diff --git a/includes/amp-helper-functions.php b/includes/amp-helper-functions.php
index 4718918448c..35e5b16cc1e 100644
--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -524,6 +524,10 @@ function amp_get_content_sanitizers( $post = null ) {
 	 */
 	$sanitizers = apply_filters( 'amp_content_sanitizers',
 		array(
+			'AMP_Core_Theme_Sanitizer'        => array(
+				'template'   => get_template(),
+				'stylesheet' => get_stylesheet(),
+			),
 			'AMP_Img_Sanitizer'               => array(),
 			'AMP_Form_Sanitizer'              => array(),
 			'AMP_Comments_Sanitizer'          => array(),
diff --git a/includes/class-amp-autoloader.php b/includes/class-amp-autoloader.php
index 416b8e42081..1ad252863eb 100644
--- a/includes/class-amp-autoloader.php
+++ b/includes/class-amp-autoloader.php
@@ -82,6 +82,7 @@ class AMP_Autoloader {
 		'AMP_Style_Sanitizer'                         => 'includes/sanitizers/class-amp-style-sanitizer',
 		'AMP_Tag_And_Attribute_Sanitizer'             => 'includes/sanitizers/class-amp-tag-and-attribute-sanitizer',
 		'AMP_Video_Sanitizer'                         => 'includes/sanitizers/class-amp-video-sanitizer',
+		'AMP_Core_Theme_Sanitizer'                    => 'includes/sanitizers/class-amp-core-theme-sanitizer',
 		'AMP_Customizer_Design_Settings'              => 'includes/settings/class-amp-customizer-design-settings',
 		'AMP_Customizer_Settings'                     => 'includes/settings/class-amp-customizer-settings',
 		'AMP_Content'                                 => 'includes/templates/class-amp-content',
diff --git a/includes/class-amp-theme-support.php b/includes/class-amp-theme-support.php
index 2ecb8ebc9dc..5097844e350 100644
--- a/includes/class-amp-theme-support.php
+++ b/includes/class-amp-theme-support.php
@@ -159,6 +159,12 @@ public static function finish_init() {
 		self::$sanitizer_classes = amp_get_content_sanitizers();
 		self::$sanitizer_classes = AMP_Validation_Manager::filter_sanitizer_args( self::$sanitizer_classes );
 		self::$embed_handlers    = self::register_content_embed_handlers();
+
+		foreach ( self::$sanitizer_classes as $sanitizer_class => $args ) {
+			if ( method_exists( $sanitizer_class, 'add_buffering_hooks' ) ) {
+				call_user_func( array( $sanitizer_class, 'add_buffering_hooks' ), $args );
+			}
+		}
 	}
 
 	/**
@@ -303,6 +309,10 @@ public static function add_hooks() {
 		add_action( 'comment_form', array( __CLASS__, 'amend_comment_form' ), 100 );
 		remove_action( 'comment_form', 'wp_comment_form_unfiltered_html_nonce' );
 		add_filter( 'wp_kses_allowed_html', array( __CLASS__, 'whitelist_layout_in_wp_kses_allowed_html' ), 10 );
+		add_filter( 'get_header_image_tag', array( __CLASS__, 'conditionally_output_header' ), 10, 3 );
+		add_action( 'wp_enqueue_scripts', function() {
+			wp_dequeue_script( 'comment-reply' ); // Handled largely by AMP_Comments_Sanitizer and *reply* methods in this class.
+		} );
 
 		// @todo Add character conversion.
 	}
@@ -1264,4 +1274,89 @@ public static function enqueue_assets() {
 		// Enqueue default styles expected by sanitizer.
 		wp_enqueue_style( 'amp-default', amp_get_asset_url( 'css/amp-default.css' ), array(), AMP__VERSION );
 	}
+
+	/**
+	 * Conditionally replace the header image markup with a header video or image.
+	 *
+	 * This is JS-driven in Core themes like Twenty Sixteen and Twenty Seventeen.
+	 * So in order for the header video to display,
+	 * this replaces the markup of the header image.
+	 *
+	 * @since 1.0
+	 * @link https://github.com/WordPress/wordpress-develop/blob/d002fde80e5e3a083e5f950313163f566561517f/src/wp-includes/js/wp-custom-header.js#L54
+	 * @param string $html The image markup to filter.
+	 * @param array  $header The header config array.
+	 * @param array  $atts The image markup attributes.
+	 * @return string $html Filtered markup.
+	 */
+	public static function conditionally_output_header( $html, $header, $atts ) {
+		unset( $header );
+		if ( ! is_header_video_active() ) {
+			return $html;
+		};
+
+		if ( ! has_header_video() ) {
+			return AMP_HTML_Utils::build_tag( 'amp-img', $atts );
+		}
+
+		return self::output_header_video( $atts );
+	}
+
+	/**
+	 * Replace the header image markup with a header video.
+	 *
+	 * @since 1.0
+	 * @link https://github.com/WordPress/wordpress-develop/blob/d002fde80e5e3a083e5f950313163f566561517f/src/wp-includes/js/wp-custom-header.js#L54
+	 * @link https://github.com/WordPress/wordpress-develop/blob/d002fde80e5e3a083e5f950313163f566561517f/src/wp-includes/js/wp-custom-header.js#L78
+	 *
+	 * @param array $atts The header tag attributes array.
