Transformer;
 
 use AmpProject\Dom\Document;
-use AmpProject\Optimizer\Configurable;
 use AmpProject\Optimizer\Configuration\TransformedIdentifierConfiguration;
 use AmpProject\Optimizer\ErrorCollection;
 use AmpProject\Optimizer\Transformer;
@@ -25,7 +24,7 @@
  *
  * @package ampproject/optimizer
  */
-final class TransformedIdentifier implements Transformer, Configurable
+final class TransformedIdentifier implements Transformer
 {
 
     /**
diff --git a/lib/optimizer/tests/SpecTest.php b/lib/optimizer/tests/SpecTest.php
index 17a779bdf50..8d417ea6078 100644
--- a/lib/optimizer/tests/SpecTest.php
+++ b/lib/optimizer/tests/SpecTest.php
@@ -3,6 +3,7 @@
 namespace AmpProject\Optimizer;
 
 use AmpProject\Dom\Document;
+use AmpProject\Optimizer\Configuration\AmpRuntimeCssConfiguration;
 use AmpProject\Optimizer\Tests\MarkupComparison;
 use AmpProject\Optimizer\Tests\TestMarkup;
 use AmpProject\Optimizer\Transformer\AmpRuntimeCss;
@@ -11,6 +12,8 @@
 use AmpProject\RemoteRequest\StubbedRemoteGetRequest;
 use DirectoryIterator;
 use PHPUnit\Framework\TestCase;
+use ReflectionClass;
+use ReflectionException;
 
 /**
  * Test the individual transformers against the NodeJS spec test suite.
@@ -32,20 +35,19 @@ final class SpecTest extends TestCase
     const CLASS_SKIP_TEST = '__SKIP__';
 
     /**
-     * Associative array of mapping data for stubbing remote requests for specific tests.
+     * Regular expression to match the leading HTML comment that provides the configuration for a spec test.
      *
-     * @todo This is a temporary fix only to get the test to pass with our current transformer.
-     *       We'll need to adapt the transformer to take the following changes into account:
-     *       https://github.com/ampproject/amp-toolbox/commit/b154a73c6dc9231e4060434c562a90d983e2a46d
+     * @see https://regex101.com/r/ImDtxI/2
      *
-     * @var array
+     * @var string
      */
-    const STUBBED_REMOTE_REQUESTS_FOR_TESTS = [
-        'AmpRuntimeCss - always_inlines_v0css' => [
-            'https://cdn.ampproject.org/v0.css' => '/* v0-prod.css */',
-        ],
-    ];
+    const LEADING_HTML_COMMENT_REGEX_PATTERN = '/^\s*<!--\s*(?<json>{(?>[^}]*})*)\s*-->/';
 
+    /**
+     * Provide the data for running the spec tests.
+     *
+     * @return array Scenarios to test.
+     */
     public function dataTransformerSpecFiles()
     {
         $scenarios = [];
@@ -106,10 +108,12 @@ public function testTransformerSpecFiles($scenario, $transformerClass, $source,
             $this->markTestSkipped("Skipping {$source}, {$expected}");
         }
 
+        $configuration = $this->mapConfigurationData($this->extractConfigurationData($source));
+
         $document = Document::fromHtmlFragment($source);
 
-        $transformer = $this->getTransformer($scenario, $transformerClass);
-        $errors      = new ErrorCollection();
+        $transformer   = $this->getTransformer($scenario, $transformerClass, $configuration);
+        $errors        = new ErrorCollection();
 
         $transformer->transform($document, $errors);
 
@@ -117,30 +121,106 @@ public function testTransformerSpecFiles($scenario, $transformerClass, $source,
     }
 
     /**
-     * Get the transformer to test.
+     * Map spec test input file configuration data to configuration arguments as needed by the PHP transformers.
      *
-     * @param string $scenario         Test scenario.
-     * @param string $transformerClass Class of the transformer to get.
-     * @return Transformer Instantiated transformer object.
+     * @param array $configurationData Associative array of configuration data coming from the spec test input file.
+     * @return Configuration Configuration object to use for the transformation engine.
      */
-    private function getTransformer($scenario, $transformerClass)
+    public function mapConfigurationData($configurationData)
     {
-        $arguments = [];
+        $mappedConfiguration = [];
+
+        foreach ($configurationData as $key => $value) {
+            switch ($key) {
+                case 'ampRuntimeStyles':
+                    $mappedConfiguration[AmpRuntimeCss::class][AmpRuntimeCssConfiguration::STYLES] = $value;
+                    break;
+                case 'ampRuntimeVersion':
+                    $mappedConfiguration[AmpRuntimeCss::class][AmpRuntimeCssConfiguration::VERSION] = $value;
+                    break;
+
+                // @TODO: Known configuration arguments used in spec tests that are not implemented yet.
+                case 'ampUrlPrefix':
+                case 'ampUrl':
+                case 'canonical':
+                case 'experimentBindAttribute':
+                case 'geoApiUrl':
+                case 'lts':
+                case 'preloadHeroImage':
+                case 'rtv':
+                default:
+                    $this->fail("Configuration argument not yet implemented: {$key}.");
+            }
+        }
 
-        if (is_a($transformerClass, MakesRemoteRequests::class, true)) {
-            $stubbedRequests = TestMarkup::STUBBED_REMOTE_REQUESTS;
+        return new Configuration($mappedConfiguration);
+    }
 
-            if (array_key_exists($scenario, self::STUBBED_REMOTE_REQUESTS_FOR_TESTS)) {
-                $stubbedRequests = array_merge($stubbedRequests, self::STUBBED_REMOTE_REQUESTS_FOR_TESTS[$scenario]);
-            }
+    /**
+     * Parse the input source file and extract the configuration data.
+     *
+     * Input HTML files can contain a leading HTML comment that provides configuration arguments in the form of a JSON
+     * object.
+     *
+     * @param string $source Input source file to parse for a configuration snippet.
+     * @return array Associative array of configuration data found in the input HTML file.
+     */
+    public function extractConfigurationData(&$source)
+    {
+        $matches = [];
+        if (!preg_match(self::LEADING_HTML_COMMENT_REGEX_PATTERN, $source, $matches)) {
+            return [];
+        }
 
-            $arguments[] = new StubbedRemoteGetRequest($stubbedRequests);
+        $json   = trim($matches['json']);
+        $source = substr($source, strlen($matches[0]));
+
+        if (empty($json)) {
+            return [];
         }
 
-        if (is_a($transformerClass, Configurable::class, true)) {
-            $arguments[] = (new Configuration())->getTransformerConfiguration($transformerClass);
+        $configurationData = (array)json_decode($json, true);
+        if (empty($configurationData) || json_last_error() !== JSON_ERROR_NONE) {
+            return [];
         }
 
-        return new $transformerClass(...$arguments);
+        return $configurationData;
+    }
+
+    /**
+     * Get the transformer to test.
+     *
+     * @param string        $scenario         Test scenario.
+     * @param string        $transformerClass Class of the transformer to get.
+     * @param Configuration $configuration    Configuration to use.
+     * @return Transformer Instantiated transformer object.
+     */
+    private function getTransformer($scenario, $transformerClass, $configuration)
+    {
+        $stubbedRequests = TestMarkup::STUBBED_REMOTE_REQUESTS;
+
+        $transformationEngine = new TransformationEngine(
+            $configuration,
+            new StubbedRemoteGetRequest($stubbedRequests)
+        );
+
+        return new $transformerClass(...$this->callPrivateMethod($transformationEngine, 'getTransformerDependencies', [$transformerClass]));
+    }
+
+    /**
+     * Call a private method as if it was public.
+     *
+     * @param object|string $object     Object instance or class string to call the method of.
+     * @param string        $methodName Name of the method to call.
+     * @param array         $args       Optional. Array of arguments to pass to the method.
+     * @return mixed Return value of the method call.
+     * @throws ReflectionException If the object could not be reflected upon.
+     */
+    private function callPrivateMethod($object, $methodName, $args = [])
+    {
+        $method = (new ReflectionClass($object))->getMethod($methodName);
+        $method->setAccessible(true);
+
+        return $method->invokeArgs($object, $args);
     }
 }
diff --git a/lib/optimizer/tests/TransformationEngineTest.php b/lib/optimizer/tests/TransformationEngineTest.php
index 09440f6d5d0..adffdca778a 100644
--- a/lib/optimizer/tests/TransformationEngineTest.php
+++ b/lib/optimizer/tests/TransformationEngineTest.php
@@ -5,8 +5,11 @@
 use AmpProject\Dom\Document;
 use AmpProject\Optimizer\Tests\MarkupComparison;
 use AmpProject\Optimizer\Tests\TestMarkup;
+use AmpProject\Optimizer\Tests\TestTransformer;
 use AmpProject\RemoteRequest\StubbedRemoteGetRequest;
 use PHPUnit\Framework\TestCase;
+use ReflectionClass;
+use ReflectionException;
 
 /**
  * Test the transformation engine as a whole.
@@ -79,16 +82,74 @@ public function testOptimizeDom()
         $this->assertCount(0, $errors);
     }
 
