diff --git a/includes/amp-helper-functions.php b/includes/amp-helper-functions.php
index a408add9e57..b06a2bbebbc 100644
--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -293,7 +293,7 @@ function amp_get_content_sanitizers( $post = null ) {
 	 * @param array   $handlers Handlers.
 	 * @param WP_Post $post     Post. Deprecated.
 	 */
-	return apply_filters( 'amp_content_sanitizers',
+	$sanitizers = apply_filters( 'amp_content_sanitizers',
 		array(
 			'AMP_Img_Sanitizer'               => array(),
 			'AMP_Form_Sanitizer'              => array(),
@@ -309,6 +309,17 @@ function amp_get_content_sanitizers( $post = null ) {
 		),
 		$post
 	);
+
+	// Force style sanitizer and whitelist sanitizer to be at end.
+	foreach ( array( 'AMP_Style_Sanitizer', 'AMP_Tag_And_Attribute_Sanitizer' ) as $class_name ) {
+		if ( isset( $sanitizers[ $class_name ] ) ) {
+			$sanitizer = $sanitizers[ $class_name ];
+			unset( $sanitizers[ $class_name ] );
+			$sanitizers[ $class_name ] = $sanitizer;
+		}
+	}
+
+	return $sanitizers;
 }
 
 /**
diff --git a/includes/class-amp-autoloader.php b/includes/class-amp-autoloader.php
index f9dfb3dc175..16880ed1325 100644
--- a/includes/class-amp-autoloader.php
+++ b/includes/class-amp-autoloader.php
@@ -31,7 +31,6 @@ class AMP_Autoloader {
 	private static $_classmap = array(
 		'AMP_Theme_Support'                           => 'includes/class-amp-theme-support',
 		'AMP_Comment_Walker'                          => 'includes/class-amp-comment-walker',
-		'AMP_WP_Styles'                               => 'includes/class-amp-wp-styles',
 		'AMP_Template_Customizer'                     => 'includes/admin/class-amp-customizer',
 		'AMP_Post_Meta_Box'                           => 'includes/admin/class-amp-post-meta-box',
 		'AMP_Post_Type_Support'                       => 'includes/class-amp-post-type-support',
diff --git a/includes/class-amp-theme-support.php b/includes/class-amp-theme-support.php
index ffae0b69ea0..9a71c112404 100644
--- a/includes/class-amp-theme-support.php
+++ b/includes/class-amp-theme-support.php
@@ -19,13 +19,6 @@ class AMP_Theme_Support {
 	 */
 	const SCRIPTS_PLACEHOLDER = '<!-- AMP:SCRIPTS_PLACEHOLDER -->';
 
-	/**
-	 * Replaced with the necessary styles.
-	 *
-	 * @var string
-	 */
-	const STYLES_PLACEHOLDER = '<!-- AMP:STYLES_PLACEHOLDER -->';
-
 	/**
 	 * Sanitizer classes.
 	 *
@@ -152,16 +145,11 @@ public static function register_hooks() {
 
 		// Remove core actions which are invalid AMP.
 		remove_action( 'wp_head', 'wp_post_preview_js', 1 );
-		remove_action( 'wp_head', 'locale_stylesheet' ); // Replaced below in add_amp_styles_placeholder() method.
 		remove_action( 'wp_head', 'print_emoji_detection_script', 7 );
-		remove_action( 'wp_head', 'wp_print_styles', 8 ); // Replaced below in add_amp_styles_placeholder() method.
 		remove_action( 'wp_head', 'wp_print_head_scripts', 9 );
-		remove_action( 'wp_head', 'wp_custom_css_cb', 101 ); // Replaced below in add_amp_styles_placeholder() method.
 		remove_action( 'wp_footer', 'wp_print_footer_scripts', 20 );
 		remove_action( 'wp_print_styles', 'print_emoji_styles' );
 
-		add_action( 'wp_enqueue_scripts', array( __CLASS__, 'override_wp_styles' ), -1 );
-
 		/*
 		 * Add additional markup required by AMP <https://www.ampproject.org/docs/reference/spec#required-markup>.
 		 * Note that the meta[name=viewport] is not added here because a theme may want to define one with additional
@@ -171,8 +159,8 @@ public static function register_hooks() {
 		 * in this case too we should defer to the theme as well to output the meta charset because it is possible the
 		 * install is not on utf-8 and we may need to do a encoding conversion.
 		 */
-		add_action( 'wp_head', array( __CLASS__, 'add_amp_styles_placeholder' ), 8 ); // Because wp_print_styles() normally happens at 8.
 		add_action( 'wp_head', array( __CLASS__, 'add_amp_component_scripts' ), 10 );
+		add_action( 'wp_head', array( __CLASS__, 'print_amp_styles' ) );
 		add_action( 'wp_head', 'amp_add_generator_metadata', 20 );
 		add_action( 'wp_head', 'amp_print_schemaorg_metadata' );
 
@@ -313,21 +301,6 @@ public static function register_widgets() {
 		}
 	}
 
-	/**
-	 * Override $wp_styles as AMP_WP_Styles, ideally before first instantiated as WP_Styles.
-	 *
-	 * @see wp_styles()
-	 * @global AMP_WP_Styles $wp_styles
-	 * @return AMP_WP_Styles Instance.
-	 */
-	public static function override_wp_styles() {
-		global $wp_styles;
-		if ( ! ( $wp_styles instanceof AMP_WP_Styles ) ) {
-			$wp_styles = new AMP_WP_Styles(); // WPCS: global override ok.
-		}
-		return $wp_styles;
-	}
-
 	/**
 	 * Register content embed handlers.
 	 *
@@ -621,53 +594,11 @@ public static function filter_cancel_comment_reply_link( $formatted_link, $link,
 	}
 
 	/**
-	 * Print placeholder for Custom AMP styles.
-	 *
-	 * The actual styles for the page injected into the placeholder when output buffering is completed.
-	 *
-	 * @see AMP_Theme_Support::finish_output_buffering()
+	 * Print AMP boilerplate and custom styles.
 	 */
-	public static function add_amp_styles_placeholder() {
-		echo self::STYLES_PLACEHOLDER; // WPCS: XSS OK.
-
-		$wp_styles = wp_styles();
-		if ( ! ( $wp_styles instanceof AMP_WP_Styles ) ) {
-			trigger_error( esc_html__( 'wp_styles() does not return an instance of AMP_WP_Styles as required.', 'amp' ), E_USER_WARNING ); // phpcs:ignore
-			return;
-		}
-
-		$wp_styles->do_items(); // Normally done at wp_head priority 8.
-		$wp_styles->do_locale_stylesheet(); // Normally done at wp_head priority 10.
-		$wp_styles->do_custom_css(); // Normally done at wp_head priority 101.
-	}
-
-	/**
-	 * Get AMP boilerplate and custom styles.
-	 *
-	 * @param string[] $stylesheets Initial stylesheets.
-	 * @see wp_custom_css_cb()
-	 * @return string Concatenated stylesheets.
-	 */
-	public static function get_amp_styles( $stylesheets ) {
-		$css = wp_styles()->print_code;
-
-		$css .= join( $stylesheets );
-
-		/**
-		 * Filters AMP custom CSS before it is injected onto the output buffer for the response.
-		 *
-		 * Plugins may add their own styles, such as for rendered widgets, by amending them via this filter.
-		 *
-		 * @since 0.7
-		 *
-		 * @param string $css AMP CSS.
-		 */
-		$css = apply_filters( 'amp_custom_styles', $css );
-
-		$css = wp_strip_all_tags( $css );
-
-		return amp_get_boilerplate_code() . "\n"
-			. '<style amp-custom>' . $css . '</style>';
+	public static function print_amp_styles() {
+		echo amp_get_boilerplate_code() . "\n"; // WPCS: XSS OK.
+		echo "<style amp-custom></style>\n"; // This will by populated by AMP_Style_Sanitizer.
 	}
 