+	 * @return string $html Filtered markup.
+	 */
+	public static function output_header_video( $atts ) {
+		// Remove the script for video.
+		wp_deregister_script( 'wp-custom-header' );
+		$video_settings = get_header_video_settings();
+
+		$parsed_url       = wp_parse_url( $video_settings['videoUrl'] );
+		$query            = isset( $parsed_url['query'] ) ? wp_parse_args( $parsed_url['query'] ) : null;
+		$video_attributes = array(
+			'media'    => '(min-width: ' . $video_settings['minWidth'] . 'px)',
+			'width'    => $video_settings['width'],
+			'height'   => $video_settings['height'],
+			'layout'   => 'responsive',
+			'autoplay' => '',
+			'id'       => 'wp-custom-header-video',
+		);
+
+		// Create image banner to stay behind the video.
+		$image_header = AMP_HTML_Utils::build_tag( 'amp-img', $atts );
+
+		// If the video URL is for YouTube, return an <amp-youtube> element.
+		if ( isset( $parsed_url['host'], $query['v'] ) && ( false !== strpos( $parsed_url['host'], 'youtube' ) ) ) {
+			$video_header = AMP_HTML_Utils::build_tag(
+				'amp-youtube',
+				array_merge(
+					$video_attributes,
+					array(
+						'data-videoid'        => $query['v'],
+						'data-param-rel'      => '0', // Don't show related videos.
+						'data-param-showinfo' => '0', // Don't show video title at the top.
+						'data-param-controls' => '0', // Don't show video controls.
+					)
+				)
+			);
+		} else {
+			$video_header = AMP_HTML_Utils::build_tag(
+				'amp-video',
+				array_merge(
+					$video_attributes,
+					array(
+						'src' => $video_settings['videoUrl'],
+					)
+				)
+			);
+		}
+
+		return $image_header . $video_header;
+	}
 }
diff --git a/includes/sanitizers/class-amp-base-sanitizer.php b/includes/sanitizers/class-amp-base-sanitizer.php
index 71aaec157a6..55e42505339 100644
--- a/includes/sanitizers/class-amp-base-sanitizer.php
+++ b/includes/sanitizers/class-amp-base-sanitizer.php
@@ -121,6 +121,22 @@ public function __construct( $dom, $args = array() ) {
 		}
 	}
 
+	/**
+	 * Add filters to manipulate output during output buffering before the DOM is constructed.
+	 *
+	 * Add actions and filters before the page is rendered so that the sanitizer can fix issues during output buffering.
+	 * This provides an alternative to manipulating the DOM in the sanitize method. This is a static function because
+	 * it is invoked before the class is instantiated, as the DOM is not available yet. This method is only called
+	 * when 'amp' theme support is present. It is conceptually similar to the AMP_Base_Embed_Handler class's register_embed
+	 * method.
+	 *
+	 * @since 1.0
+	 * @see \AMP_Base_Embed_Handler::register_embed()
+	 *
+	 * @param array $args Args.
+	 */
+	public static function add_buffering_hooks( $args = array() ) {}
+
 	/**
 	 * Get mapping of HTML selectors to the AMP component selectors which they may be converted into.
 	 *
diff --git a/includes/sanitizers/class-amp-core-theme-sanitizer.php b/includes/sanitizers/class-amp-core-theme-sanitizer.php
new file mode 100644
index 00000000000..b07854bb1a7
--- /dev/null
+++ b/includes/sanitizers/class-amp-core-theme-sanitizer.php
@@ -0,0 +1,943 @@
+<?php
+/**
+ * Class AMP_Core_Theme_Sanitizer.
+ *
+ * @package AMP
+ * @since 1.0
+ */
+
+/**
+ * Class AMP_Core_Theme_Sanitizer
+ *
+ * Fixes up common issues in core themes and others.
+ *
+ * @see AMP_Validation_Error_Taxonomy::accept_core_theme_validation_errors()
+ * @since 1.0
+ */
+class AMP_Core_Theme_Sanitizer extends AMP_Base_Sanitizer {
+
+	/**
+	 * Array of flags used to control sanitization.
+	 *
+	 * @since 1.0
+	 * @var array {
+	 *      @type string $stylesheet     Stylesheet slug.
+	 *      @type string $template       Template slug.
+	 *      @type array  $theme_features List of theme features that need to be applied. Features are method names,
+	 * }
+	 */
+	protected $args;
+
+	/**
+	 * Body element.
+	 *
+	 * @since 1.0
+	 * @var DOMElement
+	 */
+	protected $body;
+
+	/**
+	 * XPath.
+	 *
+	 * @since 1.0
+	 * @var DOMXPath
+	 */
+	protected $xpath;
+
+	/**
+	 * Config for features needed by themes.
+	 *
+	 * @since 1.0
+	 * @var array
+	 */
+	protected static $theme_features = array(
+		// Twenty Seventeen.
+		'twentyseventeen' => array(
+			// @todo Try to implement belowEntryMetaClass().
+			'dequeue_scripts'                     => array(
+				'twentyseventeen-html5', // Only relevant for IE<9.
+				'twentyseventeen-global', // There are somethings not yet implemented in AMP. See todos below.
+				'jquery-scrollto', // Implemented via add_smooth_scrolling().
+				'twentyseventeen-navigation', // Handled by add_nav_menu_styles, add_nav_menu_toggle, add_nav_sub_menu_buttons.