+    public function dataDependencyResolution()
+    {
+        return [
+            'no_constructor' => [ TestTransformer\NoConstructor::class ],
+
+            'no_dependencies' => [ TestTransformer\NoDependencies::class ],
+
+            'remote_request_only' => [ TestTransformer\RemoteRequestOnly::class ],
+
+            'configuration_only' => [ TestTransformer\ConfigurationOnly::class ],
+
+            'configuration_then_remote_request' => [ TestTransformer\ConfigurationThenRemoteRequest::class ],
+
+            'remote_request_then_configuration' => [ TestTransformer\RemoteRequestThenConfiguration::class ],
+        ];
+    }
+    /**
+     * Test dependency resolution.
+     *
+     * @covers \AmpProject\Optimizer\TransformationEngine::getTransformerDependencies()
+     * @dataProvider dataDependencyResolution
+     *
+     * @param string $transformerClass Transformer class to use for testing.
+     */
+    public function testDependencyResolution($transformerClass)
+    {
+        $configurationData = [
+            Configuration::KEY_TRANSFORMERS => [
+                $transformerClass,
+            ],
+        ];
+
+        $configuration = new Configuration($configurationData);
+        $configuration->registerConfigurationClass($transformerClass, TestTransformer\DummyTransformerConfiguration::class);
+
+        $transformationEngine = $this->getTransformationEngine($configuration);
+        $this->assertInstanceof(TransformationEngine::class, $transformationEngine);
+        $transformers = $this->getPrivateProperty($transformationEngine, 'transformers');
+        $this->assertCount(1, $transformers);
+        $this->assertInstanceof($transformerClass, array_pop($transformers));
+    }
+
     /**
      * Get the transformation engine instance to test against.
      *
+     * @param Configuration|null $configuration Optional. Configuration object to use.
      * @return TransformationEngine Transformation engine instance to test against.
      */
-    private function getTransformationEngine()
+    private function getTransformationEngine(Configuration $configuration = null)
     {
         return new TransformationEngine(
-            new Configuration(),
+            $configuration,
             new StubbedRemoteGetRequest(TestMarkup::STUBBED_REMOTE_REQUESTS)
         );
     }
+
+    /**
+     * Get a private property as if it was public.
+     *
+     * @param object|string $object       Object instance or class string to get the property of.
+     * @param string        $propertyName Name of the property to get.
+     * @return mixed Return value of the property.
+     * @throws ReflectionException If the object could not be reflected upon.
+     */
+    private function getPrivateProperty($object, $propertyName)
+    {
+        $property = ( new ReflectionClass($object) )->getProperty($propertyName);
+        $property->setAccessible(true);
+        return $property->getValue($object);
+    }
 }
diff --git a/lib/optimizer/tests/Transformer/AmpRuntimeCssTest.php b/lib/optimizer/tests/Transformer/AmpRuntimeCssTest.php
index eec01690b06..925188d064f 100644
--- a/lib/optimizer/tests/Transformer/AmpRuntimeCssTest.php
+++ b/lib/optimizer/tests/Transformer/AmpRuntimeCssTest.php
@@ -42,7 +42,37 @@ public function dataTransform()
                 '<style amp-runtime="" i-amphtml-version="023456789000000">/* v0.css */</style>' .
                 '</head><body></body></html>',
 
-                ['canary' => true],
+                [AmpRuntimeCssConfiguration::CANARY => true],
+            ],
+
+            'override version via config' => [
+                TestMarkup::DOCTYPE . '<html><head>' . TestMarkup::META_CHARSET . TestMarkup::STYLE_AMPRUNTIME . '</head></html>',
+
+                TestMarkup::DOCTYPE . '<html><head>' . TestMarkup::META_CHARSET .
+                '<style amp-runtime="" i-amphtml-version="012345678901234">/* 012345678901234/v0.css */</style>' .
+                '</head><body></body></html>',
+
+                [AmpRuntimeCssConfiguration::VERSION => '012345678901234'],
+            ],
+
+            'override styles via config' => [
+                TestMarkup::DOCTYPE . '<html><head>' . TestMarkup::META_CHARSET . TestMarkup::STYLE_AMPRUNTIME . '</head></html>',
+
+                TestMarkup::DOCTYPE . '<html><head>' . TestMarkup::META_CHARSET .
+                '<style amp-runtime="" i-amphtml-version="012345678900000">/* custom-styles-v0.css */</style>' .
+                '</head><body></body></html>',
+
+                [AmpRuntimeCssConfiguration::STYLES => '/* custom-styles-v0.css */'],
+            ],
+
+            'trying to inline invalid version results in linked stylesheet' => [
+                TestMarkup::DOCTYPE . '<html><head>' . TestMarkup::META_CHARSET . TestMarkup::STYLE_AMPRUNTIME . '</head></html>',
+
+                TestMarkup::DOCTYPE . '<html><head>' . TestMarkup::META_CHARSET .
+                '<link rel="stylesheet" href="https://cdn.ampproject.org/v0.css">' .
+                '</head><body></body></html>',
+
+                [AmpRuntimeCssConfiguration::VERSION => '0000000000000000'],
             ],
         ];
     }
@@ -61,7 +91,7 @@ public function testTransform($source, $expectedHtml, $config = [])
     {
         $document      = Document::fromHtml($source);
         $remoteRequest = new StubbedRemoteGetRequest(TestMarkup::STUBBED_REMOTE_REQUESTS);
-        $transformer   = new AmpRuntimeCss($remoteRequest, new AmpRuntimeCssConfiguration($config));
+        $transformer   = new AmpRuntimeCss(new AmpRuntimeCssConfiguration($config), $remoteRequest);
         $errors        = new ErrorCollection();
 