 	/**
@@ -880,14 +811,6 @@ public static function prepare_response( $response ) {
 			1
 		);
 
-		// Inject styles.
-		$response = preg_replace(
-			'#' . preg_quote( self::STYLES_PLACEHOLDER, '#' ) . '#',
-			self::get_amp_styles( $assets['stylesheets'] ),
-			$response,
-			1
-		);
-
 		return $response;
 	}
 }
diff --git a/includes/class-amp-wp-styles.php b/includes/class-amp-wp-styles.php
deleted file mode 100644
index c397ce4c28b..00000000000
--- a/includes/class-amp-wp-styles.php
+++ /dev/null
@@ -1,251 +0,0 @@
-<?php
-/**
- * Class AMP_WP_Styles
- *
- * @package AMP
- */
-
-/**
- * Extend WP_Styles with handling of CSS for AMP.
- *
- * @since 0.7
- */
-class AMP_WP_Styles extends WP_Styles {
-
-	/**
-	 * Concatenation is always enabled for AMP.
-	 *
-	 * @since 0.7
-	 * @var bool
-	 */
-	public $do_concat = true;
-
-	/**
-	 * Whether the locale stylesheet was done.
-	 *
-	 * @since 0.7
-	 * @var bool
-	 */
-	protected $did_locale_stylesheet = false;
-
-	/**
-	 * Whether the Custom CSS was done.
-	 *
-	 * @since 0.7
-	 * @var bool
-	 */
-	protected $did_custom_css = false;
-
-	/**
-	 * Regex for allowed font stylesheet URL.
-	 *
-	 * @var string
-	 */
-	protected $allowed_font_src_regex;
-
-	/**
-	 * Constructor.
-	 */
-	public function __construct() {
-		parent::__construct();
-
-		$spec_name = 'link rel=stylesheet for fonts'; // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
-		foreach ( AMP_Allowed_Tags_Generated::get_allowed_tag( 'link' ) as $spec_rule ) {
-			if ( isset( $spec_rule[ AMP_Rule_Spec::TAG_SPEC ]['spec_name'] ) && $spec_name === $spec_rule[ AMP_Rule_Spec::TAG_SPEC ]['spec_name'] ) {
-				$this->allowed_font_src_regex = '@^(' . $spec_rule[ AMP_Rule_Spec::ATTR_SPEC_LIST ]['href']['value_regex'] . ')$@';
-				break;
-			}
-		}
-	}
-
-	/**
-	 * Generates an enqueued style's fully-qualified file path.
-	 *
-	 * @since 0.7
-	 * @see WP_Styles::_css_href()
-	 *
-	 * @param string $src The source URL of the enqueued style.
-	 * @param string $handle The style's registered handle.
-	 * @return string|WP_Error Style's absolute validated filesystem path, or WP_Error when error.
-	 */
-	public function get_validated_css_file_path( $src, $handle ) {
-		$needs_base_url = (
-			! is_bool( $src )
-			&&
-			! preg_match( '|^(https?:)?//|', $src )
-			&&
-			! ( $this->content_url && 0 === strpos( $src, $this->content_url ) )
-		);
-		if ( $needs_base_url ) {
-			$src = $this->base_url . $src;
-		}
-
-		/** This filter is documented in wp-includes/class.wp-styles.php */
-		$src = apply_filters( 'style_loader_src', $src, $handle );
-		$src = esc_url_raw( $src );
-
-		// Strip query and fragment from URL.
-		$src = preg_replace( ':[\?#].*$:', '', $src );
-
-		if ( ! preg_match( '/\.(css|less|scss|sass)$/i', $src ) ) {
-			/* translators: %1$s is stylesheet handle, %2$s is stylesheet URL */
-			return new WP_Error( 'amp_css_bad_file_extension', sprintf( __( 'Skipped stylesheet %1$s which does not have recognized CSS file extension (%2$s).', 'amp' ), $handle, $src ) );
-		}
-
-		$includes_url = includes_url( '/' );
-		$content_url  = content_url( '/' );
-		$admin_url    = get_admin_url( null, '/' );
-		$css_path     = null;
-		if ( 0 === strpos( $src, $content_url ) ) {
-			$css_path = WP_CONTENT_DIR . substr( $src, strlen( $content_url ) - 1 );
-		} elseif ( 0 === strpos( $src, $includes_url ) ) {
-			$css_path = ABSPATH . WPINC . substr( $src, strlen( $includes_url ) - 1 );
-		} elseif ( 0 === strpos( $src, $admin_url ) ) {
-			$css_path = ABSPATH . 'wp-admin' . substr( $src, strlen( $admin_url ) - 1 );
-		}
-
-		if ( ! $css_path || false !== strpos( '../', $css_path ) || 0 !== validate_file( $css_path ) || ! file_exists( $css_path ) ) {
-			/* translators: %1$s is stylesheet handle, %2$s is stylesheet URL */
-			return new WP_Error( 'amp_css_path_not_found', sprintf( __( 'Unable to locate filesystem path for stylesheet %1$s (%2$s).', 'amp' ), $handle, $src ) );
-		}
-
-		return $css_path;
-	}
-
-	/**
-	 * Processes a style dependency.
-	 *
-	 * @since 0.7
-	 * @see WP_Styles::do_item()
-	 *
-	 * @param string $handle The style's registered handle.
-	 * @return bool True on success, false on failure.
-	 */
-	public function do_item( $handle ) {
-		if ( ! WP_Dependencies::do_item( $handle ) ) {
-			return false;
-		}
-		$obj = $this->registered[ $handle ];
-
-		// Conditional styles and alternate styles aren't supported in AMP.
-		if ( isset( $obj->extra['conditional'] ) || isset( $obj->extra['alt'] ) ) {
-			return false;
-		}
-
-		if ( isset( $obj->args ) ) {
-			$media = esc_attr( $obj->args );
-		} else {
-			$media = 'all';
-		}
-
-		// A single item may alias a set of items, by having dependencies, but no source.
-		if ( ! $obj->src ) {
-			$inline_style = $this->print_inline_style( $handle, false );
-			if ( $inline_style ) {
-				$this->print_code .= $inline_style;
-			}
-			return true;
-		}
-
-		// Allow font URLs.
-		if ( $this->allowed_font_src_regex && preg_match( $this->allowed_font_src_regex, $obj->src ) ) {
-			$this->do_concat = false;
-			$result          = parent::do_item( $handle );
-			$this->do_concat = true;
-			return $result;
-		}
-
-		$css_file_path = $this->get_validated_css_file_path( $obj->src, $handle );
-		if ( is_wp_error( $css_file_path ) ) {
-			trigger_error( esc_html( $css_file_path->get_error_message() ), E_USER_WARNING ); // phpcs:ignore
-			return false;
-		}
-		$css_rtl_file_path = '';
-
-		// Handle RTL styles.
-		if ( 'rtl' === $this->text_direction && isset( $obj->extra['rtl'] ) && $obj->extra['rtl'] ) {
-			if ( is_bool( $obj->extra['rtl'] ) || 'replace' === $obj->extra['rtl'] ) {
-				$suffix            = isset( $obj->extra['suffix'] ) ? $obj->extra['suffix'] : '';
-				$css_rtl_file_path = $this->get_validated_css_file_path(
-					str_replace( "{$suffix}.css", "-rtl{$suffix}.css", $obj->src ),
-					"$handle-rtl"
-				);
-			} else {
-				$css_rtl_file_path = $this->get_validated_css_file_path( $obj->extra['rtl'], "$handle-rtl" );
-			}
-
-			if ( is_wp_error( $css_rtl_file_path ) ) {
-				trigger_error( esc_html( $css_rtl_file_path->get_error_message() ), E_USER_WARNING ); // phpcs:ignore
-				$css_rtl_file_path = null;
-			} elseif ( 'replace' === $obj->extra['rtl'] ) {
-				$css_file_path = null;
-			}
-		}
-
-		// Load the CSS from the filesystem.
-		foreach ( array_filter( array( $css_file_path, $css_rtl_file_path ) ) as $css_path ) {
-			$css = file_get_contents( $css_path ); // phpcs:ignore -- It's a local filesystem path not a remote request.
-			if ( 'all' !== $media ) {
-				$css = sprintf( '@media %s { %s }', $media, $css );
-			}
-			$this->print_code .= $css;
-		}
-
-		// Add inline styles.
-		$inline_style = $this->print_inline_style( $handle, false );
-		if ( $inline_style ) {
-			$this->print_code .= $inline_style;
-		}
-
-		return true;
-	}
-
-	/**
-	 * Get the locale stylesheet if it exists.
-	 *
-	 * @since 0.7
-	 * @see locale_stylesheet()
-	 * @return bool Whether locale stylesheet was done.
-	 */
-	public function do_locale_stylesheet() {
-		if ( $this->did_locale_stylesheet ) {
-			return true;
-		}
-
-		$src = get_locale_stylesheet_uri();
-		if ( ! $src ) {
-			return false;
-		}
-		$path = $this->get_validated_css_file_path( $src, get_stylesheet() . '-' . get_locale() );
-		if ( is_wp_error( $path ) ) {
-			return false;
-		}
-		$this->print_code .= file_get_contents( $path ); // phpcs:ignore -- The path has been validated, and it is not a remote path.
-		$this->did_locale_stylesheet = true;
-		return true;
-	}
-
-	/**
-	 * Append Customizer Custom CSS.
-	 *
-	 * @since 0.7
-	 * @see wp_custom_css()
-	 * @see wp_custom_css_cb()
-	 * @return bool Whether locale Custom CSS was done.
-	 */
-	public function do_custom_css() {
-		if ( $this->did_custom_css ) {
-			return true;
-		}
-
-		$css = trim( wp_get_custom_css() );
-		if ( ! $css ) {
-			return false;
-		}
-
-		$this->print_code .= $css;
-
-		$this->did_custom_css = true;
-		return true;
-	}
-}
diff --git a/includes/sanitizers/class-amp-style-sanitizer.php b/includes/sanitizers/class-amp-style-sanitizer.php
index 6e926518b1e..f643a1d7f0b 100644
--- a/includes/sanitizers/class-amp-style-sanitizer.php
+++ b/includes/sanitizers/class-amp-style-sanitizer.php
@@ -8,7 +8,6 @@
 /**
  * Class AMP_Style_Sanitizer
  *
- * @todo This needs to also run on the CSS that is gathered for amp-custom.
  * Collects inline styles and outputs them in the amp-custom stylesheet.
  */
 class AMP_Style_Sanitizer extends AMP_Base_Sanitizer {
@@ -40,6 +39,44 @@ class AMP_Style_Sanitizer extends AMP_Base_Sanitizer {
 	 */
 	private $keyframes_max_size;
 
