diff --git a/includes/amp-helper-functions.php b/includes/amp-helper-functions.php
index b5dbd116ee4..d5f53c694f4 100644
--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -361,8 +361,23 @@ function amp_get_asset_url( $file ) {
  * @return string Boilerplate code.
  */
 function amp_get_boilerplate_code() {
-	return '<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>';
+	$stylesheets = amp_get_boilerplate_stylesheets();
+	return sprintf( '<style amp-boilerplate>%s</style><noscript><style amp-boilerplate>%s</style></noscript>', $stylesheets[0], $stylesheets[1] );
+}
+
+/**
+ * Get AMP boilerplate stylesheets.
+ *
+ * @since 1.3
+ * @link https://www.ampproject.org/docs/reference/spec#boilerplate
+ *
+ * @return string[] Stylesheets, where first is contained in style[amp-boilerplate] and the second in noscript>style[amp-boilerplate].
+ */
+function amp_get_boilerplate_stylesheets() {
+	return [
+		'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}}',
+		'body{-webkit-animation:none;-moz-animation:none;-ms-animation:none;animation:none}',
+	];
 }
 
 /**
diff --git a/includes/class-amp-story-post-type.php b/includes/class-amp-story-post-type.php
index 5f8ba10dc2b..79b499ea4ad 100644
--- a/includes/class-amp-story-post-type.php
+++ b/includes/class-amp-story-post-type.php
@@ -197,6 +197,30 @@ public static function register() {
 
 		add_action( 'wp_enqueue_scripts', [ __CLASS__, 'add_custom_stories_styles' ] );
 
+		add_action(
+			'amp_story_head',
+			function() {
+				// Theme support for title-tag is implied for stories. See _wp_render_title_tag().
+				echo '<title>' . esc_html( wp_get_document_title() ) . '</title>' . "\n";
+			},
+			1
+		);
+		add_action( 'amp_story_head', 'wp_enqueue_scripts', 1 );
+		add_action(
+			'amp_story_head',
+			function() {
+				/*
+				 * Same as wp_print_styles() but importantly omitting the wp_print_styles action, which themes/plugins
+				 * can use to output arbitrary styling. Styling is constrained in story template via the
+				 * \AMP_Story_Post_Type::filter_frontend_print_styles_array() method.
+				 */
+				wp_styles()->do_items();
+			},
+			8
+		);
+		add_action( 'amp_story_head', 'rel_canonical' );
+		add_action( 'amp_story_head', 'amp_add_generator_metadata' );
+
 		// Remove unnecessary settings.
 		add_filter( 'block_editor_settings', [ __CLASS__, 'filter_block_editor_settings' ], 10, 2 );
 
diff --git a/includes/class-amp-theme-support.php b/includes/class-amp-theme-support.php
index e02cf242fbf..799a6ec4354 100644
--- a/includes/class-amp-theme-support.php
+++ b/includes/class-amp-theme-support.php
@@ -1016,34 +1016,6 @@ static function( $html ) {
 			1
 		);
 