         $transformer->transform($document, $errors);
diff --git a/lib/optimizer/tests/spec/end-to-end/body-only/expected_output.valid.html b/lib/optimizer/tests/spec/end-to-end/body-only/expected_output.valid.html
index b4cfeb2c140..70aa8be8e57 100644
--- a/lib/optimizer/tests/spec/end-to-end/body-only/expected_output.valid.html
+++ b/lib/optimizer/tests/spec/end-to-end/body-only/expected_output.valid.html
@@ -1,7 +1,7 @@
 <!doctype html>
 <html amp i-amphtml-layout i-amphtml-no-boilerplate transformed="self;v=1">
 <head>
-  <meta data-auto charset="utf-8"><style amp-runtime i-amphtml-version="123456789000000">/* v0.css */</style>
+  <meta data-auto charset="utf-8"><style amp-runtime i-amphtml-version="123456789000000">/* ampproject.org/rtv v0.css */</style>
   <meta data-auto name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1">
   <link rel="preload" href="https://cdn.ampproject.org/v0.js" as="script">
   <script data-auto async src="https://cdn.ampproject.org/v0.js"></script>
diff --git a/lib/optimizer/tests/spec/end-to-end/hello-world/expected_output.html b/lib/optimizer/tests/spec/end-to-end/hello-world/expected_output.html
index 96c971120af..2ad5227e436 100644
--- a/lib/optimizer/tests/spec/end-to-end/hello-world/expected_output.html
+++ b/lib/optimizer/tests/spec/end-to-end/hello-world/expected_output.html
@@ -1,17 +1,17 @@
 <!doctype html>
 <html i-amphtml-layout i-amphtml-no-boilerplate transformed="self;v=1">
 <head>
-  <meta charset="utf-8"><style amp-runtime i-amphtml-version="123456789000000">/* v0.css */</style>
+  <meta charset="utf-8"><style amp-runtime i-amphtml-version="123456789000000">/* example.com v0.css */</style>
   <meta name="viewport" content="width=device-width,minimum-scale=1">
-  <link rel="preload" href="/amp/rtv/123456789000000/v0.js" as="script">
-  <meta name="runtime-host" content="/amp">
+  <link rel="preload" href="https://example.com/amp/rtv/123456789000000/v0.js" as="script">
+  <meta name="runtime-host" content="https://example.com/amp">
   <meta name="amp-geo-api" content="/geo">
-  <script async src="/amp/rtv/123456789000000/v0.js"></script>
-  <script async custom-element="amp-experiment" src="/amp/rtv/123456789000000/v0/amp-experiment-0.1.js"></script>
-  <script async custom-element="amp-video" src="/amp/rtv/123456789000000/v0/amp-video-0.1.js"></script>
-  <script async custom-template="amp-mustache" src="/amp/rtv/123456789000000/v0/amp-mustache-0.1.js"></script>
+  <script async src="https://example.com/amp/rtv/123456789000000/v0.js"></script>
+  <script async custom-element="amp-experiment" src="https://example.com/amp/rtv/123456789000000/v0/amp-experiment-0.1.js"></script>
+  <script async custom-element="amp-video" src="https://example.com/amp/rtv/123456789000000/v0/amp-video-0.1.js"></script>
+  <script async custom-template="amp-mustache" src="https://example.com/amp/rtv/123456789000000/v0/amp-mustache-0.1.js"></script>
   <link rel="canonical" href="self.html"><style amp-custom>h1{margin:16px}</style>
-  <link rel="amphtml" href="https://example.com/amp-version.html">
+  <link rel="amphtml" href="/amp-version.html">
 </head>
 <body>
   <h1>Hello, AMP world!</h1>
diff --git a/lib/optimizer/tests/spec/end-to-end/hello-world/expected_output.valid.html b/lib/optimizer/tests/spec/end-to-end/hello-world/expected_output.valid.html
index 82bc3f473f1..ce3cb19953b 100644
--- a/lib/optimizer/tests/spec/end-to-end/hello-world/expected_output.valid.html
+++ b/lib/optimizer/tests/spec/end-to-end/hello-world/expected_output.valid.html
@@ -1,7 +1,7 @@
 <!doctype html>
 <html ⚡ i-amphtml-layout i-amphtml-no-boilerplate transformed="self;v=1">
 <head>
-  <meta charset="utf-8"><style amp-runtime i-amphtml-version="123456789000000">/* v0.css */</style>
+  <meta charset="utf-8"><style amp-runtime i-amphtml-version="123456789000000">/* ampproject.org/rtv v0.css */</style>
   <meta name="viewport" content="width=device-width,minimum-scale=1">
   <link rel="preload" href="https://cdn.ampproject.org/v0.js" as="script">
   <script async src="https://cdn.ampproject.org/v0.js"></script>
diff --git a/lib/optimizer/tests/spec/end-to-end/hello-world/input.html b/lib/optimizer/tests/spec/end-to-end/hello-world/input.html
index 5d3c471c95d..0216462639f 100644
--- a/lib/optimizer/tests/spec/end-to-end/hello-world/input.html
+++ b/lib/optimizer/tests/spec/end-to-end/hello-world/input.html
@@ -1,8 +1,9 @@
 <!--
 {
-  "ampUrlPrefix": "/amp",
+  "ampUrlPrefix": "https://example.com/amp",
   "ampRuntimeVersion": "123456789000000",
-  "ampUrl": "https://example.com/amp-version.html",
+  "ampUrl": "/amp-version.html",
+  "rtv": true,
   "geoApiUrl": "/geo"
 }
 -->
diff --git a/lib/optimizer/tests/spec/end-to-end/markdown/expected_output.valid.html b/lib/optimizer/tests/spec/end-to-end/markdown/expected_output.valid.html
index d0d4ff7b869..3d2832b54f2 100644
--- a/lib/optimizer/tests/spec/end-to-end/markdown/expected_output.valid.html
+++ b/lib/optimizer/tests/spec/end-to-end/markdown/expected_output.valid.html
@@ -1,7 +1,7 @@
 <!doctype html>
 <html amp i-amphtml-layout i-amphtml-no-boilerplate transformed="self;v=1">
 <head>
-  <meta data-auto charset="utf-8"><style amp-runtime i-amphtml-version="123456789000000">/* v0.css */</style>
+  <meta data-auto charset="utf-8"><style amp-runtime i-amphtml-version="123456789000000">/* ampproject.org/rtv v0.css */</style>
   <meta data-auto name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1">
   <link rel="preload" href="https://cdn.ampproject.org/v0.js" as="script">
   <script data-auto async src="https://cdn.ampproject.org/v0.js"></script>
diff --git a/lib/optimizer/tests/spec/transformers/experimental/AmpBoilerplateTransformer/uses_given_runtime_version/expected_output.html b/lib/optimizer/tests/spec/transformers/experimental/AmpBoilerplateTransformer/uses_given_runtime_version/expected_output.html
deleted file mode 100644
index a0aef2ac27e..00000000000
--- a/lib/optimizer/tests/spec/transformers/experimental/AmpBoilerplateTransformer/uses_given_runtime_version/expected_output.html
+++ /dev/null
@@ -1,12 +0,0 @@
-<!doctype html>
-<html ⚡>
-<head>
-  <meta charset="utf-8">
-  <style amp-runtime i-amphtml-version="001515617716922">/* v0-rtv.css */</style>
-  <script async src="https://cdn.ampproject.org/v0.js"></script>
-  <link href="https://example.com/favicon.ico" rel="icon">
-  <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>
-</head>
-<body>
-</body>
-</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/experimental/AmpBoilerplateTransformer/uses_given_runtime_version/input.html b/lib/optimizer/tests/spec/transformers/experimental/AmpBoilerplateTransformer/uses_given_runtime_version/input.html
deleted file mode 100644
index 27b0f61a906..00000000000
--- a/lib/optimizer/tests/spec/transformers/experimental/AmpBoilerplateTransformer/uses_given_runtime_version/input.html
+++ /dev/null
@@ -1,17 +0,0 @@
-<!--
-{
-  "ampRuntimeVersion": "001515617716922"
-}
--->
-<!doctype html>
-<html ⚡>
-  <head>
-    <meta charset="utf-8">
-    <style amp-runtime></style>
-    <script async src=https://cdn.ampproject.org/v0.js></script>
-    <link href=https://example.com/favicon.ico rel=icon>
-    <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>
-  </head>
-  <body>
-  </body>
-</html>
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_esm/expected_output.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_esm/expected_output.html
new file mode 100644
index 00000000000..65663c3ff66
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_esm/expected_output.html
@@ -0,0 +1,15 @@
+<!doctype html>
+<html ⚡>
+<head>
+  <script async nomodule src="https://cdn.ampproject.org/v0/amp-experiment-0.1.js"></script>
+  <script async custom-element="amp-experiment" src="https://cdn.ampproject.org/v0/amp-experiment-0.1.mjs" type="module" crossorigin="anonymous"></script>
+  <link as="script" crossorigin="anonymous" href="https://cdn.ampproject.org/v0.mjs" rel="preload">
+  <script async nomodule src="https://cdn.ampproject.org/v0.js"></script>
+  <script async src="https://cdn.ampproject.org/v0.mjs" type="module" crossorigin="anonymous"></script>
+  <link rel="stylesheet" href="https://cdn.ampproject.org/v0.css">
+  <link href="https://fonts.googleapis.com/css?foobar" rel="stylesheet" type="text/css">
+  <link href="https://example.com/favicon.ico" rel="icon">
+  <link rel="preload" href="https://cdn.ampproject.org/v0.css" as="style">
+</head>
+<body></body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_esm/input.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_esm/input.html
new file mode 100644
index 00000000000..f3ab7ec0dc3
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_esm/input.html
@@ -0,0 +1,11 @@
+<!doctype html>
+<html ⚡>
+<head>
+  <script async custom-element=amp-experiment src=https://cdn.ampproject.org/v0/amp-experiment-0.1.js></script>
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+  <link rel=stylesheet href=https://cdn.ampproject.org/v0.css>
+  <link href=https://fonts.googleapis.com/css?foobar rel=stylesheet type=text/css>
+  <link href=https://example.com/favicon.ico rel=icon>
+</head>
+<body></body>
+</html>
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_geoapi/expected_output.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_geoapi/expected_output.html
new file mode 100644
index 00000000000..1a0397c9302
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_geoapi/expected_output.html
@@ -0,0 +1,16 @@
+<!doctype html>
+<html ⚡>
+<head>
+  <meta name="amp-geo-api" content="/geo">
+  <script async nomodule src="https://cdn.ampproject.org/v0/amp-experiment-0.1.js"></script>
+  <script async custom-element="amp-experiment" src="https://cdn.ampproject.org/v0/amp-experiment-0.1.mjs" type="module" crossorigin="anonymous"></script>
+  <link as="script" crossorigin="anonymous" href="https://cdn.ampproject.org/v0.mjs" rel="preload">
+  <script async nomodule src="https://cdn.ampproject.org/v0.js"></script>
+  <script async src="https://cdn.ampproject.org/v0.mjs" type="module" crossorigin="anonymous"></script>
+  <link rel="stylesheet" href="https://cdn.ampproject.org/v0.css">
+  <link href="https://fonts.googleapis.com/css?foobar" rel="stylesheet" type="text/css">
+  <link href="https://example.com/favicon.ico" rel="icon">
+  <link rel="preload" href="https://cdn.ampproject.org/v0.css" as="style">
+</head>
+<body></body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_geoapi/input.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_geoapi/input.html
new file mode 100644
index 00000000000..7cece1eb1a9
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_geoapi/input.html
@@ -0,0 +1,16 @@
+<!--
+{
+  "geoApiUrl": "/geo"
+}
+-->
+<!doctype html>
+<html ⚡>
+<head>
+  <script async custom-element=amp-experiment src=https://cdn.ampproject.org/v0/amp-experiment-0.1.js></script>
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+  <link rel=stylesheet href=https://cdn.ampproject.org/v0.css>
+  <link href=https://fonts.googleapis.com/css?foobar rel=stylesheet type=text/css>
+  <link href=https://example.com/favicon.ico rel=icon>
+</head>
+<body></body>
+</html>
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_lts/expected_output.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_lts/expected_output.html
new file mode 100644
index 00000000000..d24376bc103
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_lts/expected_output.html
@@ -0,0 +1,15 @@
+<!doctype html>
+<html ⚡>
+<head>
+  <script async nomodule src="https://cdn.ampproject.org/lts/v0/amp-experiment-0.1.js"></script>
+  <script async custom-element="amp-experiment" src="https://cdn.ampproject.org/lts/v0/amp-experiment-0.1.mjs" type="module" crossorigin="anonymous"></script>
+  <link as="script" crossorigin="anonymous" href="https://cdn.ampproject.org/lts/v0.mjs" rel="preload">
+  <script async nomodule src="https://cdn.ampproject.org/lts/v0.js"></script>
+  <script async src="https://cdn.ampproject.org/lts/v0.mjs" type="module" crossorigin="anonymous"></script>
+  <link rel="stylesheet" href="https://cdn.ampproject.org/lts/v0.css">
+  <link href="https://fonts.googleapis.com/css?foobar" rel="stylesheet" type="text/css">
+  <link href="https://example.com/favicon.ico" rel="icon">
+  <link rel="preload" href="https://cdn.ampproject.org/lts/v0.css" as="style">
+</head>
+<body></body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_lts/input.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_lts/input.html
new file mode 100644
index 00000000000..c99fddab9b5
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_lts/input.html
@@ -0,0 +1,16 @@
+<!--
+{
+  "lts": true
+}
+-->
+<!doctype html>
+<html ⚡>
+<head>
+  <script async custom-element=amp-experiment src=https://cdn.ampproject.org/v0/amp-experiment-0.1.js></script>
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+  <link rel=stylesheet href=https://cdn.ampproject.org/v0.css>
+  <link href=https://fonts.googleapis.com/css?foobar rel=stylesheet type=text/css>
+  <link href=https://example.com/favicon.ico rel=icon>
+</head>
+<body></body>
+</html>
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_preloads/expected_output.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_preloads/expected_output.html
index 4a160fd163d..65663c3ff66 100644
--- a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_preloads/expected_output.html
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_preloads/expected_output.html
@@ -1,12 +1,14 @@
 <!doctype html>
 <html ⚡>
 <head>
-  <script async custom-element="amp-experiment" src="https://cdn.ampproject.org/v0/amp-experiment-0.1.js"></script>
-  <script async src="https://cdn.ampproject.org/v0.js"></script>
+  <script async nomodule src="https://cdn.ampproject.org/v0/amp-experiment-0.1.js"></script>
+  <script async custom-element="amp-experiment" src="https://cdn.ampproject.org/v0/amp-experiment-0.1.mjs" type="module" crossorigin="anonymous"></script>
+  <link as="script" crossorigin="anonymous" href="https://cdn.ampproject.org/v0.mjs" rel="preload">
+  <script async nomodule src="https://cdn.ampproject.org/v0.js"></script>
+  <script async src="https://cdn.ampproject.org/v0.mjs" type="module" crossorigin="anonymous"></script>
   <link rel="stylesheet" href="https://cdn.ampproject.org/v0.css">
   <link href="https://fonts.googleapis.com/css?foobar" rel="stylesheet" type="text/css">
   <link href="https://example.com/favicon.ico" rel="icon">
-  <link rel="preload" href="https://cdn.ampproject.org/v0.js" as="script">
   <link rel="preload" href="https://cdn.ampproject.org/v0.css" as="style">
 </head>
 <body></body>
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_runtime_version/expected_output.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_runtime_version/expected_output.html
index 7863bd10304..91daa629cbf 100644
--- a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_runtime_version/expected_output.html
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_runtime_version/expected_output.html
@@ -1,12 +1,14 @@
 <!doctype html>
 <html ⚡>
 <head>
-  <script async custom-element="amp-experiment" src="https://cdn.ampproject.org/rtv/001515617716922/v0/amp-experiment-0.1.js"></script>
-  <script async src="https://cdn.ampproject.org/rtv/001515617716922/v0.js"></script>
+  <script async nomodule src="https://cdn.ampproject.org/rtv/001515617716922/v0/amp-experiment-0.1.js"></script>
+  <script async custom-element="amp-experiment" src="https://cdn.ampproject.org/rtv/001515617716922/v0/amp-experiment-0.1.mjs" type="module" crossorigin="anonymous"></script>
+  <link as="script" crossorigin="anonymous" href="https://cdn.ampproject.org/rtv/001515617716922/v0.mjs" rel="preload">
+  <script async nomodule src="https://cdn.ampproject.org/rtv/001515617716922/v0.js"></script>
+  <script async src="https://cdn.ampproject.org/rtv/001515617716922/v0.mjs" type="module" crossorigin="anonymous"></script>
   <link rel="stylesheet" href="https://cdn.ampproject.org/rtv/001515617716922/v0.css">
   <link href="https://fonts.googleapis.com/css?foobar" rel="stylesheet" type="text/css">
   <link href="https://example.com/favicon.ico" rel="icon">
-  <link rel="preload" href="https://cdn.ampproject.org/rtv/001515617716922/v0.js" as="script">
   <link rel="preload" href="https://cdn.ampproject.org/rtv/001515617716922/v0.css" as="style">
 </head>
 <body></body>
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_runtime_version/input.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_runtime_version/input.html
index c7a43086ae3..287a0bf187c 100644
--- a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_runtime_version/input.html
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/adds_runtime_version/input.html
@@ -1,5 +1,6 @@
 <!--
 {
+  "rtv": true,
   "ampRuntimeVersion": "001515617716922"
 }
 -->
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_host_and_adds_version/expected_output.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_host_and_adds_version/expected_output.html
index 9c6be4a0b83..11b7b9586fd 100644
--- a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_host_and_adds_version/expected_output.html
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_host_and_adds_version/expected_output.html
@@ -2,12 +2,14 @@
 <html ⚡>
 <head>
   <meta name="runtime-host" content="/amp">
-  <script async custom-element="amp-experiment" src="/amp/rtv/001515617716922/v0/amp-experiment-0.1.js"></script>
-  <script async src="/amp/rtv/001515617716922/v0.js"></script>
+  <script async nomodule src="/amp/rtv/001515617716922/v0/amp-experiment-0.1.js"></script>
+  <script async custom-element="amp-experiment" src="/amp/rtv/001515617716922/v0/amp-experiment-0.1.mjs" type="module" crossorigin="anonymous"></script>
+  <link as="script" crossorigin="anonymous" href="/amp/rtv/001515617716922/v0.mjs" rel="preload">
+  <script async nomodule src="/amp/rtv/001515617716922/v0.js"></script>
+  <script async src="/amp/rtv/001515617716922/v0.mjs" type="module" crossorigin="anonymous"></script>
   <link rel="stylesheet" href="/amp/rtv/001515617716922/v0.css">
   <link href="https://fonts.googleapis.com/css?foobar" rel="stylesheet" type="text/css">
   <link href="https://example.com/favicon.ico" rel="icon">
-  <link rel="preload" href="/amp/rtv/001515617716922/v0.js" as="script">
   <link rel="preload" href="/amp/rtv/001515617716922/v0.css" as="style">
 </head>
 <body></body>
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_host_and_adds_version/input.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_host_and_adds_version/input.html
index c1ac9a6b700..bbdc56c84ca 100644
--- a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_host_and_adds_version/input.html
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_host_and_adds_version/input.html
@@ -1,5 +1,6 @@
 <!--
 {
+  "rtv": true,
   "ampUrlPrefix": "/amp",
   "ampRuntimeVersion": "001515617716922"
 }
diff --git a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_hosts/expected_output.html b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_hosts/expected_output.html
index 0e977493a51..ed1afb8aecf 100644
--- a/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_hosts/expected_output.html
+++ b/lib/optimizer/tests/spec/transformers/experimental/RewriteAmpUrls/rewrites_hosts/expected_output.html
@@ -2,12 +2,14 @@
 <html ⚡>
 <head>
   <meta name="runtime-host" content="/amp">
-  <script async custom-element="amp-experiment" src="/amp/v0/amp-experiment-0.1.js"></script>
-  <script async src="/amp/v0.js"></script>
+  <script async nomodule src="/amp/v0/amp-experiment-0.1.js"></script>
+  <script async custom-element="amp-experiment" src="/amp/v0/amp-experiment-0.1.mjs" type="module" crossorigin="anonymous"></script>
+  <link as="script" crossorigin="anonymous" href="/amp/v0.mjs" rel="preload">
+  <script async nomodule src="/amp/v0.js"></script>
+  <script async src="/amp/v0.mjs" type="module" crossorigin="anonymous"></script>