+	/**
+	 * Maximum number of bytes allowed for a AMP Custom style.
+	 *
+	 * @since 0.7
+	 * @var int
+	 */
+	private $custom_max_size;
+
+	/**
+	 * The style[amp-custom] element.
+	 *
+	 * @var DOMElement
+	 */
+	private $amp_custom_style_element;
+
+	/**
+	 * Regex for allowed font stylesheet URL.
+	 *
+	 * @var string
+	 */
+	private $allowed_font_src_regex;
+
+	/**
+	 * Base URL for styles.
+	 *
+	 * Full URL with trailing slash.
+	 *
+	 * @var string
+	 */
+	private $base_url;
+
+	/**
+	 * URL of the content directory.
+	 *
+	 * @var string
+	 */
+	private $content_url;
+
 	/**
 	 * AMP_Base_Sanitizer constructor.
 	 *
@@ -58,6 +95,29 @@ public function __construct( DOMDocument $dom, array $args = array() ) {
 				break;
 			}
 		}
+
+		$spec_name = 'style amp-custom';
+		foreach ( AMP_Allowed_Tags_Generated::get_allowed_tag( 'style' ) as $spec_rule ) {
+			if ( isset( $spec_rule[ AMP_Rule_Spec::TAG_SPEC ]['spec_name'] ) && $spec_name === $spec_rule[ AMP_Rule_Spec::TAG_SPEC ]['spec_name'] ) {
+				$this->custom_max_size = $spec_rule[ AMP_Rule_Spec::CDATA ]['max_bytes'];
+				break;
+			}
+		}
+
+		$spec_name = 'link rel=stylesheet for fonts'; // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
+		foreach ( AMP_Allowed_Tags_Generated::get_allowed_tag( 'link' ) as $spec_rule ) {
+			if ( isset( $spec_rule[ AMP_Rule_Spec::TAG_SPEC ]['spec_name'] ) && $spec_name === $spec_rule[ AMP_Rule_Spec::TAG_SPEC ]['spec_name'] ) {
+				$this->allowed_font_src_regex = '@^(' . $spec_rule[ AMP_Rule_Spec::ATTR_SPEC_LIST ]['href']['value_regex'] . ')$@';
+				break;
+			}
+		}
+
+		$guessurl = site_url();
+		if ( ! $guessurl ) {
+			$guessurl = wp_guess_url();
+		}
+		$this->base_url    = $guessurl;
+		$this->content_url = WP_CONTENT_URL;
 	}
 
 	/**
@@ -81,7 +141,7 @@ public function get_styles() {
 	 * @returns array Values are the CSS stylesheets. Keys are MD5 hashes of the stylesheets.
 	 */
 	public function get_stylesheets() {
-		return array_merge( parent::get_stylesheets(), $this->stylesheets );
+		return array_merge( $this->stylesheets, parent::get_stylesheets() );
 	}
 
 	/**
@@ -90,50 +150,225 @@ public function get_stylesheets() {
 	 * @since 0.4
 	 */
 	public function sanitize() {
-		$body = $this->root_element;
+		$elements = array();
 
-		$this->collect_style_elements();
+		/*
+		 * Note that xpath is used to query the DOM so that the link and style elements will be
+		 * in document order. DOMNode::compareDocumentPosition() is not yet implemented.
+		 */
+		$xpath = new DOMXPath( $this->dom );
+
+		$lower_case = 'translate( %s, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz" )'; // In XPath 2.0 this is lower-case().
+		$predicates = array(
+			sprintf( '( self::style and not( @amp-boilerplate ) and ( not( @type ) or %s = "text/css" ) )', sprintf( $lower_case, '@type' ) ),
+			sprintf( '( self::link and @href and %s = "stylesheet" )', sprintf( $lower_case, '@rel' ) ),
+		);
+
+		foreach ( $xpath->query( '//*[ ' . implode( ' or ', $predicates ) . ' ]' ) as $element ) {
+			$elements[] = $element;
+		}
 