+				'twentyseventeen-skip-link-focus-fix', // Only needed by IE11 and when admin bar is present.
+			),
+			'remove_actions'                      => array(
+				'wp_head' => array(
+					'twentyseventeen_javascript_detection', // AMP is essentially no-js, with any interactively added explicitly via amp-bind.
+				),
+			),
+			'force_svg_support'                   => array(),
+			'force_fixed_background_support'      => array(),
+			'add_twentyseventeen_masthead_styles' => array(),
+			'add_twentyseventeen_sticky_nav_menu' => array(),
+			'add_has_header_video_body_class'     => array(),
+			'add_nav_menu_styles'                 => array(),
+			'add_nav_menu_toggle'                 => array(),
+			'add_nav_sub_menu_buttons'            => array(),
+			'add_smooth_scrolling'                => array(
+				'//header[@id = "masthead"]//a[ contains( @class, "menu-scroll-down" ) ]',
+			),
+			'set_twentyseventeen_quotes_icon'     => array(),
+		),
+
+		// Twenty Sixteen.
+		'twentysixteen'   => array(
+			// @todo Figure out an AMP solution for onResizeARIA().
+			// @todo Try to implement belowEntryMetaClass().
+			'dequeue_scripts'          => array(
+				'twentysixteen-script',
+				'twentysixteen-html5', // Only relevant for IE<9.
+				'twentysixteen-keyboard-image-navigation', // AMP does not yet allow for listening to keydown events.
+				'twentysixteen-skip-link-focus-fix', // Only needed by IE11 and when admin bar is present.
+			),
+			'remove_actions'           => array(
+				'wp_head' => array(
+					'twentysixteen_javascript_detection', // AMP is essentially no-js, with any interactively added explicitly via amp-bind.
+				),
+			),
+			'add_nav_menu_styles'      => array(),
+			'add_nav_menu_toggle'      => array(),
+			'add_nav_sub_menu_buttons' => array(),
+		),
+
+		// Twenty Fifteen.
+		'twentyfifteen'   => array(
+			// @todo Figure out an AMP solution for onResizeARIA().
+			'dequeue_scripts'          => array(
+				'twentyfifteen-script',
+				'twentyfifteen-keyboard-image-navigation', // AMP does not yet allow for listening to keydown events.
+				'twentyfifteen-skip-link-focus-fix', // Only needed by IE11 and when admin bar is present.
+			),
+			'remove_actions'           => array(
+				'wp_head' => array(
+					'twentyfifteen_javascript_detection', // AMP is essentially no-js, with any interactively added explicitly via amp-bind.
+				),
+			),
+			'add_nav_menu_styles'      => array(),
+			'add_nav_menu_toggle'      => array(),
+			'add_nav_sub_menu_buttons' => array(),
+		),
+	);
+
+	/**
+	 * Get the acceptable validation errors.
+	 *
+	 * @since 1.0
+	 *
+	 * @param string $template Template.
+	 * @return array Acceptable errors.
+	 */
+	public static function get_acceptable_errors( $template ) {
+		switch ( $template ) {
+			case 'twentyfifteen':
+				return array(
+					'removed_unused_css_rules' => true,
+					'illegal_css_at_rule'      => array(
+						array(
+							'at_rule'         => 'viewport',
+							'node_attributes' => array(
+								'id' => 'twentyfifteen-style-css',
+							),
+						),
+						array(
+							'at_rule'         => '-ms-viewport',
+							'node_attributes' => array(
+								'id' => 'twentyfifteen-style-css',
+							),
+						),
+					),
+					'invalid_element'          => array(
+						array(
+							'node_name'       => 'meta',
+							'parent_name'     => 'head',
+							'node_attributes' => array(
+								'name'    => 'viewport',
+								'content' => 'width=device-width',
+							),
+						),
+					),
+				);
+			case 'twentysixteen':
+				return array(
+					'removed_unused_css_rules' => true,
+					'illegal_css_at_rule'      => array(
+						array(
+							'at_rule'         => 'viewport',
+							'node_attributes' => array(
+								'id' => 'twentysixteen-style-css',
+							),
+						),
+						array(
+							'at_rule'         => '-ms-viewport',
+							'node_attributes' => array(
+								'id' => 'twentysixteen-style-css',
+							),
+						),
+					),
+					'invalid_element'          => array(
+						array(
+							'node_name'       => 'meta',
+							'parent_name'     => 'head',
+							'node_attributes' => array(
+								'name'    => 'viewport',
+								'content' => 'width=device-width, initial-scale=1',
+							),
+						),
+					),
+				);
+			case 'twentyseventeen':
+				return array(
+					'removed_unused_css_rules' => true,
+					'invalid_element'          => array(
+						array(
+							'node_name'       => 'meta',
+							'parent_name'     => 'head',
+							'node_attributes' => array(
+								'name'    => 'viewport',
+								'content' => 'width=device-width, initial-scale=1',
+							),
+						),
+					),
+				);
+		}
+		return array();
+	}
+
+	/**
+	 * Get theme config.
+	 *
+	 * @since 1.0
+	 *
+	 * @param string $theme Theme slug.
+	 * @return array Class names.