   <link rel="stylesheet" href="/amp/v0.css">
   <link href="https://fonts.googleapis.com/css?foobar" rel="stylesheet" type="text/css">
   <link href="https://example.com/favicon.ico" rel="icon">
-  <link rel="preload" href="/amp/v0.js" as="script">
   <link rel="preload" href="/amp/v0.css" as="style">
 </head>
 <body></body>
diff --git a/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/always_inlines_v0css/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/always_inlines_v0css/expected_output.html
index 3c82bfe1ac3..6abea808543 100644
--- a/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/always_inlines_v0css/expected_output.html
+++ b/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/always_inlines_v0css/expected_output.html
@@ -2,7 +2,7 @@
 <html ⚡>
 <head>
   <meta charset="utf-8">
-  <style amp-runtime i-amphtml-version="012345678900000">/* v0-prod.css */</style>
+  <style amp-runtime i-amphtml-version="012345678901234">/* v0-prod.css */</style>
   <script async src="https://cdn.ampproject.org/v0.js"></script>
   <link href="https://example.com/favicon.ico" rel="icon">
   <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>
diff --git a/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/always_inlines_v0css/input.html b/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/always_inlines_v0css/input.html
index 7890f7d4fe0..c80df2b8a82 100644
--- a/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/always_inlines_v0css/input.html
+++ b/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/always_inlines_v0css/input.html
@@ -1,3 +1,9 @@
+<!--
+{
+  "ampRuntimeVersion": "012345678901234",
+  "ampRuntimeStyles": "/* v0-prod.css */"
+}
+-->
 <!doctype html>
 <html ⚡>
   <head>
diff --git a/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/does_not_add_v0.css_if_style_amp-runtime_not_present/input.html b/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/does_not_add_v0.css_if_style_amp-runtime_not_present/input.html
index b0f3319e908..8fb601e80f0 100644
--- a/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/does_not_add_v0.css_if_style_amp-runtime_not_present/input.html
+++ b/lib/optimizer/tests/spec/transformers/valid/AmpBoilerplateTransformer/does_not_add_v0.css_if_style_amp-runtime_not_present/input.html
@@ -1,3 +1,9 @@
+<!--
+{
+  "ampRuntimeVersion": "012345678901234",
+  "ampRuntimeStyles": "v0.css"
+}
+-->
 <!doctype html>
 <html ⚡>
   <head>
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/amp-video_with_missing_poster/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/amp-video_with_missing_poster/expected_output.html
new file mode 100644
index 00000000000..44369b0f433
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/amp-video_with_missing_poster/expected_output.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <amp-video width="400" height="400">
+    <source src="foo.mp4">
+  </amp-video>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/amp-video_with_missing_poster/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/amp-video_with_missing_poster/input.html
new file mode 100644
index 00000000000..8063e5e6c92
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/amp-video_with_missing_poster/input.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <amp-video width="400" height="400">
+    <source src="foo.mp4" />
+  </amp-video>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/disable_via_param/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/disable_via_param/expected_output.html
new file mode 100644
index 00000000000..9f87ae9b515
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/disable_via_param/expected_output.html
@@ -0,0 +1,9 @@
+<html>
+<head>
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+</head>
+<body>
+  <amp-img width="500" height="400" src="/foo.png">
+  </amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/disable_via_param/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/disable_via_param/input.html
new file mode 100644
index 00000000000..933972c2435
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/disable_via_param/input.html
@@ -0,0 +1,13 @@
+<!--
+{
+  "preloadHeroImage": false
+}
+-->
+<html>
+<head>
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+</head>
+<body>
+  <amp-img width="500" height="400" src="/foo.png">
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimensions_from_parent_container/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimensions_from_parent_container/expected_output.html
new file mode 100644
index 00000000000..11f05d60796
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimensions_from_parent_container/expected_output.html
@@ -0,0 +1,10 @@
+<html>
+<head>
+  <link rel="preload" href="https://example-com.cdn.ampproject.org/foo.png" as="image" data-hero>
+</head>
+<body>
+  <div width="500" height="500">
+    <amp-img layout="fill" src="https://example-com.cdn.ampproject.org/foo.png"></amp-img>
+  </div>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimensions_from_parent_container/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimensions_from_parent_container/input.html
new file mode 100644
index 00000000000..dd2bab99115
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimensions_from_parent_container/input.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <div width="500" height="500">
+    <amp-img layout="fill" src=https://example-com.cdn.ampproject.org/foo.png></amp-img>
+  </div>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimesions_from_parent_container-_too_small/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimesions_from_parent_container-_too_small/expected_output.html
new file mode 100644
index 00000000000..eebe67d850f
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimesions_from_parent_container-_too_small/expected_output.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <div width="50" height="50">
+    <amp-img layout="fill" src="https://example-com.cdn.ampproject.org/foo.png"></amp-img>
+  </div>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimesions_from_parent_container-_too_small/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimesions_from_parent_container-_too_small/input.html
new file mode 100644
index 00000000000..f0f83a30ac1
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/hero_image_dimesions_from_parent_container-_too_small/input.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <div width="50" height="50">
+    <amp-img layout="fill" src=https://example-com.cdn.ampproject.org/foo.png></amp-img>
+  </div>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset/expected_output.html
new file mode 100644
index 00000000000..f3a932d8c42
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset/expected_output.html
@@ -0,0 +1,9 @@
+<html>
+<head>
+  <link rel="preload" href="https://example-com.cdn.ampproject.org/foo.png" as="image" data-hero>
+</head>
+<body>
+  <amp-img width="500" height="400" src="https://example-com.cdn.ampproject.org/foo.png" srcset="test 100w test2 3dpr">
+  </amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset/input.html
new file mode 100644
index 00000000000..58b59838578
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset/input.html
@@ -0,0 +1,6 @@
+<html>
+<head></head>
+<body>
+  <amp-img width="500" height="400" src="https://example-com.cdn.ampproject.org/foo.png" srcset="test 100w test2 3dpr">
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset_duplicates/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset_duplicates/expected_output.html
new file mode 100644
index 00000000000..d18dad5bb02
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset_duplicates/expected_output.html
@@ -0,0 +1,9 @@
+<html>
+<head>
+  <link rel="preload" href="https://example-com.cdn.ampproject.org/foo.png" as="image" data-hero>
+</head>
+<body>
+  <amp-img width="500" height="400" src="https://example-com.cdn.ampproject.org/foo.png" srcset="foo 10w, bar 10w, baz 100w">
+  </amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset_duplicates/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset_duplicates/input.html
new file mode 100644
index 00000000000..67ed4fd1f1f
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/invalid_srcset_duplicates/input.html
@@ -0,0 +1,6 @@
+<html>
+<head></head>
+<body>
+  <amp-img width="500" height="400" src="https://example-com.cdn.ampproject.org/foo.png" srcset="foo 10w, bar 10w, baz 100w">
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimension_from_parent_because_layout_is_not_responsive_or_fill/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimension_from_parent_because_layout_is_not_responsive_or_fill/expected_output.html
new file mode 100644
index 00000000000..1f9035ecedf
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimension_from_parent_because_layout_is_not_responsive_or_fill/expected_output.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <div>
+    <amp-img src="https://example-com.cdn.ampproject.org/foo.png"></amp-img>
+  </div>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimension_from_parent_because_layout_is_not_responsive_or_fill/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimension_from_parent_because_layout_is_not_responsive_or_fill/input.html
new file mode 100644
index 00000000000..b38e210ab9c
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimension_from_parent_because_layout_is_not_responsive_or_fill/input.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <div>
+    <amp-img src=https://example-com.cdn.ampproject.org/foo.png></amp-img>
+  </div>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimensions_in_parent_containers/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimensions_in_parent_containers/expected_output.html
new file mode 100644
index 00000000000..511cbc39a4c
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimensions_in_parent_containers/expected_output.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <div>
+    <amp-img layout="fill" src="https://example-com.cdn.ampproject.org/foo.png"></amp-img>
+  </div>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimensions_in_parent_containers/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimensions_in_parent_containers/input.html
new file mode 100644
index 00000000000..7ef2d9afff4
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_dimensions_in_parent_containers/input.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <div>
+    <amp-img layout="fill" src=https://example-com.cdn.ampproject.org/foo.png></amp-img>
+  </div>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_display_layout/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_display_layout/expected_output.html
new file mode 100644
index 00000000000..a2b48e83416
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_display_layout/expected_output.html
@@ -0,0 +1,6 @@
+<html>
+<head></head>
+<body>
+  <amp-img height="500" src="https://example-com.cdn.ampproject.org/foo.png" width="500" layout="nodisplay"></amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_display_layout/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_display_layout/input.html
new file mode 100644
index 00000000000..00f99979107
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/no_display_layout/input.html
@@ -0,0 +1,6 @@
+<html>
+<head></head>
+<body>
+  <amp-img height=500 src=https://example-com.cdn.ampproject.org/foo.png width=500 layout=nodisplay></amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/preloads_hero_image/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/preloads_hero_image/expected_output.html
new file mode 100644
index 00000000000..12e3305cc69
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/preloads_hero_image/expected_output.html
@@ -0,0 +1,9 @@
+<html>
+<head>
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+  <link rel="preload" href="/foo.png" as="image" data-hero>
+</head>
+<body>
+  <amp-img width="500" height="400" src="/foo.png"></amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/preloads_hero_image/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/preloads_hero_image/input.html
new file mode 100644
index 00000000000..34d566811dd
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/preloads_hero_image/input.html
@@ -0,0 +1,8 @@
+<html>
+<head>
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+</head>
+<body>
+  <amp-img width="500" height="400" src="/foo.png"></amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/same_as_above_with_nodisplay_layout_removed/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/same_as_above_with_nodisplay_layout_removed/expected_output.html
new file mode 100644
index 00000000000..8cd5fc8fe5c
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/same_as_above_with_nodisplay_layout_removed/expected_output.html
@@ -0,0 +1,8 @@
+<html>
+<head>
+  <link rel="preload" href="https://example-com.cdn.ampproject.org/foo.png" as="image" data-hero>
+</head>
+<body>
+  <amp-img height="500" src="https://example-com.cdn.ampproject.org/foo.png" width="500"></amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/same_as_above_with_nodisplay_layout_removed/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/same_as_above_with_nodisplay_layout_removed/input.html
new file mode 100644
index 00000000000..5673d503db0
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/same_as_above_with_nodisplay_layout_removed/input.html
@@ -0,0 +1,6 @@
+<html>
+<head></head>
+<body>
+  <amp-img height=500 src=https://example-com.cdn.ampproject.org/foo.png width=500></amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/srcset_validity_empty_srcset/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/srcset_validity_empty_srcset/expected_output.html
new file mode 100644
index 00000000000..40186ece231
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/srcset_validity_empty_srcset/expected_output.html
@@ -0,0 +1,9 @@
+<html>
+<head>
+  <link rel="preload" href="https://example-com.cdn.ampproject.org/foo.png" as="image" data-hero>
+</head>
+<body>
+  <amp-img width="500" height="400" src="https://example-com.cdn.ampproject.org/foo.png" srcset>
+  </amp-img>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/srcset_validity_empty_srcset/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/srcset_validity_empty_srcset/input.html
new file mode 100644
index 00000000000..fa8751631bb
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/srcset_validity_empty_srcset/input.html
@@ -0,0 +1,6 @@
+<html>
+<head></head>
+<body>
+  <amp-img width="500" height="400" src="https://example-com.cdn.ampproject.org/foo.png" srcset="">
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/video_posters/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/video_posters/expected_output.html
new file mode 100644
index 00000000000..8703c83b57f
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/video_posters/expected_output.html
@@ -0,0 +1,10 @@
+<html>
+<head>
+  <link rel="preload" href="https://example-com.cdn.ampproject.org/foo.png" as="image" data-hero>
+</head>
+<body>
+  <amp-video poster="https://example-com.cdn.ampproject.org/foo.png" width="400" height="400">
+    <source src="foo.mp4">
+  </amp-video>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/video_posters/input.html b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/video_posters/input.html
new file mode 100644
index 00000000000..d622ed596f6
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/PreloadHeroImage/video_posters/input.html
@@ -0,0 +1,8 @@
+<html>
+<head></head>
+<body>
+  <amp-video poster="https://example-com.cdn.ampproject.org/foo.png" width="400" height="400">
+    <source src="foo.mp4" />
+  </amp-video>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/SeparateKeyframes/ignores_amp_stories/expected_output.html b/lib/optimizer/tests/spec/transformers/valid/SeparateKeyframes/ignores_amp_stories/expected_output.html
new file mode 100644
index 00000000000..3751fd95705
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/SeparateKeyframes/ignores_amp_stories/expected_output.html
@@ -0,0 +1,40 @@
+<!doctype html>
+<html amp lang="en">
+<head>
+  <meta charset="utf-8">
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+  <title>My Story</title>
+  <meta name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1">
+  <link rel="canonical" href="helloworld.html">
+  <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>
+  <style amp-custom>
+    .another{display:block}@keyframes anim1{from{opacity:0}to{opacity:1}}.foo{height:0}@keyframes anim2{from{opacity:1;}to{opacity:0;}}
+  </style>
+</head>
+
+<body>
+  <amp-story standalone>
+
+    <amp-story-page id="cover">
+      <amp-story-grid-layer template="vertical">
+        <h1>Hello World</h1>
+        <p>This is the cover page of this story.</p>
+      </amp-story-grid-layer>
+    </amp-story-page>
+
+    <amp-story-page id="page-1">
+      <amp-story-grid-layer template="vertical">
+        <h1>First Page</h1>
+        <p>This is the first page of this story.</p>
+      </amp-story-grid-layer>
+    </amp-story-page>
+
+    <amp-story-page id="page-2">
+      <amp-story-grid-layer template="vertical">
+        <h1>Second Page</h1>
+        <p>This is the second page of this story.</p>
+      </amp-story-grid-layer>
+    </amp-story-page>
+  </amp-story>
+</body>
+</html>
\ No newline at end of file
diff --git a/lib/optimizer/tests/spec/transformers/valid/SeparateKeyframes/ignores_amp_stories/input.html b/lib/optimizer/tests/spec/transformers/valid/SeparateKeyframes/ignores_amp_stories/input.html
new file mode 100644
index 00000000000..74a7805e5ad
--- /dev/null
+++ b/lib/optimizer/tests/spec/transformers/valid/SeparateKeyframes/ignores_amp_stories/input.html
@@ -0,0 +1,41 @@
+<!doctype html>
+<html amp lang="en">
+  <head>
+    <meta charset="utf-8">
+    <script async src="https://cdn.ampproject.org/v0.js"></script>
+    <title>My Story</title>
+    <meta name="viewport"
+          content="width=device-width,minimum-scale=1,initial-scale=1">
+    <link rel="canonical" href="helloworld.html">
+    <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>
+  <style amp-custom>
+    .another{display:block}@keyframes anim1{from{opacity:0}to{opacity:1}}.foo{height:0}@keyframes anim2{from{opacity:1;}to{opacity:0;}}
+  </style>
+  </head>
+
+  <body>
+    <amp-story standalone>
+
+      <amp-story-page id="cover">
+        <amp-story-grid-layer template="vertical">
+          <h1>Hello World</h1>
+          <p>This is the cover page of this story.</p>
+        </amp-story-grid-layer>
+      </amp-story-page>
+
+      <amp-story-page id="page-1">
+        <amp-story-grid-layer template="vertical">
+          <h1>First Page</h1>
+          <p>This is the first page of this story.</p>
+        </amp-story-grid-layer>
+      </amp-story-page>
+
+      <amp-story-page id="page-2">
+        <amp-story-grid-layer template="vertical">
+          <h1>Second Page</h1>
+          <p>This is the second page of this story.</p>
+        </amp-story-grid-layer>
+      </amp-story-page>
+    </amp-story>
+  </body>
+</html>
diff --git a/lib/optimizer/tests/src/TestMarkup.php b/lib/optimizer/tests/src/TestMarkup.php
index a111d99dd3a..425c66faac3 100644
--- a/lib/optimizer/tests/src/TestMarkup.php
+++ b/lib/optimizer/tests/src/TestMarkup.php
@@ -24,6 +24,8 @@ final class TestMarkup
     const STUBBED_REMOTE_REQUESTS = [
         'https://cdn.ampproject.org/rtv/metadata'               => '{"ampRuntimeVersion":"012345678900000","ampCssUrl":"https://cdn.ampproject.org/rtv/012345678900000/v0.css","canaryPercentage":"0.1","diversions":["023456789000000","034567890100000","045678901200000"]}',
         'https://cdn.ampproject.org/v0.css'                     => '/* v0.css */',
+        'https://cdn.ampproject.org/rtv/012345678900000/v0.css' => '/* 012345678900000/v0.css */',
+        'https://cdn.ampproject.org/rtv/012345678901234/v0.css' => '/* 012345678901234/v0.css */',
     ];
 