-		$this->collect_styles_recursive( $body );
+		/**
+		 * Element.
+		 *
+		 * @var DOMElement $element
+		 */
+		foreach ( $elements as $element ) {
+			$node_name = strtolower( $element->nodeName );
+			if ( 'style' === $node_name ) {
+				$this->process_style_element( $element );
+			} elseif ( 'link' === $node_name ) {
+				$this->process_link_element( $element );
+			}
+		}
+
+		$elements = array();
+		foreach ( $xpath->query( '//*[ @style ]' ) as $element ) {
+			$elements[] = $element;
+		}
+		foreach ( $elements as $element ) {
+			$this->collect_inline_styles( $element );
+		}
 		$this->did_convert_elements = true;
+
+		// Now make sure the amp-custom style is in the DOM and populated, if we're working with the document element.
+		if ( ! empty( $this->args['use_document_element'] ) ) {
+			if ( ! $this->amp_custom_style_element ) {
+				$this->amp_custom_style_element = $this->dom->createElement( 'style' );
+				$this->amp_custom_style_element->setAttribute( 'amp-custom', '' );
+				$this->dom->getElementsByTagName( 'head' )->item( 0 )->appendChild( $this->amp_custom_style_element );
+			}
+
+			// Gather stylesheets to print as long as they don't surpass the limit.
+			$skipped    = array();
+			$css        = '';
+			$total_size = 0;
+			foreach ( $this->get_stylesheets() as $key => $stylesheet ) {
+				$sheet_size = strlen( $stylesheet );
+				if ( $total_size + $sheet_size > $this->custom_max_size ) {
+					$skipped[] = $key;
+				} else {
+					if ( $total_size ) {
+						$css .= ' ';
+					}
+					$css        .= $stylesheet;
+					$total_size += $sheet_size;
+				}
+			}
+
+			/*
+			 * Let the style[amp-custom] be populated with the concatenated CSS.
+			 * !important: Updating the contents of this style element by setting textContent is not
+			 * reliable across PHP/libxml versions, so this is why the children are removed and the
+			 * text node is then explicitly added containing the CSS.
+			 */
+			while ( $this->amp_custom_style_element->firstChild ) {
+				$this->amp_custom_style_element->removeChild( $this->amp_custom_style_element->firstChild );
+			}
+			$this->amp_custom_style_element->appendChild( $this->dom->createTextNode( $css ) );
+
+			// @todo This would be a candidate for sanitization reporting.
+			// Add comments to indicate which sheets were not included.
+			foreach ( array_reverse( $skipped ) as $skip ) {
+				$this->amp_custom_style_element->parentNode->insertBefore(
+					$this->dom->createComment( sprintf( 'Skipped including %s stylesheet since too large.', $skip ) ),
+					$this->amp_custom_style_element->nextSibling
+				);
+			}
+		}
 	}
 
 	/**
-	 * Collect and sanitize all style elements.
+	 * Generates an enqueued style's fully-qualified file path.
+	 *
+	 * @since 0.7
+	 * @see WP_Styles::_css_href()
+	 *
+	 * @param string $src The source URL of the enqueued style.
+	 * @return string|WP_Error Style's absolute validated filesystem path, or WP_Error when error.
 	 */
-	public function collect_style_elements() {
-		$style_elements  = $this->dom->getElementsByTagName( 'style' );
-		$nodes_to_remove = array();
-
-		foreach ( $style_elements as $style_element ) {
-			/**
-			 * Style element.
-			 *
-			 * @var DOMElement $style_element
-			 */
+	public function get_validated_css_file_path( $src ) {
+		$needs_base_url = (
+			! is_bool( $src )
+			&&
+			! preg_match( '|^(https?:)?//|', $src )
+			&&
+			! ( $this->content_url && 0 === strpos( $src, $this->content_url ) )
+		);
+		if ( $needs_base_url ) {
+			$src = $this->base_url . $src;
+		}
 
-			if ( 'body' === $style_element->parentNode->nodeName && $style_element->hasAttribute( 'amp-keyframes' ) ) {
-				$validity = $this->validate_amp_keyframe( $style_element );
-				if ( true === $validity ) {
-					continue;
-				}
+		// Strip query and fragment from URL.
+		$src = preg_replace( ':[\?#].*$:', '', $src );
+
+		if ( ! preg_match( '/\.(css|less|scss|sass)$/i', $src ) ) {
+			/* translators: %s is stylesheet URL */
+			return new WP_Error( 'amp_css_bad_file_extension', sprintf( __( 'Skipped stylesheet which does not have recognized CSS file extension (%s).', 'amp' ), $src ) );
+		}
+
+		$includes_url = includes_url( '/' );
+		$content_url  = content_url( '/' );
+		$admin_url    = get_admin_url( null, '/' );
+		$css_path     = null;
+		if ( 0 === strpos( $src, $content_url ) ) {
+			$css_path = WP_CONTENT_DIR . substr( $src, strlen( $content_url ) - 1 );
+		} elseif ( 0 === strpos( $src, $includes_url ) ) {
+			$css_path = ABSPATH . WPINC . substr( $src, strlen( $includes_url ) - 1 );
+		} elseif ( 0 === strpos( $src, $admin_url ) ) {
+			$css_path = ABSPATH . 'wp-admin' . substr( $src, strlen( $admin_url ) - 1 );
+		}
+
+		if ( ! $css_path || false !== strpos( '../', $css_path ) || 0 !== validate_file( $css_path ) || ! file_exists( $css_path ) ) {
+			/* translators: %s is stylesheet URL */
+			return new WP_Error( 'amp_css_path_not_found', sprintf( __( 'Unable to locate filesystem path for stylesheet %s.', 'amp' ), $src ) );
+		}
+
+		return $css_path;
+	}
+
+	/**
+	 * Process style element.
+	 *
+	 * @param DOMElement $element Style element.
+	 */
+	private function process_style_element( DOMElement $element ) {
+		if ( 'body' === $element->parentNode->nodeName && $element->hasAttribute( 'amp-keyframes' ) ) {
+			$validity = $this->validate_amp_keyframe( $element );
+			if ( true !== $validity ) {
+				$element->parentNode->removeChild( $element ); // @todo Add reporting.
 			}
+			return;
+		}
 
-			$nodes_to_remove[] = $style_element;
+		$rules = trim( $element->textContent );
+		$rules = $this->remove_illegal_css( $rules );
 
-			// @todo This should perhaps be done in document order to ensure proper cascade.
-			$rules = trim( $style_element->textContent );
+		$this->stylesheets[ md5( $rules ) ] = $rules;
 
-			// @todo This needs proper CSS parser, and de-duplication with \AMP_Style_Sanitizer::filter_style().
-			$rules = preg_replace( '/\s*!important\s*(?=\s*;|})/', '', $rules );
-			$rules = preg_replace( '/overflow\s*:\s*(auto|scroll)\s*;?\s*/', '', $rules );
+		if ( $element->hasAttribute( 'amp-custom' ) ) {
+			if ( ! $this->amp_custom_style_element ) {
+				$this->amp_custom_style_element = $element;
+			} else {
+				$element->parentNode->removeChild( $element ); // There can only be one. #highlander.
+			}
+		} else {
 
-			$this->stylesheets[ md5( $rules ) ] = $rules;
+			// Remove from DOM since we'll be adding it to amp-custom.
+			$element->parentNode->removeChild( $element );
 		}
+	}
 
-		foreach ( $nodes_to_remove as $node_to_remove ) {
-			$node_to_remove->parentNode->removeChild( $node_to_remove );
+	/**
+	 * Process link element.
+	 *
+	 * @param DOMElement $element Link element.
+	 */
+	private function process_link_element( DOMElement $element ) {
+		$href = $element->getAttribute( 'href' );
+
+		// Allow font URLs.
+		if ( $this->allowed_font_src_regex && preg_match( $this->allowed_font_src_regex, $href ) ) {
+			return;
 		}
+
+		$css_file_path = $this->get_validated_css_file_path( $href );
+		if ( is_wp_error( $css_file_path ) ) {
+			$element->parentNode->removeChild( $element ); // @todo Report removal. Show HTML comment?
+			return;
+		}
+
+		// Load the CSS from the filesystem.
+		$css  = "\n/* $href */\n";
+		$css .= file_get_contents( $css_file_path ); // phpcs:ignore -- It's a local filesystem path not a remote request.
+
+		$css = $this->remove_illegal_css( $css );
+
+		$media = $element->getAttribute( 'media' );
+		if ( $media && 'all' !== $media ) {
+			$css = sprintf( '@media %s { %s }', $media, $css );
+		}
+
+		$this->stylesheets[ $href ] = $css;
+
+		// Remove now that styles have been processed.
+		$element->parentNode->removeChild( $element );
+	}
+
+	/**
+	 * Remove illegal CSS from the stylesheet.
+	 *
+	 * @since 0.7
+	 *
+	 * @todo This needs proper CSS parser and to take an alternative approach to removing !important by extracting
+	 * the rule into a separate style rule with a very specific selector.
+	 * @param string $stylesheet Stylesheet.
+	 * @return string Scrubbed stylesheet.
+	 */
+	private function remove_illegal_css( $stylesheet ) {
+		$stylesheet = preg_replace( '/\s*!important/', '', $stylesheet ); // Note this has to also replace inside comments to be valid.
+		$stylesheet = preg_replace( '/overflow\s*:\s*(auto|scroll)\s*;?\s*/', '', $stylesheet );
+		return $stylesheet;
 	}
 