-		/*
-		 * "AMP HTML documents MUST contain the AMP boilerplate code (head > style[amp-boilerplate] and noscript > style[amp-boilerplate])
-		 * in their head tag." {@link https://www.ampproject.org/docs/fundamentals/spec#required-markup AMP Required markup}
-		 *
-		 * After "Specify the <link> tag for your favicon.", then
-		 * "Specify any custom styles by using the <style amp-custom> tag."
-		 *
-		 * Note that the boilerplate is added at the very end because:
-		 * "Finally, specify the AMP boilerplate code. By putting the boilerplate code last, it prevents custom styles from accidentally
-		 * overriding the boilerplate css rules." {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit AMP Hosting Guide}
-		 *
-		 * Other required markup is added in the ensure_required_markup method, including meta charset, meta viewport, and rel=canonical link.
-		 */
-		add_action(
-			'wp_head',
-			static function() {
-				echo '<style amp-custom></style>';
-			},
-			0
-		);
-		add_action(
-			'wp_head',
-			static function() {
-				echo amp_get_boilerplate_code(); // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
-			},
-			PHP_INT_MAX
-		);
-
 		add_action( 'admin_bar_init', [ __CLASS__, 'init_admin_bar' ] );
 		add_action( 'wp_head', 'amp_add_generator_metadata', 20 );
 		add_action( 'wp_enqueue_scripts', [ __CLASS__, 'enqueue_assets' ], 0 ); // Enqueue before theme's styles.
@@ -1434,8 +1406,9 @@ static function( $body_classes ) {
 	 *
 	 * @since 0.7
 	 * @link https://www.ampproject.org/docs/reference/spec#required-markup
-	 * @link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit#heading=h.2ha259c3ffos
+	 * @link https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/
 	 * @todo All of this might be better placed inside of a sanitizer.
+	 * @todo Consider removing any scripts that are not among the $script_handles.
 	 *
 	 * @param DOMDocument $dom            Document.
 	 * @param string[]    $script_handles AMP script handles for components identified during output buffering.
@@ -1447,6 +1420,8 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		 * @var DOMElement $meta
 		 * @var DOMElement $script
 		 * @var DOMElement $link
+		 * @var DOMElement $style
+		 * @var DOMElement $noscript
 		 */
 
 		$xpath = new DOMXPath( $dom );
@@ -1496,9 +1471,9 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		 * there are a few basic optimizations that you can apply. The key is to structure the <head> section
 		 * in a way so that all render-blocking scripts and custom fonts load as fast as possible."
 		 *
-		 * "The first tag should be the meta charset tag, followed by any remaining meta tags."
+		 * "1. The first tag should be the meta charset tag, followed by any remaining meta tags."
 		 *
-		 * {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit#heading=h.2ha259c3ffos Optimize the AMP Runtime loading}
+		 * {@link https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/ Optimize the AMP Runtime loading}
 		 */
 		$meta_charset  = null;
 		$meta_viewport = null;
@@ -1606,10 +1581,10 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 
 		/* phpcs:ignore Squiz.PHP.CommentedOutCode.Found
 		 *
-		 * "Next, preload the AMP runtime v0.js <script> tag with <link as=script href=https://cdn.ampproject.org/v0.js rel=preload>.
+		 * "2. Next, preload the AMP runtime v0.js <script> tag with <link as=script href=https://cdn.ampproject.org/v0.js rel=preload>.
 		 * The AMP runtime should start downloading as soon as possible because the AMP boilerplate hides the document via body { visibility:hidden }
 		 * until the AMP runtime has loaded. Preloading the AMP runtime tells the browser to download the script with a higher priority."
-		 * {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit#heading=h.2ha259c3ffos Optimize the AMP Runtime loading}
+		 * {@link https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/ Optimize the AMP Runtime loading}
 		 */
 		$prioritized_preloads = [];
 		if ( ! isset( $links['preload'] ) ) {
@@ -1626,6 +1601,10 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 			]
 		);
 
+		/*
+		 * "3. If your page includes render-delaying extensions (e.g., amp-experiment, amp-dynamic-css-classes, amp-story),
+		 * preload those extensions as they're required by the AMP runtime for rendering the page."
+		 */
 		$amp_script_handles = array_keys( $amp_scripts );
 		foreach ( array_intersect( $render_delaying_extensions, $amp_script_handles ) as $script_handle ) {
 			if ( ! in_array( $script_handle, $render_delaying_extensions, true ) ) {
@@ -1643,6 +1622,12 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		}
 		$links['preload'] = array_merge( $prioritized_preloads, $links['preload'] );
 
+		/*
+		 * "4. Use preconnect to speedup the connection to other origin where the full resource URL is not known ahead of time,
+		 * for example, when using Google Fonts."
+		 *
+		 * Note that \AMP_Style_Sanitizer::process_link_element() will ensure preconnect links for Google Fonts are present.
+		 */
 		$link_relations = [ 'preconnect', 'dns-prefetch', 'preload', 'prerender', 'prefetch' ];
 		foreach ( $link_relations as $rel ) {
 			if ( ! isset( $links[ $rel ] ) ) {
@@ -1657,15 +1642,22 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 			}
 		}
 