     // Doctype is the doctype expected for AMP documents.
diff --git a/lib/optimizer/tests/src/TestTransformer/ConfigurationOnly.php b/lib/optimizer/tests/src/TestTransformer/ConfigurationOnly.php
new file mode 100644
index 00000000000..fa051b50da6
--- /dev/null
+++ b/lib/optimizer/tests/src/TestTransformer/ConfigurationOnly.php
@@ -0,0 +1,20 @@
+<?php
+
+namespace AmpProject\Optimizer\Tests\TestTransformer;
+
+use AmpProject\Dom\Document;
+use AmpProject\Optimizer\ErrorCollection;
+use AmpProject\Optimizer\Transformer;
+use AmpProject\Optimizer\TransformerConfiguration;
+
+final class ConfigurationOnly implements Transformer
+{
+
+    public function __construct(TransformerConfiguration $configuration)
+    {
+    }
+
+    public function transform(Document $document, ErrorCollection $errors)
+    {
+    }
+}
diff --git a/lib/optimizer/tests/src/TestTransformer/ConfigurationThenRemoteRequest.php b/lib/optimizer/tests/src/TestTransformer/ConfigurationThenRemoteRequest.php
new file mode 100644
index 00000000000..70666aefc5d
--- /dev/null
+++ b/lib/optimizer/tests/src/TestTransformer/ConfigurationThenRemoteRequest.php
@@ -0,0 +1,21 @@
+<?php
+
+namespace AmpProject\Optimizer\Tests\TestTransformer;
+
+use AmpProject\Dom\Document;
+use AmpProject\Optimizer\ErrorCollection;
+use AmpProject\Optimizer\Transformer;
+use AmpProject\Optimizer\TransformerConfiguration;
+use AmpProject\RemoteGetRequest;
+
+final class ConfigurationThenRemoteRequest implements Transformer
+{
+
+    public function __construct(TransformerConfiguration $configuration, RemoteGetRequest $remoteRequest)
+    {
+    }
+
+    public function transform(Document $document, ErrorCollection $errors)
+    {
+    }
+}
diff --git a/lib/optimizer/tests/src/TestTransformer/DummyTransformerConfiguration.php b/lib/optimizer/tests/src/TestTransformer/DummyTransformerConfiguration.php
new file mode 100644
index 00000000000..6c98c5dd502
--- /dev/null
+++ b/lib/optimizer/tests/src/TestTransformer/DummyTransformerConfiguration.php
@@ -0,0 +1,13 @@
+<?php
+
+namespace AmpProject\Optimizer\Tests\TestTransformer;
+
+use AmpProject\Optimizer\TransformerConfiguration;
+
+final class DummyTransformerConfiguration implements TransformerConfiguration
+{
+
+    public function get($key)
+    {
+    }
+}
diff --git a/lib/optimizer/tests/src/TestTransformer/NoConstructor.php b/lib/optimizer/tests/src/TestTransformer/NoConstructor.php
new file mode 100644
index 00000000000..4c09c9871f5
--- /dev/null
+++ b/lib/optimizer/tests/src/TestTransformer/NoConstructor.php
@@ -0,0 +1,15 @@
+<?php
+
+namespace AmpProject\Optimizer\Tests\TestTransformer;
+
+use AmpProject\Dom\Document;
+use AmpProject\Optimizer\ErrorCollection;
+use AmpProject\Optimizer\Transformer;
+
+final class NoConstructor implements Transformer
+{
+
+    public function transform(Document $document, ErrorCollection $errors)
+    {
+    }
+}
diff --git a/lib/optimizer/tests/src/TestTransformer/NoDependencies.php b/lib/optimizer/tests/src/TestTransformer/NoDependencies.php
new file mode 100644
index 00000000000..01f4834e300
--- /dev/null
+++ b/lib/optimizer/tests/src/TestTransformer/NoDependencies.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace AmpProject\Optimizer\Tests\TestTransformer;
+
+use AmpProject\Dom\Document;
+use AmpProject\Optimizer\ErrorCollection;
+use AmpProject\Optimizer\Transformer;
+
+final class NoDependencies implements Transformer
+{
+
+    public function __construct()
+    {
+    }
+
+    public function transform(Document $document, ErrorCollection $errors)
+    {
+    }
+}
diff --git a/lib/optimizer/tests/src/TestTransformer/RemoteRequestOnly.php b/lib/optimizer/tests/src/TestTransformer/RemoteRequestOnly.php
new file mode 100644
index 00000000000..3340b848bd7
--- /dev/null
+++ b/lib/optimizer/tests/src/TestTransformer/RemoteRequestOnly.php
@@ -0,0 +1,20 @@
+<?php
+
+namespace AmpProject\Optimizer\Tests\TestTransformer;
+
+use AmpProject\Dom\Document;
+use AmpProject\Optimizer\ErrorCollection;
+use AmpProject\Optimizer\Transformer;
+use AmpProject\RemoteGetRequest;
+
+final class RemoteRequestOnly implements Transformer
+{
+
+    public function __construct(RemoteGetRequest $remoteRequest)
+    {
+    }
+
+    public function transform(Document $document, ErrorCollection $errors)
+    {
+    }
+}
diff --git a/lib/optimizer/tests/src/TestTransformer/RemoteRequestThenConfiguration.php b/lib/optimizer/tests/src/TestTransformer/RemoteRequestThenConfiguration.php
new file mode 100644
index 00000000000..fd11d2c1363
--- /dev/null
+++ b/lib/optimizer/tests/src/TestTransformer/RemoteRequestThenConfiguration.php
@@ -0,0 +1,21 @@
+<?php
+
+namespace AmpProject\Optimizer\Tests\TestTransformer;
+
+use AmpProject\Dom\Document;
+use AmpProject\Optimizer\ErrorCollection;
+use AmpProject\Optimizer\Transformer;
+use AmpProject\Optimizer\TransformerConfiguration;
+use AmpProject\RemoteGetRequest;
+
+final class RemoteRequestThenConfiguration implements Transformer
+{
+
+    public function __construct(RemoteGetRequest $remoteRequest, TransformerConfiguration $configuration)
+    {
+    }
+
+    public function transform(Document $document, ErrorCollection $errors)
+    {
+    }
+}
diff --git a/src/Transformer/AmpSchemaOrgMetadata.php b/src/Transformer/AmpSchemaOrgMetadata.php
index 14de4c202ba..15a9557a544 100644
--- a/src/Transformer/AmpSchemaOrgMetadata.php
+++ b/src/Transformer/AmpSchemaOrgMetadata.php
@@ -9,7 +9,6 @@
 