 	/**
@@ -171,39 +406,28 @@ private function validate_amp_keyframe( $style ) {
 	 * @see Retrieve array of styles using $this->get_styles() after calling this method.
 	 *
 	 * @since 0.4
+	 * @since 0.7 Modified to use element passed by XPath query.
 	 *
 	 * @note Uses recursion to traverse down the tree of DOMDocument nodes.
-	 * @todo This could use XPath to more efficiently find all elements with style attributes.
 	 *
-	 * @param DOMNode $node Node.
+	 * @param DOMElement $element Node.
 	 */
-	private function collect_styles_recursive( $node ) {
-		if ( XML_ELEMENT_NODE !== $node->nodeType ) {
+	private function collect_inline_styles( $element ) {
+		$style = $element->getAttribute( 'style' );
+		if ( ! $style ) {
 			return;
 		}
+		$class = $element->getAttribute( 'class' );
 
-		if ( $node->hasAttributes() && $node instanceof DOMElement ) {
-			$style = $node->getAttribute( 'style' );
-			$class = $node->getAttribute( 'class' );
-
-			if ( $style ) {
-				$style = $this->process_style( $style );
-				if ( ! empty( $style ) ) {
-					$class_name = $this->generate_class_name( $style );
-					$new_class  = trim( $class . ' ' . $class_name );
-
-					$node->setAttribute( 'class', $new_class );
-					$this->styles[ '.' . $class_name ] = $style;
-				}
-				$node->removeAttribute( 'style' );
-			}
-		}
+		$style = $this->process_style( $style );
+		if ( ! empty( $style ) ) {
+			$class_name = $this->generate_class_name( $style );
+			$new_class  = trim( $class . ' ' . $class_name );
 
-		$length = $node->childNodes->length;
-		for ( $i = $length - 1; $i >= 0; $i -- ) {
-			$child_node = $node->childNodes->item( $i );
-			$this->collect_styles_recursive( $child_node );
+			$element->setAttribute( 'class', $new_class );
+			$this->styles[ '.' . $class_name ] = $style;
 		}
+		$element->removeAttribute( 'style' );
 	}
 
 	/**
diff --git a/includes/utils/class-amp-dom-utils.php b/includes/utils/class-amp-dom-utils.php
index d60aa6da72a..5288c044f2f 100644
--- a/includes/utils/class-amp-dom-utils.php
+++ b/includes/utils/class-amp-dom-utils.php
@@ -42,13 +42,21 @@ class AMP_DOM_Utils {
 		'wbr',
 	);
 
+	/**
+	 * Stored noscript/comment replacements for libxml<2.8.
+	 *
+	 * @since 0.7
+	 * @var array
+	 */
+	public static $noscript_placeholder_comments = array();
+
 	/**
 	 * Return a valid DOMDocument representing HTML document passed as a parameter.
 	 *
 	 * @since 0.7
+	 * @see AMP_DOM_Utils::get_content_from_dom_node()
 	 *
 	 * @param string $document Valid HTML document to be represented by a DOMDocument.
-	 *
 	 * @return DOMDocument|false Returns DOMDocument, or false if conversion failed.
 	 */
 	public static function get_dom( $document ) {
@@ -82,6 +90,24 @@ public static function get_dom( $document ) {
 			$document
 		);
 
+		/*
+		 * Replace noscript elements with placeholders since libxml<2.8 can parse them incorrectly.
+		 * When appearing in the head element, a noscript can cause the head to close prematurely
+		 * and the noscript gets moved to the body and anything after it which was in the head.
+		 * See <https://stackoverflow.com/questions/39013102/why-does-noscript-move-into-body-tag-instead-of-head-tag>.
+		 */
+		if ( version_compare( LIBXML_DOTTED_VERSION, '2.8', '<' ) ) {
+			$document = preg_replace_callback(
+				'#<noscript[^>]*>.*?</noscript>#si',
+				function( $matches ) {
+					$placeholder = sprintf( '<!--noscript:%s-->', (string) wp_rand() );
+					AMP_DOM_Utils::$noscript_placeholder_comments[ $placeholder ] = $matches[0];
+					return $placeholder;
+				},
+				$document
+			);
+		}
+
 		/*
 		 * Wrap in dummy tags, since XML needs one parent node.
 		 * It also makes it easier to loop through nodes.
@@ -266,15 +292,14 @@ public static function get_dom_from_content( $content ) {
 	}
 
 	/**
-	 * Return valid HTML content extracted from the DOMDocument passed as a parameter.
-	 *
-	 * @see Reciprocal function get_dom_from_content()
+	 * Return valid HTML *body* content extracted from the DOMDocument passed as a parameter.
 	 *
 	 * @since 0.2
+	 * @see AMP_DOM_Utils::get_content_from_dom_node() Reciprocal function.
 	 *
 	 * @param DOMDocument $dom Represents an HTML document from which to extract HTML content.
 	 *
-	 * @return string Returns the HTML content represented in the DOMDocument
+	 * @return string Returns the HTML content of the body element represented in the DOMDocument.
 	 */
 	public static function get_content_from_dom( $dom ) {
 
@@ -305,9 +330,9 @@ public static function get_content_from_dom( $dom ) {
 	/**
 	 * Return valid HTML content extracted from the DOMNode passed as a parameter.
 	 *
-	 * @see Called by function get_content_from_dom()
-	 *
 	 * @since 0.6
+	 * @see AMP_DOM_Utils::get_dom() Where the operations in this method are mirrored.
+	 * @see AMP_DOM_Utils::get_content_from_dom() Reciprocal function.
 	 * @todo In the future consider an AMP_DOMDocument subclass that does this automatically at saveHTML(). See <https://github.com/Automattic/amp-wp/pull/895/files#r163825513>.
 	 *
 	 * @param DOMDocument $dom  Represents an HTML document.
@@ -338,6 +363,15 @@ public static function get_content_from_dom_node( $dom, $node ) {
 			return '';
 		}
 
+		// Restore noscript elements which were temporarily removed to prevent libxml<2.8 parsing problems.
+		if ( version_compare( LIBXML_DOTTED_VERSION, '2.8', '<' ) ) {
+			$html = str_replace(
+				array_keys( self::$noscript_placeholder_comments ),
+				array_values( self::$noscript_placeholder_comments ),
+				$html
+			);
+		}
+
 		$html = self::restore_amp_bind_attributes( $html );
 
 		// Restore amp-mustache placeholders which were replaced to prevent URL-encoded corruption by saveHTML.
diff --git a/tests/test-amp-helper-functions.php b/tests/test-amp-helper-functions.php
index f7cde3d0e7a..d6846c33708 100644
--- a/tests/test-amp-helper-functions.php
+++ b/tests/test-amp-helper-functions.php
@@ -196,6 +196,16 @@ public function test_amp_get_content_sanitizers() {
 		$this->assertEquals( 'amp_content_sanitizers', $this->last_filter_call['current_filter'] );
 		$this->assertEquals( $handlers, $this->last_filter_call['args'][0] );
 		$this->assertEquals( $post, $this->last_filter_call['args'][1] );
+
+		// Make sure the style and whitelist sanitizers are always at the end, even after filtering.
+		add_filter( 'amp_content_sanitizers', function( $classes ) {
+			$classes['Even_After_Whitelist_Sanitizer'] = array();
+			return $classes;
+		} );
+		$orderd_sanitizers = array_keys( amp_get_content_sanitizers() );
+		$this->assertEquals( 'Even_After_Whitelist_Sanitizer', $orderd_sanitizers[ count( $orderd_sanitizers ) - 3 ] );
+		$this->assertEquals( 'AMP_Style_Sanitizer', $orderd_sanitizers[ count( $orderd_sanitizers ) - 2 ] );
+		$this->assertEquals( 'AMP_Tag_And_Attribute_Sanitizer', $orderd_sanitizers[ count( $orderd_sanitizers ) - 1 ] );
 	}
 