-		/*
-		 * "Specify the <script> tags for render-delaying extensions (e.g., amp-experiment, amp-dynamic-css-classes, and amp-story)."
-		 * "Specify the <script> tags for remaining extensions (e.g., amp-bind, ...). These extensions are not render-delaying and therefore
-		 * should not be preloaded because they might take away important bandwidth for the initial render."
-		 * {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit AMP Hosting Guide}
-		 */
+		// "5. Load the AMP runtime."
 		if ( isset( $amp_scripts['amp-runtime'] ) ) {
 			$ordered_scripts['amp-runtime'] = $amp_scripts['amp-runtime'];
+			unset( $amp_scripts['amp-runtime'] );
+		} else {
+			$script = $dom->createElement( 'script' );
+			$script->setAttribute( 'async', '' );
+			$script->setAttribute( 'src', $runtime_src );
+			$ordered_scripts['amp-runtime'] = $script;
 		}
+
+		/*
+		 * "6. Specify the <script> tags for render-delaying extensions (e.g., amp-experiment amp-dynamic-css-classes and amp-story"
+		 *
+		 * {@link https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/optimize_amp/ AMP Hosting Guide}
+		 */
 		foreach ( $render_delaying_extensions as $extension ) {
 			if ( isset( $amp_scripts[ $extension ] ) ) {
 				$ordered_scripts[ $extension ] = $amp_scripts[ $extension ];
@@ -1673,6 +1665,10 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 			}
 		}
 
+		/*
+		 * "7. Specify the <script> tags for remaining extensions (e.g., amp-bind ...). These extensions are not render-delaying
+		 * and therefore should not be preloaded as they might take away important bandwidth for the initial render."
+		 */
 		$ordered_scripts = array_merge( $ordered_scripts, $amp_scripts );
 		foreach ( $ordered_scripts as $ordered_script ) {
 			$head->insertBefore( $ordered_script, $previous_node->nextSibling );
@@ -1680,18 +1676,54 @@ public static function ensure_required_markup( DOMDocument $dom, $script_handles
 		}
 
 		/*
-		 * "Specify the <link> tag for your favicon."
-		 * {@link https://docs.google.com/document/d/169XUxtSSEJb16NfkrCr9y5lqhUR7vxXEAsNxBzg07fM/edit AMP Hosting Guide}
+		 * "8. Specify any custom styles by using the <style amp-custom> tag."
 		 */
-		if ( isset( $links['icon'] ) ) {
-			foreach ( $links['icon'] as $link ) {
-				$link->parentNode->removeChild( $link ); // So we can move it.
-				$head->insertBefore( $link, $previous_node->nextSibling );
-				$previous_node = $link;
+		$style = $xpath->query( './style[ @amp-custom ]', $head )->item( 0 );
+		if ( $style ) {
+			// Ensure the CSS manifest comment remains before style[amp-custom].
+			if ( $style->previousSibling instanceof DOMComment ) {
+				$comment = $style->previousSibling;
+				$comment->parentNode->removeChild( $comment );
+				$head->insertBefore( $comment, $previous_node->nextSibling );
+				$previous_node = $comment;
 			}
+
+			$style->parentNode->removeChild( $style );
+			$head->insertBefore( $style, $previous_node->nextSibling );
+			$previous_node = $style;
+		}
+
+		/*
+		 * "9. Add any other tags allowed in the <head> section. In particular, any external fonts should go last since
+		 * they block rendering."
+		 */
+
+		/*
+		 * "10. Finally, specify the AMP boilerplate code. By putting the boilerplate code last, it prevents custom styles
+		 * from accidentally overriding the boilerplate css rules."
+		 */
+		$style = $xpath->query( './style[ @amp-boilerplate ]', $head )->item( 0 );
+		if ( ! $style ) {
+			$style = $dom->createElement( 'style' );
+			$style->setAttribute( 'amp-boilerplate', '' );
+			$style->appendChild( $dom->createTextNode( amp_get_boilerplate_stylesheets()[0] ) );
+		} else {
+			$style->parentNode->removeChild( $style ); // So we can move it.
+		}
+		$head->appendChild( $style );
+
+		$noscript = $xpath->query( './noscript[ style[ @amp-boilerplate ] ]', $head )->item( 0 );
+		if ( ! $noscript ) {
+			$noscript = $dom->createElement( 'noscript' );
+			$style    = $dom->createElement( 'style' );
+			$style->setAttribute( 'amp-boilerplate', '' );
+			$style->appendChild( $dom->createTextNode( amp_get_boilerplate_stylesheets()[1] ) );
+			$noscript->appendChild( $style );
+		} else {
+			$noscript->parentNode->removeChild( $noscript ); // So we can move it.
 		}
+		$head->appendChild( $noscript );
 