 use AmpProject\Attribute;
 use AmpProject\Dom\Document;
-use AmpProject\Optimizer\Configurable;
 use AmpProject\Optimizer\ErrorCollection;
 use AmpProject\Optimizer\Transformer;
 use AmpProject\Optimizer\TransformerConfiguration;
@@ -20,7 +19,7 @@
  *
  * @package AmpProject\AmpWP
  */
-final class AmpSchemaOrgMetadata implements Transformer, Configurable {
+final class AmpSchemaOrgMetadata implements Transformer {
 
 	/**
 	 * XPath query to use for fetching the schema.org meta script.
diff --git a/tests/php/test-amp-gallery-embed.php b/tests/php/test-amp-gallery-embed.php
index dabf61f6d49..23c350f6205 100644
--- a/tests/php/test-amp-gallery-embed.php
+++ b/tests/php/test-amp-gallery-embed.php
@@ -41,7 +41,7 @@ public function tearDown() {
 	 */
 	public function get_conversion_data() {
 		$amp_carousel_caption = '<span class="amp-wp-gallery-caption"><span>' . self::CAPTION_TEXT . '</span></span>';
-		$loading_attribute    = version_compare( get_bloginfo( 'version' ), '5.5', '>' ) ? 'loading="lazy"' : '';
+		$loading_attribute    = version_compare( get_bloginfo( 'version' ), '5.5-alpha', '>' ) ? 'loading="lazy"' : '';
 