 	/**
diff --git a/tests/test-amp-style-sanitizer.php b/tests/test-amp-style-sanitizer.php
index 98d8e868f53..9280a2117b4 100644
--- a/tests/test-amp-style-sanitizer.php
+++ b/tests/test-amp-style-sanitizer.php
@@ -5,6 +5,8 @@
  * @package AMP
  */
 
+// phpcs:disable WordPress.Arrays.MultipleStatementAlignment.DoubleArrowNotAligned
+
 /**
  * Test AMP_Style_Sanitizer.
  */
@@ -15,7 +17,7 @@ class AMP_Style_Sanitizer_Test extends WP_UnitTestCase {
 	 *
 	 * @return array
 	 */
-	public function get_data() {
+	public function get_body_style_attribute_data() {
 		return array(
 			'empty' => array(
 				'',
@@ -116,10 +118,41 @@ public function get_data() {
 					'div > span { font-weight:bold; font-style: italic; }',
 				),
 			),
+		);
+	}
+
+	/**
+	 * Test sanitizer for style attributes that appear in the body.
+	 *
+	 * @dataProvider get_body_style_attribute_data
+	 * @param string $source               Source.
+	 * @param string $expected_content     Expected content.
+	 * @param string $expected_stylesheets Expected stylesheets.
+	 */
+	public function test_body_style_attribute_sanitizer( $source, $expected_content, $expected_stylesheets ) {
+		$dom = AMP_DOM_Utils::get_dom_from_content( $source );
 
-			'styles_in_head_and_body_both_handled' => array(
-				'<html amp><head><meta charset="utf-8"><style amp-custom>b {color:red !important}</style><style amp-custom>i {color:blue}</style><style>u {color:green}</style></head><body><style>s {color:yellow}</style></body></html>',
-				'<html amp><head><meta charset="utf-8"></head><body></body></html>',
+		$sanitizer = new AMP_Style_Sanitizer( $dom );
+		$sanitizer->sanitize();
+
+		// Test content.
+		$content = AMP_DOM_Utils::get_content_from_dom( $dom );
+		$content = preg_replace( '/(?<=>)\s+(?=<)/', '', $content );
+		$this->assertEquals( $expected_content, $content );
+
+		// Test stylesheet.
+		$this->assertEquals( $expected_stylesheets, array_values( $sanitizer->get_stylesheets() ) );
+	}
+
+	/**
+	 * Get link and style test data.
+	 *
+	 * @return array
+	 */
+	public function get_link_and_style_test_data() {
+		return array(
+			'multiple_amp_custom_and_other_styles' => array(
+				'<html amp><head><meta charset="utf-8"><style amp-custom>b {color:red !important}</style><style amp-custom>i {color:blue}</style><style type="text/css">u {color:green}</style></head><body><style>s {color:yellow} /* So !important! */</style></body></html>',
 				array(
 					'b {color:red}',
 					'i {color:blue}',
@@ -127,24 +160,29 @@ public function get_data() {
 					's {color:yellow}',
 				),
 			),
+			'style_eleemnts_with_link_elements' => array(
+				sprintf(
+					'<html amp><head><meta charset="utf-8"><style type="text/css">strong.before-dashicon {color:green}</style><link rel="stylesheet" href="%s"><style type="text/css">strong.after-dashicon {color:green}</style></head><body><style>s {color:yellow !important}</style></body></html>', // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
+					includes_url( 'css/dashicons.css' )
+				),
+				array(
+					'strong.before-dashicon',
+					'.dashicons-dashboard:before',
+					'strong.after-dashicon',
+					's {color:yellow}',
+				),
+			),
 		);
 	}
 
 	/**
-	 * Test sanitizer.
+	 * Test style elements and link elements.
 	 *
-	 * @dataProvider get_data
+	 * @dataProvider get_link_and_style_test_data
 	 * @param string $source               Source.
-	 * @param string $expected_content     Expected content.
-	 * @param string $expected_stylesheets Expected stylesheets.
+	 * @param array  $expected_stylesheets Expected stylesheets.
 	 */
-	public function test_sanitizer( $source, $expected_content, $expected_stylesheets ) {
-		$html_doc_format = '<html amp><head><meta charset="utf-8"></head><body><!-- before -->%s<!-- after --></body></html>';
-		if ( false === strpos( $source, '<html' ) ) {
-			$source           = sprintf( $html_doc_format, $source );
-			$expected_content = sprintf( $html_doc_format, $expected_content );
-		}
-
+	public function test_link_and_style_elements( $source, $expected_stylesheets ) {
 		$dom = AMP_DOM_Utils::get_dom( $source );
 
 		$sanitizer = new AMP_Style_Sanitizer( $dom, array(
@@ -152,16 +190,18 @@ public function test_sanitizer( $source, $expected_content, $expected_stylesheet
 		) );
 		$sanitizer->sanitize();
 
-		$whitelist_sanitizer = new AMP_Tag_And_Attribute_Sanitizer( $dom );
+		$whitelist_sanitizer = new AMP_Tag_And_Attribute_Sanitizer( $dom, array(
+			'use_document_element' => true,
+		) );
 		$whitelist_sanitizer->sanitize();
 
-		// Test content.
-		$content = AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
-		$content = preg_replace( '/(?<=>)\s+(?=<)/', '', $content );
-		$this->assertEquals( $expected_content, $content );
-
-		// Test stylesheet.
-		$this->assertEquals( $expected_stylesheets, array_values( $sanitizer->get_stylesheets() ) );
+		$sanitized_html     = AMP_DOM_Utils::get_content_from_dom_node( $dom, $dom->documentElement );
+		$actual_stylesheets = array_values( $sanitizer->get_stylesheets() );
+		$this->assertCount( count( $expected_stylesheets ), $actual_stylesheets );
+		foreach ( $expected_stylesheets as $i => $expected_stylesheet ) {
+			$this->assertContains( $expected_stylesheet, $actual_stylesheets[ $i ] );
+			$this->assertContains( $expected_stylesheet, $sanitized_html );
+		}
 	}
 