+	 */
+	protected static function get_theme_config( $theme ) {
+		// phpcs:disable WordPress.WP.I18n.TextDomainMismatch
+		$config = array(
+			'sub_menu_button_class' => 'dropdown-toggle',
+		);
+		switch ( $theme ) {
+			case 'twentyfifteen':
+				return array_merge(
+					$config,
+					array(
+						'nav_container_id'             => 'secondary',
+						'nav_container_toggle_class'   => 'toggled-on',
+						'menu_button_class'            => 'secondary-toggle',
+						'menu_button_xpath'            => '//header[ @id = "masthead" ]//button[ contains( @class, "secondary-toggle" ) ]',
+						'menu_button_toggle_class'     => 'toggled-on',
+						'sub_menu_button_toggle_class' => 'toggle-on',
+						'expand_text '                 => __( 'expand child menu', 'twentyfifteen' ),
+						'collapse_text'                => __( 'collapse child menu', 'twentyfifteen' ),
+					)
+				);
+
+			case 'twentysixteen':
+				return array_merge(
+					$config,
+					array(
+						'nav_container_id'             => 'site-header-menu',
+						'nav_container_toggle_class'   => 'toggled-on',
+						'menu_button_class'            => 'menu-toggle',
+						'menu_button_xpath'            => '//header[@id = "masthead"]//button[ @id = "menu-toggle" ]',
+						'menu_button_toggle_class'     => 'toggled-on',
+						'sub_menu_button_toggle_class' => 'toggled-on',
+						'expand_text '                 => __( 'expand child menu', 'twentysixteen' ),
+						'collapse_text'                => __( 'collapse child menu', 'twentysixteen' ),
+					)
+				);
+
+			case 'twentyseventeen':
+			default:
+				return array_merge(
+					$config,
+					array(
+						'nav_container_id'             => 'site-navigation',
+						'nav_container_toggle_class'   => 'toggled-on',
+						'menu_button_class'            => 'menu-toggle',
+						'menu_button_xpath'            => '//nav[@id = "site-navigation"]//button[ contains( @class, "menu-toggle" ) ]',
+						'menu_button_toggle_class'     => 'toggled-on',
+						'sub_menu_button_toggle_class' => 'toggled-on',
+						'expand_text '                 => __( 'expand child menu', 'twentyseventeen' ),
+						'collapse_text'                => __( 'collapse child menu', 'twentyseventeen' ),
+					)
+				);
+		}
+		// phpcs:enable WordPress.WP.I18n.TextDomainMismatch
+	}
+
+	/**
+	 * Find theme features for core theme.
+	 *
+	 * @since 1.0
+	 *
+	 * @param array $args   Args.
+	 * @param bool  $static Static. that is, whether should run during output buffering.
+	 * @return array Theme features.
+	 */
+	protected static function get_theme_features( $args, $static = false ) {
+		$theme_features   = array();
+		$theme_candidates = wp_array_slice_assoc( $args, array( 'stylesheet', 'template' ) );
+		foreach ( $theme_candidates as $theme_candidate ) {
+			if ( isset( self::$theme_features[ $theme_candidate ] ) ) {
+				$theme_features = self::$theme_features[ $theme_candidate ];
+				break;
+			}
+		}
+
+		// Allow specific theme features to be requested even if the theme is not in core.
+		if ( isset( $args['theme_features'] ) ) {
+			$theme_features = array_merge( $args['theme_features'], $theme_features );
+		}
+
+		$final_theme_features = array();
+		foreach ( $theme_features as $theme_feature => $feature_args ) {
+			if ( ! method_exists( __CLASS__, $theme_feature ) ) {
+				continue;
+			}
+			try {
+				$reflection = new ReflectionMethod( __CLASS__, $theme_feature );
+				if ( $reflection->isStatic() === $static ) {
+					$final_theme_features[ $theme_feature ] = $feature_args;
+				}
+			} catch ( Exception $e ) {
+				unset( $e );
+			}
+		}
+		return $final_theme_features;
+	}
+
+	/**
+	 * Add filters to manipulate output during output buffering before the DOM is constructed.
+	 *
+	 * @since 1.0
+	 *
+	 * @param array $args Args.
+	 */
+	public static function add_buffering_hooks( $args = array() ) {
+		$theme_features = self::get_theme_features( $args, true );
+		foreach ( $theme_features as $theme_feature => $feature_args ) {
+			if ( method_exists( __CLASS__, $theme_feature ) ) {
+				call_user_func( array( __CLASS__, $theme_feature ), $feature_args );
+			}
+		}
+	}
+
+	/**
+	 * Add filter to output the quote icons in front of the article content.
+	 *
+	 * This is only used in Twenty Seventeen.
+	 *
+	 * @since 1.0
+	 * @link https://github.com/WordPress/wordpress-develop/blob/f4580c122b7d0d2d66d22f806c6fe6e11023c6f0/src/wp-content/themes/twentyseventeen/assets/js/global.js#L105-L108
+	 */
+	public static function set_twentyseventeen_quotes_icon() {
+		add_filter( 'the_content', function ( $content ) {
+
+			// Why isn't Twenty Seventeen doing this to begin with? Why is it using JS to add the quote icon?
+			if ( function_exists( 'twentyseventeen_get_svg' ) && 'quote' === get_post_format() ) {
+				$icon    = twentyseventeen_get_svg( array( 'icon' => 'quote-right' ) );
+				$content = preg_replace( '#(<blockquote.*?>)#s', '$1' . $icon, $content );
+			}
+
+			return $content;
+		} );
+	}
+
+	/**
+	 * Fix up core themes to do things in the AMP way.