-		// Note the style[amp-custom] and style[amp-boilerplate] are output in the add_hooks() method.
 		unset( $previous_node );
 	}
 
@@ -1840,8 +1872,11 @@ public static function prepare_response( $response, $args = [] ) {
 			);
 		}
 
-		// Abort if the response was not HTML.
-		if ( 'text/html' !== substr( AMP_HTTP::get_response_content_type(), 0, 9 ) || '<' !== substr( ltrim( $response ), 0, 1 ) ) {
+		/*
+		 * Abort if the response was not HTML. To be post-processed as an AMP page, the output-buffered document must
+		 * have the HTML mime type and it must start with <html> followed by <head> tag (with whitespace, doctype, and comments optionally interspersed).
+		 */
+		if ( 'text/html' !== substr( AMP_HTTP::get_response_content_type(), 0, 9 ) || ! preg_match( '#^(?:<!.*?>|\s+)*<html.*?>(?:<!.*?>|\s+)*<head\b(.*?)>#is', $response ) ) {
 			return $response;
 		}
 
@@ -2025,11 +2060,14 @@ public static function prepare_response( $response, $args = [] ) {
 		 * See <https://www.w3.org/International/questions/qa-html-encoding-declarations>.
 		 */
 		if ( ! preg_match( '#<meta[^>]+charset=#i', substr( $response, 0, 1024 ) ) ) {
+			$meta_charset = sprintf( '<meta charset="%s">', esc_attr( get_bloginfo( 'charset' ) ) );
+
 			$response = preg_replace(
-				'/(<head[^>]*>)/i',
-				'$1' . sprintf( '<meta charset="%s">', esc_attr( get_bloginfo( 'charset' ) ) ),
+				'/(<head\b.*?>)/is',
+				'$1' . $meta_charset,
 				$response,
-				1
+				1,
+				$count
 			);
 		}
 
@@ -2253,8 +2291,6 @@ public static function whitelist_layout_in_wp_kses_allowed_html( $context ) {
 	 * @return void
 	 */
 	public static function enqueue_assets() {
-		wp_enqueue_script( 'amp-runtime' );
-
 		// Enqueue default styles expected by sanitizer.
 		wp_enqueue_style( 'amp-default', amp_get_asset_url( 'css/amp-default.css' ), [], AMP__VERSION );
 		wp_styles()->add_data( 'amp-default', 'rtl', 'replace' );
diff --git a/includes/templates/single-amp_story.php b/includes/templates/single-amp_story.php
index 312c487913b..6b3353daf0f 100644
--- a/includes/templates/single-amp_story.php
+++ b/includes/templates/single-amp_story.php
@@ -7,26 +7,25 @@
 
 the_post();
 