 	/**
@@ -212,4 +252,124 @@ public function test_keyframe_sanitizer( $source, $expected = null ) {
 		$content = preg_replace( '/(?<=>)\s+(?=<)/', '', $content );
 		$this->assertEquals( $expected, $content );
 	}
+
+	/**
+	 * Get stylesheet URLs.
+	 *
+	 * @returns array Stylesheet URL data.
+	 */
+	public function get_stylesheet_urls() {
+		return array(
+			'theme_stylesheet_without_host' => array(
+				'/wp-content/themes/twentyseventeen/style.css',
+				WP_CONTENT_DIR . '/themes/twentyseventeen/style.css',
+			),
+			'theme_stylesheet_with_host' => array(
+				WP_CONTENT_URL . '/themes/twentyseventeen/style.css',
+				WP_CONTENT_DIR . '/themes/twentyseventeen/style.css',
+			),
+			'dashicons_without_host' => array(
+				'/wp-includes/css/dashicons.css',
+				ABSPATH . WPINC . '/css/dashicons.css',
+			),
+			'dashicons_with_host' => array(
+				includes_url( 'css/dashicons.css' ),
+				ABSPATH . WPINC . '/css/dashicons.css',
+			),
+			'admin_without_host' => array(
+				'/wp-admin/css/common.css',
+				ABSPATH . 'wp-admin/css/common.css',
+			),
+			'admin_with_host' => array(
+				admin_url( 'css/common.css' ),
+				ABSPATH . 'wp-admin/css/common.css',
+			),
+			'amp_css_bad_file_extension' => array(
+				content_url( 'themes/twentyseventeen/index.php' ),
+				null,
+				'amp_css_bad_file_extension',
+			),
+			'amp_css_path_not_found' => array(
+				content_url( 'themes/twentyseventeen/404.css' ),
+				null,
+				'amp_css_path_not_found',
+			),
+		);
+	}
+
+	/**
+	 * Tests get_validated_css_file_path.
+	 *
+	 * @dataProvider get_stylesheet_urls
+	 * @covers AMP_Style_Sanitizer::get_validated_css_file_path()
+	 * @param string      $source     Source URL.
+	 * @param string|null $expected   Expected path or null if error.
+	 * @param string      $error_code Error code. Optional.
+	 */
+	public function test_get_validated_css_file_path( $source, $expected, $error_code = null ) {
+		$dom = AMP_DOM_Utils::get_dom( '<html></html>' );
+
+		$sanitizer = new AMP_Style_Sanitizer( $dom );
+		$actual    = $sanitizer->get_validated_css_file_path( $source );
+		if ( isset( $error_code ) ) {
+			$this->assertInstanceOf( 'WP_Error', $actual );
+			$this->assertEquals( $error_code, $actual->get_error_code() );
+		} else {
+			$this->assertEquals( $expected, $actual );
+		}
+	}
+
+	/**
+	 * Get font url test data.
+	 *
+	 * @return array Data.
+	 */
+	public function get_font_urls() {
+		return array(
+			'tangerine'   => array(
+				'https://fonts.googleapis.com/css?family=Tangerine',
+				true,
+			),
+			'typekit'     => array(
+				'https://use.typekit.net/abc.css',
+				true,
+			),
+			'fontscom'    => array(
+				'https://fast.fonts.net/abc.css',
+				true,
+			),
+			'fontawesome' => array(
+				'https://maxcdn.bootstrapcdn.com/font-awesome/123/css/font-awesome.min.css',
+				true,
+			),
+			'fontbad' => array(
+				'https://bad.example.com/font.css',
+				false,
+			),
+		);
+	}
+
+	/**
+	 * Tests that font URLs get validated.
+	 *
+	 * @dataProvider get_font_urls
+	 * @param string $url  Font URL.
+	 * @param bool   $pass Whether the font URL is ok.
+	 */
+	public function test_font_urls( $url, $pass ) {
+		$dom = AMP_DOM_Utils::get_dom( sprintf( '<html><head><link rel="stylesheet" href="%s"></head></html>', $url ) ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
+
+		$sanitizer = new AMP_Style_Sanitizer( $dom, array(
+			'use_document_element' => true,
+		) );
+		$sanitizer->sanitize();
+
+		$link = $dom->getElementsByTagName( 'link' )->item( 0 );
+		if ( $pass ) {
+			$this->assertInstanceOf( 'DOMElement', $link );
+			$this->assertEquals( $url, $link->getAttribute( 'href' ) );
+		} else {
+			$this->assertEmpty( $link );
+		}
+	}
 }
diff --git a/tests/test-class-amp-theme-support.php b/tests/test-class-amp-theme-support.php
index 84dc66300be..3429ab836cb 100644
--- a/tests/test-class-amp-theme-support.php
+++ b/tests/test-class-amp-theme-support.php
@@ -113,6 +113,8 @@ public function test_prepare_response() {
 					data-aax_pubname="test123"
 					data-aax_src="302"></amp-ad>
 				<?php wp_footer(); ?>
+
+				<style>body { background: black; }</style>
 			</body>
 		</html>
 		<?php
@@ -122,7 +124,7 @@ public function test_prepare_response() {
 		$this->assertContains( '<meta charset="' . get_bloginfo( 'charset' ) . '">', $sanitized_html );
 		$this->assertContains( '<meta name="viewport" content="width=device-width,minimum-scale=1">', $sanitized_html );
 		$this->assertContains( '<style amp-boilerplate>', $sanitized_html );
-		$this->assertContains( '<style amp-custom>', $sanitized_html );
+		$this->assertContains( '<style amp-custom>body { background: black; }', $sanitized_html );
 		$this->assertContains( '<script async src="https://cdn.ampproject.org/v0.js"', $sanitized_html ); // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedScript
 		$this->assertContains( '<meta name="generator" content="AMP Plugin', $sanitized_html );
 
@@ -203,25 +205,6 @@ public function test_purge_amp_query_vars() {
 		// phpcs:enable WordPress.Security.NonceVerification.NoNonceVerification
 	}
 