+	 *
+	 * @since 1.0
+	 */
+	public function sanitize() {
+		$this->body = $this->dom->getElementsByTagName( 'body' )->item( 0 );
+		if ( ! $this->body ) {
+			return;
+		}
+
+		$this->xpath = new DOMXPath( $this->dom );
+
+		$theme_features = self::get_theme_features( $this->args, false );
+		foreach ( $theme_features as $theme_feature => $feature_args ) {
+			if ( method_exists( $this, $theme_feature ) ) {
+				call_user_func( array( $this, $theme_feature ), $feature_args );
+			}
+		}
+	}
+
+	/**
+	 * Dequeue scripts.
+	 *
+	 * @since 1.0
+	 *
+	 * @param string[] $handles Handles, where each item value is the script handle.
+	 */
+	public static function dequeue_scripts( $handles = array() ) {
+		add_action( 'wp_enqueue_scripts', function() use ( $handles ) {
+			foreach ( $handles as $handle ) {
+				wp_dequeue_script( $handle );
+			}
+		}, PHP_INT_MAX );
+	}
+
+	/**
+	 * Remove actions.
+	 *
+	 * @since 1.0
+	 *
+	 * @param array $actions Actions, with action name as key and value being callback.
+	 */
+	public static function remove_actions( $actions = array() ) {
+		foreach ( $actions as $action => $callbacks ) {
+			foreach ( $callbacks as $callback ) {
+				$priority = has_action( $action, $callback );
+				if ( false !== $priority ) {
+					remove_action( $action, $callback, $priority );
+				}
+			}
+		}
+	}
+
+	/**
+	 * Add smooth scrolling from link to target element.
+	 *
+	 * @since 1.0
+	 *
+	 * @param string[] $link_xpaths XPath queries to the links that should smooth scroll.
+	 */
+	public function add_smooth_scrolling( $link_xpaths ) {
+		foreach ( $link_xpaths as $link_xpath ) {
+			foreach ( $this->xpath->query( $link_xpath ) as $link ) {
+				if ( $link instanceof DOMElement && preg_match( '/#(.+)/', $link->getAttribute( 'href' ), $matches ) ) {
+					$link->setAttribute( 'on', sprintf( 'tap:%s.scrollTo(duration=600)', $matches[1] ) );
+				}
+			}
+		}
+	}
+
+	/**
+	 * Force SVG support, replacing no-svg class name with svg class name.
+	 *
+	 * @since 1.0
+	 *
+	 * @link https://github.com/WordPress/wordpress-develop/blob/1af1f65a21a1a697fb5f33027497f9e5ae638453/src/wp-content/themes/twentyseventeen/assets/js/global.js#L211-L213
+	 * @link https://caniuse.com/#feat=svg
+	 */
+	public function force_svg_support() {
+		$this->dom->documentElement->setAttribute(
+			'class',
+			preg_replace(
+				'/(^|\s)no-svg(\s|$)/',
+				' svg ',
+				$this->dom->documentElement->getAttribute( 'class' )
+			)
+		);
+	}
+
+	/**
+	 * Force support for fixed background-attachment.
+	 *
+	 * @since 1.0
+	 *
+	 * @link https://github.com/WordPress/wordpress-develop/blob/1af1f65a21a1a697fb5f33027497f9e5ae638453/src/wp-content/themes/twentyseventeen/assets/js/global.js#L215-L217
+	 * @link https://caniuse.com/#feat=background-attachment
+	 */
+	public function force_fixed_background_support() {
+		$this->dom->documentElement->setAttribute(
+			'class',
+			$this->dom->documentElement->getAttribute( 'class' ) . ' background-fixed'
+		);
+	}
+
+	/**
+	 * Add body class when there is a header video.
+	 *
+	 * @since 1.0
+	 * @link https://github.com/WordPress/wordpress-develop/blob/a26c24226c6b131a0ed22c722a836c100d3ba254/src/wp-content/themes/twentyseventeen/assets/js/global.js#L244-L247
+	 *
+	 * @param array $args Args.
+	 */
+	public static function add_has_header_video_body_class( $args = array() ) {
+		$args = array_merge(
+			array(
+				'class_name' => 'has-header-video',
+			),
+			$args
+		);
+
+		add_filter( 'body_class', function( $body_classes ) use ( $args ) {
+			if ( has_header_video() ) {
+				$body_classes[] = $args['class_name'];
+			}
+			return $body_classes;
+		} );
+	}
+
+	/**
+	 * Add required styles for video and image headers.
+	 *
+	 * This is currently used exclusively for Twenty Seventeen.
+	 *
+	 * @since 1.0
+	 * @link https://github.com/WordPress/wordpress-develop/blob/1af1f65a21a1a697fb5f33027497f9e5ae638453/src/wp-content/themes/twentyseventeen/style.css#L1687
+	 * @link https://github.com/WordPress/wordpress-develop/blob/1af1f65a21a1a697fb5f33027497f9e5ae638453/src/wp-content/themes/twentyseventeen/style.css#L1743
+	 */
+	public static function add_twentyseventeen_masthead_styles() {
+		$args = self::get_theme_config( get_template() );
+
+		/*
+		 * The following is necessary because the styles in the theme apply to img and video,
+		 * and the CSS parser will then convert the selectors to amp-img and amp-video respectively.