-$metadata = amp_get_schemaorg_metadata();
 ?>
 <!DOCTYPE html>
 <html amp <?php language_attributes(); ?>>
 	<head>
 		<meta charset="utf-8">
 		<meta name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1">
-		<title><?php echo esc_html( wp_get_document_title() ); ?></title>
+
 		<?php
-		wp_enqueue_scripts();
-		wp_scripts()->do_items( [ 'amp-runtime' ] ); // @todo Duplicate with AMP_Theme_Support::enqueue_assets().
-		wp_styles()->do_items();
+		/**
+		 * Prints scripts or data in the head tag on the front end.
+		 *
+		 * @since 1.3
+		 */
+		do_action( 'amp_story_head' );
 		?>
-		<?php rel_canonical(); ?>
-		<?php amp_add_generator_metadata(); ?>
-		<script type="application/ld+json"><?php echo wp_json_encode( $metadata, JSON_UNESCAPED_UNICODE ); ?></script>
-		<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>
 		<?php
+		$metadata = amp_get_schemaorg_metadata();
 		if ( isset( $metadata['publisher']['logo']['url'] ) ) {
 			$publisher_logo_src = $metadata['publisher']['logo']['url'];
 		} elseif ( isset( $metadata['publisher']['logo'] ) && is_string( $metadata['publisher']['logo'] ) ) {
diff --git a/tests/php/test-amp-helper-functions.php b/tests/php/test-amp-helper-functions.php
index 52cdaf6960f..89e4e236625 100644
--- a/tests/php/test-amp-helper-functions.php
+++ b/tests/php/test-amp-helper-functions.php
@@ -480,6 +480,39 @@ public function capture_filter_call( $value ) {
 		return $value;
 	}
 
+	/**
+	 * Test amp_get_asset_url.
+	 *
+	 * @covers ::amp_get_asset_url()
+	 */
+	public function test_amp_get_asset_url() {
+		$this->assertStringEndsWith( '/assets/foo.jpg', amp_get_asset_url( 'foo.jpg' ) );
+	}
+
+	/**
+	 * Test amp_get_boilerplate_code.
+	 *
+	 * @covers ::amp_get_boilerplate_code()
+	 */
+	public function test_amp_get_boilerplate_code() {
+		$boilerplate_code = amp_get_boilerplate_code();
+		$this->assertStringStartsWith( '<style amp-boilerplate>', $boilerplate_code );
+		$this->assertContains( '<noscript><style amp-boilerplate>', $boilerplate_code );
+	}
+
+	/**
+	 * Test amp_get_boilerplate_stylesheets.
+	 *
+	 * @covers ::amp_get_boilerplate_stylesheets()
+	 */
+	public function test_amp_get_boilerplate_stylesheets() {
+		$stylesheets = amp_get_boilerplate_stylesheets();
+		$this->assertInternalType( 'array', $stylesheets );
+		$this->assertCount( 2, $stylesheets );
+		$this->assertContains( 'body{-webkit-animation:-amp-start', $stylesheets[0] );
+		$this->assertContains( 'body{-webkit-animation:none', $stylesheets[1] );
+	}
+
 	/**
 	 * Test amp_add_generator_metadata.
 	 *
diff --git a/tests/php/test-class-amp-theme-support.php b/tests/php/test-class-amp-theme-support.php
index d1b0f84d6c5..72eaa076241 100644
--- a/tests/php/test-class-amp-theme-support.php
+++ b/tests/php/test-class-amp-theme-support.php
@@ -1292,14 +1292,14 @@ public function test_finish_output_buffering() {
 		$this->assertTrue( AMP_Theme_Support::is_output_buffering() );
 		$this->assertSame( 3, ob_get_level() );
 
-		echo '<img src="test.png"><script data-test>document.write(\'Illegal\');</script>';
+		echo '<html><head></head><body><img src="test.png"><script data-test>document.write(\'Illegal\');</script>';
 
 		// Additional nested output bufferings which aren't getting closed.
 		ob_start();
 		echo 'foo';
 		ob_start(
 			static function( $response ) {
-					return strtoupper( $response );
+				return strtoupper( $response );
 			}
 		);
 		echo 'bar';
@@ -1427,15 +1427,16 @@ static function ( $url ) {
 			'<script src="https://cdn.ampproject.org/v0/amp-audio-0.1.js" async="" custom-element="amp-audio"></script>',
 			'<script src="https://cdn.ampproject.org/v0/amp-ad-0.1.js" async="" custom-element="amp-ad"></script>',
 
-			'<link rel="icon" href="https://example.org/favicon.png" sizes="32x32">',
-			'<link rel="icon" href="https://example.org/favicon.png" sizes="192x192">',
+			'#<style amp-custom(="")?>.*?body\s*{\s*background:\s*black;?\s*}.*?</style>#s',
 