 		return [
 			'shortcode_with_invalid_id'               => [
diff --git a/tests/php/test-class-amp-hulu-embed-handler.php b/tests/php/test-class-amp-hulu-embed-handler.php
index b16a0fb5e7c..743f3874843 100644
--- a/tests/php/test-class-amp-hulu-embed-handler.php
+++ b/tests/php/test-class-amp-hulu-embed-handler.php
@@ -25,7 +25,7 @@ public function setUp() {
 		add_filter(
 			'pre_http_request',
 			static function( $pre, $r, $url ) {
-				if ( in_array( 'external-http', $_SERVER['argv'], true ) ) {
+				if ( self::is_external_http_test_suite() ) {
 					return $pre;
 				}
 
@@ -55,6 +55,15 @@ static function( $pre, $r, $url ) {
 		}
 	}
 
+	/**
+	 * Whether external-http test suite is running.
+	 *
+	 * @return bool Running external-http test suite.
+	 */
+	private static function is_external_http_test_suite() {
+		return in_array( 'external-http', $_SERVER['argv'], true );
+	}
+
 	/**
 	 * Get conversion data.
 	 *
@@ -62,18 +71,13 @@ static function( $pre, $r, $url ) {
 	 */
 	public function get_conversion_data() {
 		return [
-			'no_embed'        => [
-				'<p>Hello world.</p>',
-				'<p>Hello world.</p>' . PHP_EOL,
-			],
-
 			'url_simple'      => [
-				'https://www.hulu.com/watch/771496' . PHP_EOL,
+				'https://www.hulu.com/watch/771496',
 				'<p><amp-hulu width="500" height="289" data-eid="771496"></amp-hulu></p>' . PHP_EOL,
 			],
 
 			'url_with_params' => [
-				'https://www.hulu.com/watch/771496?foo=bar' . PHP_EOL,
+				'https://www.hulu.com/watch/771496?foo=bar',
 				'<p><amp-hulu width="500" height="289" data-eid="771496"></amp-hulu></p>' . PHP_EOL,
 			],
 
@@ -83,14 +87,18 @@ public function get_conversion_data() {
 	/**
 	 * Test conversion.
 	 *
-	 * @param string $source Source.
+	 * @param string $url      URL.
 	 * @param string $expected Expected.
 	 * @dataProvider get_conversion_data
 	 */
-	public function test__conversion( $source, $expected ) {
+	public function test__conversion( $url, $expected ) {
 		$embed = new AMP_Hulu_Embed_Handler();
 		$embed->register_embed();
-		$filtered_content = apply_filters( 'the_content', $source );
+		$filtered_content = apply_filters( 'the_content', $url );
+
+		if ( self::is_external_http_test_suite() && "<p>$url</p>" === trim( $filtered_content ) ) {
+			$this->markTestSkipped( 'Endpoint is down.' );
+		}
 