-	/**
-	 * Test get_amp_styles().
-	 *
-	 * @covers AMP_Theme_Support::get_amp_styles()
-	 */
-	public function test_get_amp_styles() {
-		$styles = AMP_Theme_Support::get_amp_styles( array() );
-		$this->assertStringStartsWith( amp_get_boilerplate_code(), $styles );
-
-		$injected_css = 'b strong { color: red; }';
-		add_filter( 'amp_custom_styles', function( $css ) use ( $injected_css ) {
-			return $css . $injected_css;
-		} );
-		$styles = AMP_Theme_Support::get_amp_styles( array() );
-
-		$this->assertStringStartsWith( amp_get_boilerplate_code(), $styles );
-		$this->assertContains( $injected_css, $styles );
-	}
-
 	/**
 	 * Test get_amp_scripts().
 	 *
diff --git a/tests/test-class-amp-wp-styles.php b/tests/test-class-amp-wp-styles.php
deleted file mode 100644
index 1882acf0c13..00000000000
--- a/tests/test-class-amp-wp-styles.php
+++ /dev/null
@@ -1,221 +0,0 @@
-<?php
-/**
- * Tests for AMP_WP_Styles.
- *
- * @package AMP
- * @since 0.6
- */
-
-/**
- * Tests for AMP_WP_Styles.
- *
- * @covers AMP_WP_Styles
- */
-class Test_AMP_WP_Styles extends WP_UnitTestCase {
-
-	/**
-	 * Tear down.
-	 */
-	public function tearDown() {
-		global $wp_styles;
-		parent::tearDown();
-		$wp_styles = null;
-	}
-
-	/**
-	 * Test that wp_styles() returns AMP_WP_Styles.
-	 *
-	 * @covers wp_styles()
-	 */
-	public function test_wp_styles() {
-		AMP_Theme_Support::override_wp_styles();
-		$this->assertInstanceOf( 'AMP_WP_Styles', wp_styles() );
-	}
-
-	/**
-	 * Return bad URL.
-	 *
-	 * @return string Bad URL.
-	 */
-	public function return_bad_style_loader_src() {
-		return site_url( 'wp-config.php' );
-	}
-
-	/**
-	 * Tests get_validated_css_file_path.
-	 *
-	 * @covers AMP_WP_Styles::get_validated_css_file_path()
-	 */
-	public function test_get_validated_css_file_path() {
-		$wp_styles = AMP_Theme_Support::override_wp_styles();
-
-		// Theme.
-		$expected = WP_CONTENT_DIR . '/themes/twentyseventeen/style.css';
-		$path     = $wp_styles->get_validated_css_file_path( '/wp-content/themes/twentyseventeen/style.css', 'twentyseventeen-style' );
-		$this->assertEquals( $expected, $path );
-		$path = $wp_styles->get_validated_css_file_path( content_url( 'themes/twentyseventeen/style.css' ), 'dashicons' );
-		$this->assertEquals( $expected, $path );
-
-		add_filter( 'style_loader_src', array( $this, 'return_bad_style_loader_src' ) );
-		$r = $wp_styles->get_validated_css_file_path( content_url( 'themes/twentyseventeen/style.css' ), 'dashicons' );
-		$this->assertInstanceOf( 'WP_Error', $r );
-		$this->assertEquals( 'amp_css_bad_file_extension', $r->get_error_code() );
-		remove_filter( 'style_loader_src', array( $this, 'return_bad_style_loader_src' ) );
-
-		// Includes.
-		$expected = ABSPATH . WPINC . '/css/dashicons.css';
-		$path     = $wp_styles->get_validated_css_file_path( '/wp-includes/css/dashicons.css', 'dashicons' );
-		$this->assertEquals( $expected, $path );
-		$path = $wp_styles->get_validated_css_file_path( includes_url( 'css/dashicons.css' ), 'dashicons' );
-		$this->assertEquals( $expected, $path );
-
-		// Admin.
-		$expected = ABSPATH . 'wp-admin/css/common.css';
-		$path     = $wp_styles->get_validated_css_file_path( '/wp-admin/css/common.css', 'dashicons' );
-		$this->assertEquals( $expected, $path );
-		$path = $wp_styles->get_validated_css_file_path( admin_url( 'css/common.css' ), 'common' );
-		$this->assertEquals( $expected, $path );
-
-		// Bad URLs.
-		$r = $wp_styles->get_validated_css_file_path( content_url( 'themes/twentyseventeen/index.php' ), 'bad' );
-		$this->assertInstanceOf( 'WP_Error', $r );
-		$this->assertEquals( 'amp_css_bad_file_extension', $r->get_error_code() );
-
-		$r = $wp_styles->get_validated_css_file_path( content_url( 'themes/twentyseventeen/404.css' ), 'bad' );
-		$this->assertInstanceOf( 'WP_Error', $r );
-		$this->assertEquals( 'amp_css_path_not_found', $r->get_error_code() );
-
-		$r = $wp_styles->get_validated_css_file_path( get_template_directory() . '/style.css', 'bad' );
-		$this->assertInstanceOf( 'WP_Error', $r );
-		$this->assertEquals( 'amp_css_path_not_found', $r->get_error_code() );
-	}
-
-	/**
-	 * Tests test_do_item.
-	 *
-	 * @covers AMP_WP_Styles::do_item()
-	 */
-	public function test_do_item() {
-		$wp_styles = AMP_Theme_Support::override_wp_styles();
-		$this->assertFalse( $wp_styles->do_item( 'non-existent' ) );
-
-		// Conditional stylesheets are ignored.
-		$wp_styles->registered['buttons-conditional']                       = clone $wp_styles->registered['buttons'];
-		$wp_styles->registered['buttons-conditional']->extra['conditional'] = 'IE8';
-		$this->assertFalse( $wp_styles->do_item( 'buttons-conditional' ) );
-
-		// Alt stylesheets are ignored.
-		$wp_styles->registered['buttons-alt']               = clone $wp_styles->registered['buttons'];
-		$wp_styles->registered['buttons-alt']->extra['alt'] = true;
-		$this->assertFalse( $wp_styles->do_item( 'buttons-alt' ) );
-
-		// Media.
-		$wp_styles->registered['admin-bar-print']       = clone $wp_styles->registered['admin-bar'];
-		$wp_styles->registered['admin-bar-print']->args = 'x_virtual_reality';
-		$wp_styles->print_code                          = '';
-		$this->assertTrue( $wp_styles->do_item( 'admin-bar-print' ) );
-		$this->assertStringStartsWith( '@media x_virtual_reality {', $wp_styles->print_code );
-
-		// Inline style.
-		$wp_styles->print_code = '';
-		$inline                = '/* INLINE STYLE FOR BUTTONS */';
-		wp_add_inline_style( 'buttons', $inline );
-		$this->assertTrue( $wp_styles->do_item( 'buttons' ) );
-		$wp_styles->do_item( 'buttons' );
-		$this->assertStringEndsWith( $inline, $wp_styles->print_code );
-		$this->assertContains( '.wp-core-ui .button-link', $wp_styles->print_code );
-
-		// Buttons bad.
-		$this->setExpectedException( 'PHPUnit_Framework_Error_Warning', 'Skipped stylesheet buttons-bad which does not have recognized CSS file extension (http://example.org/wp-config.php).' );
-		$wp_styles->base_url                       = 'http://example.org';
-		$wp_styles->print_code                     = '';
-		$wp_styles->registered['buttons-bad']      = clone $wp_styles->registered['buttons'];
-		$wp_styles->registered['buttons-bad']->src = $this->return_bad_style_loader_src();
-		$this->assertFalse( $wp_styles->do_item( 'buttons-bad' ) );
-		$this->assertEmpty( $wp_styles->print_code );
-	}
-
-	/**
-	 * Tests test_do_item for stylesheets.
-	 *
-	 * @covers AMP_WP_Styles::do_item()
-	 */
-	public function test_do_item_font_stylesheet() {
-		$wp_styles = AMP_Theme_Support::override_wp_styles();
-
-		$ok_styles = array(
-			'tangerine'   => 'https://fonts.googleapis.com/css?family=Tangerine',
-			'typekit'     => 'https://use.typekit.net/abc.css',
-			'fontscom'    => 'https://fast.fonts.net/abc.css',
-			'fontawesome' => 'https://maxcdn.bootstrapcdn.com/font-awesome/123/css/font-awesome.min.css',
-		);
-		foreach ( $ok_styles as $handle => $src ) {
-			$wp_styles->add( $handle, $src );
-			ob_start();
-			$this->assertTrue( $wp_styles->do_item( $handle ) );
-			$output = ob_get_clean();
-			$this->assertContains( '<link', $output );
-			$this->assertContains( $src, $output );
-		}
-
-		$this->setExpectedException( 'PHPUnit_Framework_Error_Warning', 'Unable to locate filesystem path for stylesheet fontillegal (https://illegal.example.com/forbidden.css).' );
-		$handle = 'fontillegal';
-		$wp_styles->add( $handle, 'https://illegal.example.com/forbidden.css' );
-		$this->assertFalse( $wp_styles->do_item( $handle ) );
-	}
-
-	/**
-	 * Tests do_locale_stylesheet.
-	 *
-	 * @covers AMP_WP_Styles::do_locale_stylesheet()
-	 */
-	public function test_do_locale_stylesheet() {
-		$wp_styles = AMP_Theme_Support::override_wp_styles();
-		add_filter( 'locale_stylesheet_uri', '__return_false' );
-		$this->assertFalse( $wp_styles->do_locale_stylesheet() );
-		$this->assertEmpty( $wp_styles->print_code );
-		remove_filter( 'locale_stylesheet_uri', '__return_false' );
-
-		add_filter( 'locale_stylesheet_uri', array( $this, 'return_css_url' ) );
-		$this->assertTrue( $wp_styles->do_locale_stylesheet() );
-		$this->assertNotEmpty( $wp_styles->print_code );
-	}
-
-	/**
-	 * Tests do_custom_css.
-	 *
-	 * @covers AMP_WP_Styles::do_custom_css()
-	 */
-	public function test_do_custom_css() {
-		$wp_styles = AMP_Theme_Support::override_wp_styles();
-		$this->assertFalse( $wp_styles->do_custom_css() );
-		$this->assertEmpty( $wp_styles->print_code );
-
-		add_filter( 'wp_get_custom_css', array( $this, 'return_css_rule' ) );
-		$wp_styles = null;
-		$wp_styles = AMP_Theme_Support::override_wp_styles();
-		$wp_styles->do_custom_css();
-		$this->assertEquals( $this->return_css_rule(), $wp_styles->print_code );
-		$wp_styles->do_custom_css();
-		$this->assertEquals( $this->return_css_rule(), $wp_styles->print_code );
-		remove_filter( 'wp_get_custom_css', array( $this, 'return_css_rule' ) );
-	}
-
-	/**
-	 * Return sample CSS rule.
-	 *
-	 * @return string
-	 */
-	public function return_css_rule() {
-		return 'body { color:black; }';
-	}
-
-	/**
-	 * Return URL to valid CSS file.
-	 *
-	 * @return string
-	 */
-	public function return_css_url() {
-		return includes_url( 'css/buttons.css' );
-	}
-}