+		 * Nevertheless, object-fit does not apply on amp-img and it needs to apply on an actual img.
+		 */
+		add_action( 'wp_enqueue_scripts', function() use ( $args ) {
+			$is_front_page_layout = ( is_front_page() && 'posts' !== get_option( 'show_on_front' ) ) || ( is_home() && is_front_page() );
+			ob_start();
+			?>
+			<style>
+				.has-header-image .custom-header-media amp-img > img,
+				.has-header-video .custom-header-media amp-video > video{
+					position: fixed;
+					height: auto;
+					left: 50%;
+					max-width: 1000%;
+					min-height: 100%;
+					min-width: 100%;
+					min-width: 100vw; /* vw prevents 1px gap on left that 100% has */
+					width: auto;
+					top: 50%;
+					padding-bottom: 1px; /* Prevent header from extending beyond the footer */
+					-ms-transform: translateX(-50%) translateY(-50%);
+					-moz-transform: translateX(-50%) translateY(-50%);
+					-webkit-transform: translateX(-50%) translateY(-50%);
+					transform: translateX(-50%) translateY(-50%);
+				}
+				.has-header-image:not(.twentyseventeen-front-page):not(.home) .custom-header-media amp-img > img {
+					bottom: 0;
+					position: absolute;
+					top: auto;
+					-ms-transform: translateX(-50%) translateY(0);
+					-moz-transform: translateX(-50%) translateY(0);
+					-webkit-transform: translateX(-50%) translateY(0);
+					transform: translateX(-50%) translateY(0);
+				}
+				/* For browsers that support object-fit */
+				@supports ( object-fit: cover ) {
+					.has-header-image .custom-header-media amp-img > img,
+					.has-header-video .custom-header-media amp-video > video,
+					.has-header-image:not(.twentyseventeen-front-page):not(.home) .custom-header-media amp-img > img {
+						height: 100%;
+						left: 0;
+						-o-object-fit: cover;
+						object-fit: cover;
+						top: 0;
+						-ms-transform: none;
+						-moz-transform: none;
+						-webkit-transform: none;
+						transform: none;
+						width: 100%;
+					}
+				}
+
+				.navigation-top.site-navigation-fixed {
+					display: none;
+				}
+
+				@media screen and (min-width: 48em) {
+					/* Note that adjustHeaderHeight() is irrelevant with this change */
+					<?php if ( ! $is_front_page_layout ) : ?>
+						.navigation-top {
+							position: static;
+						}
+					<?php endif; ?>
+
+					/* Initial styles that amp-animations for navigationTopShow and navigationTopHide will override */
+					.navigation-top.site-navigation-fixed {
+						opacity: 0;
+						transform: translateY( -72px );
+						display: block;
+					}
+				}
+			</style>
+			<?php
+			$styles = str_replace( array( '<style>', '</style>' ), '', ob_get_clean() );
+			wp_add_inline_style( get_template() . '-style', $styles );
+		}, 11 );
+	}
+
+	/**
+	 * Add sticky nav menu to Twenty Seventeen.
+	 *
+	 * This is implemented by cloning the navigation-top element, giving it a fixed position outside of the viewport,
+	 * and then showing it at the top of the window as soon as the original nav begins to get scrolled out of view.
+	 * In order to improve accessibility, the cloned nav gets aria-hidden=true and all of the links get tabindex=-1
+	 * to prevent the keyboard from focusing on elements off the screen; it is not necessary to focus on the elements
+	 * in the fixed nav menu because as soon as the original nav menu is focused then the window is scrolled to the
+	 * top anyway.
+	 *
+	 * @since 1.0
+	 */
+	public function add_twentyseventeen_sticky_nav_menu() {
+		/**
+		 * Elements.
+		 *
+		 * @var DOMElement $link
+		 * @var DOMElement $navigation_top
+		 * @var DOMElement $navigation_top_fixed
+		 */
+		$navigation_top = $this->xpath->query( '//header[ @id = "masthead" ]//div[ contains( @class, "navigation-top" ) ]' )->item( 0 );
+		if ( ! $navigation_top ) {
+			return;
+		}
+
+		$navigation_top_fixed = $navigation_top->cloneNode( true );
+		$navigation_top_fixed->setAttribute( 'class', $navigation_top_fixed->getAttribute( 'class' ) . ' site-navigation-fixed' );
+
+		$navigation_top_fixed->setAttribute( 'aria-hidden', 'true' );
+		foreach ( $navigation_top_fixed->getElementsByTagName( 'a' ) as $link ) {
+			$link->setAttribute( 'tabindex', '-1' );
+		}
+
+		$navigation_top->parentNode->insertBefore( $navigation_top_fixed, $navigation_top->nextSibling );
+
+		$position_observer = AMP_DOM_Utils::create_node( $this->dom, 'amp-position-observer', array(
+			'layout'              => 'nodisplay',
+			'intersection-ratios' => 1,
+			'on'                  => implode( ';', array(
+				'exit:navigationTopShow.start',
+				'enter:navigationTopHide.start',
+			) ),
+		) );
+		$navigation_top->appendChild( $position_observer );
+
+		$animations = array(
+			'navigationTopShow' => array(
+				'duration'   => 0,
+				'fill'       => 'both',
+				'animations' => array(
+					'selector'  => '.navigation-top.site-navigation-fixed',
+					'media'     => '(min-width: 48em)',
+					'keyframes' => array(
+						'opacity'   => 1.0,
+						'transform' => 'translateY( 0 )',
+					),
+				),
+			),
+			'navigationTopHide' => array(
+				'duration'   => 0,
+				'fill'       => 'both',
+				'animations' => array(
+					'selector'  => '.navigation-top.site-navigation-fixed',
+					'media'     => '(min-width: 48em)',
+					'keyframes' => array(
+						'opacity'   => 0.0,
+						'transform' => 'translateY( -72px )',
+					),
+				),
+			),
+		);
+
+		foreach ( $animations as $animation_id => $animation ) {
+			$amp_animation   = AMP_DOM_Utils::create_node( $this->dom, 'amp-animation', array(
+				'id'     => $animation_id,
+				'layout' => 'nodisplay',
+			) );
+			$position_script = $this->dom->createElement( 'script' );
+			$position_script->setAttribute( 'type', 'application/json' );
+			$position_script->appendChild( $this->dom->createTextNode( wp_json_encode( $animation ) ) );
+			$amp_animation->appendChild( $position_script );
+			$this->body->appendChild( $amp_animation );
+		}
+	}
+
+	/**
+	 * Add styles for the nav menu specifically to deal with AMP running in a no-js context.