 		$this->assertEquals( $expected, $filtered_content );
 	}
@@ -107,7 +115,7 @@ public function get_scripts_data() {
 				[],
 			],
 			'converted'     => [
-				'https://www.hulu.com/watch/771496' . PHP_EOL,
+				'https://www.hulu.com/watch/771496',
 				[ 'amp-hulu' => true ],
 			],
 		];
@@ -116,16 +124,20 @@ public function get_scripts_data() {
 	/**
 	 * Test scripts.
 	 *
-	 * @param string $source Source.
+	 * @param string $url      URL.
 	 * @param string $expected Expected.
 	 * @dataProvider get_scripts_data
 	 */
-	public function test__get_scripts( $source, $expected ) {
+	public function test__get_scripts( $url, $expected ) {
 		$embed = new AMP_Hulu_Embed_Handler();
 		$embed->register_embed();
-		$source = apply_filters( 'the_content', $source );
+		$filtered_content = apply_filters( 'the_content', $url );
+
+		if ( self::is_external_http_test_suite() && "<p>$url</p>" === trim( $filtered_content ) ) {
+			$this->markTestSkipped( 'Endpoint is down.' );
+		}
 
-		$whitelist_sanitizer = new AMP_Tag_And_Attribute_Sanitizer( AMP_DOM_Utils::get_dom_from_content( $source ) );
+		$whitelist_sanitizer = new AMP_Tag_And_Attribute_Sanitizer( AMP_DOM_Utils::get_dom_from_content( $filtered_content ) );
 		$whitelist_sanitizer->sanitize();
 
 		$scripts = array_merge(
diff --git a/tests/php/test-class-amp-theme-support.php b/tests/php/test-class-amp-theme-support.php
index 226fd9b7d3c..98840d2c793 100644
--- a/tests/php/test-class-amp-theme-support.php
+++ b/tests/php/test-class-amp-theme-support.php
@@ -2002,11 +2002,10 @@ static function ( $url ) {
 
 			'<link rel="icon" href="https://example.org/favicon.png" sizes="32x32">',
 			'<link rel="icon" href="https://example.org/favicon.png" sizes="192x192">',
+			'<link crossorigin="anonymous" rel="stylesheet" id="my-font-css" href="https://fonts.googleapis.com/css?family=Tangerine" type="text/css" media="all">',
 
 			'#<style amp-custom(="")?>.*?body\s*{\s*background:\s*black;?\s*}.*?</style>#s',
 
-			'<link crossorigin="anonymous" rel="stylesheet" id="my-font-css" href="https://fonts.googleapis.com/css?family=Tangerine" type="text/css" media="all">',
-
 			'<script type="application/ld+json">{"@context"',
 
 			'#<style amp-boilerplate(="")?>#',
diff --git a/tests/php/test-class-amp-wordpress-tv-embed-handler.php b/tests/php/test-class-amp-wordpress-tv-embed-handler.php
index 520875c1358..2b1f8863cfb 100644
--- a/tests/php/test-class-amp-wordpress-tv-embed-handler.php
+++ b/tests/php/test-class-amp-wordpress-tv-embed-handler.php
@@ -39,20 +39,30 @@ public function mock_http_request( $preempt, $r, $url ) {
 			return $preempt;
 		}
 
-		if ( false === strpos( $url, 'wordpress.tv' ) ) {
-			return $preempt;
+		$body = null;
+
+		$host = wp_parse_url( $url, PHP_URL_HOST );
+		if ( 'wordpress.tv' === $host ) {
+			$body = '{"type":"video","version":"1.0","title":null,"width":500,"height":281,"html":"<iframe width=\'500\' height=\'281\' src=\'https:\\/\\/video.wordpress.com\\/embed\\/yFCmLMGL?hd=0\' frameborder=\'0\' allowfullscreen><\\/iframe><script src=\'https:\\/\\/v0.wordpress.com\\/js\\/next\\/videopress-iframe.js?m=1435166243\'></script>"}'; // phpcs:ignore
+		} elseif ( 'videopress.com' === $host ) {
+			$body = '{"version":"1.0","provider_name":"VideoPress","provider_url":"https:\/\/videopress.com\/","type":"video","title":"VideoPress Demo","width":560,"height":316,"thumbnail_url":"https:\/\/videos.files.wordpress.com\/kUJmAcSf\/bbb_sunflower_1080p_30fps_normal_scruberthumbnail_2.jpg","thumbnail_width":560,"thumbnail_height":316,"html":"<iframe width=\'560\' height=\'316\' src=\'https:\/\/video.wordpress.com\/embed\/kUJmAcSf?hd=0\' frameborder=\'0\' allowfullscreen><\/iframe><script src=\'https:\/\/v0.wordpress.com\/js\/next\/videopress-iframe.js?m=1435166243\'><\/script>"}'; // phpcs:ignore
+		}
+
+		if ( isset( $body ) ) {
+			return [
+				'body'          => $body,
+				'headers'       => [],
+				'response'      => [
+					'code'    => 200,
+					'message' => 'ok',
+				],
+				'cookies'       => [],
+				'http_response' => null,
+			];
 		}
+
 		unset( $r );
-		return [
-			'body'          => '{"type":"video","version":"1.0","title":null,"width":500,"height":281,"html":"<iframe width=\'500\' height=\'281\' src=\'https:\\/\\/video.wordpress.com\\/embed\\/yFCmLMGL?hd=0\' frameborder=\'0\' allowfullscreen><\\/iframe><script src=\'https:\\/\\/v0.wordpress.com\\/js\\/next\\/videopress-iframe.js?m=1435166243\'></script>"}', // phpcs:ignore
-			'headers'       => [],
-			'response'      => [
-				'code'    => 200,
-				'message' => 'ok',
-			],
-			'cookies'       => [],
-			'http_response' => null,
-		];
+		return $preempt;
 	}
 
 	/**
@@ -74,10 +84,24 @@ public function test_script_removal() {
 			<!-- /wp:core-embed/wordpress-tv -->
 		';
 
+		$videopress_block = '
+			<!-- wp:core-embed/videopress {"url":"https://videopress.com/v/kUJmAcSf","type":"video","providerNameSlug":"videopress","className":"wp-embed-aspect-16-9 wp-has-aspect-ratio"} -->
+				<figure class="wp-block-embed-videopress wp-block-embed is-type-video is-provider-videopress wp-embed-aspect-16-9 wp-has-aspect-ratio"><div class="wp-block-embed__wrapper">
+					https://videopress.com/v/kUJmAcSf
+				</div></figure>
+			<!-- /wp:core-embed/videopress -->
+		';
+
 		$handler->register_embed();
+
 		$rendered = apply_filters( 'the_content', $wordpress_tv_block );
 		$this->assertStringContains( '<iframe', $rendered );
-		$this->assertStringContains( 'video.wordpress.com/embed', $rendered );
+		$this->assertStringContains( 'https://video.wordpress.com/embed/yFCmLMGL', $rendered );
+		$this->assertStringNotContains( '<script', $rendered );
+
+		$rendered = apply_filters( 'the_content', $videopress_block );
+		$this->assertStringContains( '<iframe', $rendered );
+		$this->assertStringContains( 'https://video.wordpress.com/embed/kUJmAcSf', $rendered );
 		$this->assertStringNotContains( '<script', $rendered );
 	}
 
diff --git a/tests/php/test-class-amp-youtube-embed-handler.php b/tests/php/test-class-amp-youtube-embed-handler.php
index 8026baa187a..130577f90b7 100644
--- a/tests/php/test-class-amp-youtube-embed-handler.php
+++ b/tests/php/test-class-amp-youtube-embed-handler.php
@@ -135,7 +135,7 @@ public function test_video_override() {
 	}
 
 	public function get_conversion_data() {
-		$loading_attribute = version_compare( get_bloginfo( 'version' ), '5.5', '>' ) ? 'loading="lazy" ' : '';
+		$loading_attribute = version_compare( get_bloginfo( 'version' ), '5.5-alpha', '>' ) ? 'loading="lazy" ' : '';
 
 		return [
 			'no_embed'                         => [
diff --git a/tests/php/validation/test-class-amp-validation-manager.php b/tests/php/validation/test-class-amp-validation-manager.php
index 00b1159dca3..97f4e929831 100644
--- a/tests/php/validation/test-class-amp-validation-manager.php
+++ b/tests/php/validation/test-class-amp-validation-manager.php
@@ -1610,7 +1610,7 @@ public function test_decorate_shortcode_and_filter_source() {
 				[
 					'type'     => 'core',
 					'name'     => 'wp-includes',
-					'function' => version_compare( get_bloginfo( 'version' ), '5.5', '>' ) ? 'wp_filter_content_tags' : 'wp_make_content_images_responsive',
+					'function' => version_compare( get_bloginfo( 'version' ), '5.5-alpha', '>' ) ? 'wp_filter_content_tags' : 'wp_make_content_images_responsive',
 				],
 				[
 					'type'     => 'core',