-			'#<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">',
-			'<style amp-boilerplate>',
-			'<noscript><style amp-boilerplate>',
+			'<link rel="icon" href="https://example.org/favicon.png" sizes="32x32">',
+			'<link rel="icon" href="https://example.org/favicon.png" sizes="192x192">',
 			'<script type="application/ld+json">{"@context"',
 			'<link rel="canonical" href="',
+
+			'#<style amp-boilerplate(="")?>#',
+			'#<noscript><style amp-boilerplate(="")?>#',
 			'</head>',
 		];
 
@@ -1654,7 +1655,7 @@ private function get_original_html() {
 			'wp_enqueue_scripts',
 			static function() {
 				wp_enqueue_script( 'amp-list' );
-			wp_enqueue_style( 'my-font', 'https://fonts.googleapis.com/css?family=Tangerine', [], null ); // phpcs:ignore
+				wp_enqueue_style( 'my-font', 'https://fonts.googleapis.com/css?family=Tangerine', [], null ); // phpcs:ignore
 			}
 		);
 		add_action(
@@ -1681,7 +1682,7 @@ static function( $tag, $handle ) {
 			static function() {
 				wp_print_scripts( 'amp-mathml' );
 				?>
-			<amp-mathml layout="container" data-formula="\[x = {-b \pm \sqrt{b^2-4ac} \over 2a}.\]"></amp-mathml>
+				<amp-mathml layout="container" data-formula="\[x = {-b \pm \sqrt{b^2-4ac} \over 2a}.\]"></amp-mathml>
 				<?php
 			},
 			1
@@ -1765,11 +1766,11 @@ private function reset_post_processor_cache_effectiveness() {
 	}
 
 	/**
-	 * Test prepare_response for bad/non-HTML.
+	 * Test prepare_response for responses that may or may not be valid HTML.
 	 *
 	 * @covers AMP_Theme_Support::prepare_response()
 	 */
-	public function test_prepare_response_bad_html() {
+	public function test_prepare_response_varying_html() {
 		wp();
 		add_filter( 'amp_validation_error_sanitized', '__return_true' );
 		add_theme_support( AMP_Theme_Support::SLUG );
@@ -1784,10 +1785,25 @@ public function test_prepare_response_bad_html() {
 		$input = '';
 		$this->assertEquals( $input, AMP_Theme_Support::prepare_response( $input ) );
 
+		// HTML, but a fragment.
+		$input = '<ul><li>one</li><li>two</li><li>three</li></ul>';
+		$this->assertEquals( $input, AMP_Theme_Support::prepare_response( $input ) );
+
+		// HTML, but still a fragment.
+		$input = '<html><header><h1>HellO!</h1></header></html>';
+		$this->assertEquals( $input, AMP_Theme_Support::prepare_response( $input ) );
+
 		// HTML, but very stripped down.
-		$input  = '<html>Hello</html>';
+		$input  = '<html><head></head>Hello</html>';
+		$output = AMP_Theme_Support::prepare_response( $input );
+		$this->assertContains( '<html amp', $output );
+		$this->assertContains( '<meta charset="UTF-8">', $output );
+
+		// HTML with doctype, comments, and whitespace before head.
+		$input  = "   <!--\nHello world!\n-->\n\n<!DOCTYPE html>  <html\n\n>\n<head profile='http://www.acme.com/profiles/core'></head><body>Hello</body></html>";
 		$output = AMP_Theme_Support::prepare_response( $input );
 		$this->assertContains( '<html amp', $output );
+		$this->assertContains( '<meta charset="UTF-8">', $output );
 	}
 
 	/**
@@ -1902,12 +1918,9 @@ static function( $url ) use ( &$redirects ) {
 	 * @covers AMP_Theme_Support::enqueue_assets()
 	 */
 	public function test_enqueue_assets() {
-		$script_slug = 'amp-runtime';
-		$style_slug  = 'amp-default';
-		wp_dequeue_script( $script_slug );
+		$style_slug = 'amp-default';
 		wp_dequeue_style( $style_slug );
 		AMP_Theme_Support::enqueue_assets();
-		$this->assertContains( $script_slug, wp_scripts()->queue );
 		$this->assertContains( $style_slug, wp_styles()->queue );
 	}