+	 *
+	 * @since 1.0
+	 *
+	 * @param array $args Args.
+	 */
+	public static function add_nav_menu_styles( $args = array() ) {
+		$args = array_merge(
+			self::get_theme_config( get_template() ),
+			$args
+		);
+
+		add_action( 'wp_enqueue_scripts', function() use ( $args ) {
+			ob_start();
+			?>
+			<style>
+				/* Override no-js selector in parent theme. */
+				.no-js .main-navigation ul ul {
+					display: none;
+				}
+
+				/* Use sibling selector and re-use class on button instead of toggling toggle-on class on ul.sub-menu */
+				.main-navigation ul .<?php echo esc_html( $args['sub_menu_button_toggle_class'] ); ?> + .sub-menu {
+					display: block;
+				}
+
+				<?php if ( 'twentyseventeen' === get_template() ) : ?>
+					/* Show the button*/
+					.no-js .<?php echo esc_html( $args['menu_button_class'] ); ?> {
+						display: block;
+					}
+					.no-js .main-navigation > div > ul {
+						display: none;
+					}
+					.no-js .main-navigation.<?php echo esc_html( $args['nav_container_toggle_class'] ); ?> > div > ul {
+						display: block;
+					}
+					@media screen and (min-width: 48em) {
+						.no-js .<?php echo esc_html( $args['menu_button_class'] ); ?>,
+						.no-js .<?php echo esc_html( $args['sub_menu_button_class'] ); ?> {
+							display: none;
+						}
+						.no-js .main-navigation ul,
+						.no-js .main-navigation ul ul,
+						.no-js .main-navigation > div > ul {
+							display: block;
+						}
+						.main-navigation ul li.menu-item-has-children:focus-within:before,
+						.main-navigation ul li.menu-item-has-children:focus-within:after,
+						.main-navigation ul li.page_item_has_children:focus-within:before,
+						.main-navigation ul li.page_item_has_children:focus-within:after {
+							display: block;
+						}
+						.main-navigation ul ul li:focus-within > ul {
+							<?php if ( is_rtl() ) : ?>
+								left: auto;
+								right: 100%;
+							<?php else : ?>
+								left: 100%;
+								right: auto;
+							<?php endif; ?>
+						}
+						.main-navigation li li:focus-within {
+							background: #767676;
+						}
+						.main-navigation li li:focus-within > a,
+						.main-navigation li li a:focus-within,
+						.main-navigation li li.current_page_item a:focus-within,
+						.main-navigation li li.current-menu-item a:focus-within {
+							color: #fff;
+						}
+						.main-navigation ul li:focus-within > ul {
+							<?php if ( is_rtl() ) : ?>
+								left: auto;
+								right: 0.5em;
+							<?php else : ?>
+								left: 0.5em;
+								right: auto;
+							<?php endif; ?>
+						}
+
+						.main-navigation ul ul li.menu-item-has-children:focus-within:before,
+						.main-navigation ul ul li.menu-item-has-children:focus-within:after,
+						.main-navigation ul ul li.page_item_has_children:focus-within:before,
+						.main-navigation ul ul li.page_item_has_children:focus-within:after {
+							display: none;
+						}
+					}
+				<?php elseif ( 'twentysixteen' === get_template() ) : ?>
+					@media screen and (max-width: 56.875em) {
+						/* Show the button*/
+						.no-js .<?php echo esc_html( $args['menu_button_class'] ); ?> {
+							display: block;
+						}
+						.no-js .site-header-menu {
+							display: none;
+						}
+						.no-js .site-header-menu.toggled-on {
+							display: block;
+						}
+					}
+					@media screen and (min-width: 56.875em) {
+						.no-js .main-navigation ul ul {
+							display: block;
+						}
+						.main-navigation li:focus-within > a {
+							color: #007acc;
+						}
+						.main-navigation li:focus-within > ul {
+							<?php if ( is_rtl() ) : ?>
+								left: auto;
+								right: 0;
+							<?php else : ?>
+								left: 0;
+								right: auto;
+							<?php endif; ?>
+						}
+						.main-navigation ul ul li:focus-within > ul {
+							<?php if ( is_rtl() ) : ?>
+								left: 100%;
+								right: auto;
+							<?php else : ?>
+								left: auto;
+								right: 100%;
+							<?php endif; ?>
+						}
+					}
+				<?php elseif ( 'twentyfifteen' === get_template() ) : ?>
+					@media screen and (min-width: 59.6875em) {
+						/* Attempt to emulate https://github.com/WordPress/wordpress-develop/blob/5e9a39baa7d4368f7d3c36dcbcd53db6317677c9/src/wp-content/themes/twentyfifteen/js/functions.js#L108-L149 */
+						#sidebar {
+							position: sticky;
+							top: -9vh;
+							max-height: 109vh;
+							overflow-y: auto;
+						}
+					}
+
+				<?php endif; ?>
+			</style>
+			<?php
+			$styles = str_replace( array( '<style>', '</style>' ), '', ob_get_clean() );
+			wp_add_inline_style( get_template() . '-style', $styles );
+		}, 11 );
+	}
+
+	/**
+	 * Ensure that JS-only nav menu styles apply to AMP as well since even though scripts are not allowed, there are AMP-bind implementations.
+	 *
+	 * @since 1.0
+	 *
+	 * @param array $args Args.
+	 */
+	public function add_nav_menu_toggle( $args = array() ) {
+		$args = array_merge(
+			self::get_theme_config( get_template() ),
+			$args
+		);
+
+		$nav_el = $this->dom->getElementById( $args['nav_container_id'] );
+		if ( ! $nav_el ) {
+			return;
+		}
+
+		$button_el = $this->xpath->query( $args['menu_button_xpath'] )->item( 0 );
+		if ( ! $button_el ) {
+			return;
+		}
+
+		$state_id = 'navMenuToggledOn';
+		$expanded = false;
+
+		$nav_el->setAttribute(
+			AMP_DOM_Utils::get_amp_bind_placeholder_prefix() . 'class',
+			sprintf(
+				"%s + ( $state_id ? %s : '' )",
+				wp_json_encode( $nav_el->getAttribute( 'class' ) ),
+				wp_json_encode( ' ' . $args['nav_container_toggle_class'] )
+			)
+		);
+
+		$state_el = $this->dom->createElement( 'amp-state' );
+		$state_el->setAttribute( 'id', $state_id );
+		$script_el = $this->dom->createElement( 'script' );
+		$script_el->setAttribute( 'type', 'application/json' );
+		$script_el->appendChild( $this->dom->createTextNode( wp_json_encode( $expanded ) ) );
+		$state_el->appendChild( $script_el );
+		$nav_el->parentNode->insertBefore( $state_el, $nav_el );
+
+		$button_on = sprintf( "tap:AMP.setState({ $state_id: ! $state_id })" );
+		$button_el->setAttribute( 'on', $button_on );
+		$button_el->setAttribute( 'aria-expanded', 'false' );
+		$button_el->setAttribute( AMP_DOM_Utils::get_amp_bind_placeholder_prefix() . 'aria-expanded', "$state_id ? 'true' : 'false'" );
+		$button_el->setAttribute(
+			AMP_DOM_Utils::get_amp_bind_placeholder_prefix() . 'class',
+			sprintf( "%s + ( $state_id ? %s : '' )", wp_json_encode( $button_el->getAttribute( 'class' ) ), wp_json_encode( ' ' . $args['menu_button_toggle_class'] ) )
+		);
+	}
+
+	/**
+	 * Add buttons for nav sub-menu items.
+	 *
+	 * @since 1.0
+	 * @link https://github.com/WordPress/wordpress-develop/blob/a26c24226c6b131a0ed22c722a836c100d3ba254/src/wp-content/themes/twentyseventeen/assets/js/navigation.js#L11-L43
+	 *
+	 * @param array $args Args.
+	 */
+	public static function add_nav_sub_menu_buttons( $args = array() ) {
+		$default_args = self::get_theme_config( get_template() );
+		switch ( get_template() ) {
+			case 'twentyseventeen':
+				if ( function_exists( 'twentyseventeen_get_svg' ) ) {
+					$default_args['icon'] = twentyseventeen_get_svg( array(
+						'icon'     => 'angle-down',
+						'fallback' => true,
+					) );
+				}
+				break;
+		}
+		$args = array_merge( $default_args, $args );
+
+		/**
+		 * Filter the HTML output of a nav menu item to add the AMP dropdown button to reveal the sub-menu.
+		 *
+		 * @see twentyfifteen_amp_setup_hooks()
+		 *
+		 * @param string $item_output Nav menu item HTML.
+		 * @param object $item        Nav menu item.
+		 * @return string Modified nav menu item HTML.
+		 */
+		add_filter( 'walker_nav_menu_start_el', function( $item_output, $item, $depth, $nav_menu_args ) use ( $args ) {
+			unset( $depth );
+
+			// Skip adding buttons to nav menu widgets for now.
+			if ( empty( $nav_menu_args->theme_location ) ) {
+				return $item_output;
+			}
+
+			if ( ! in_array( 'menu-item-has-children', $item->classes, true ) ) {
+				return $item_output;
+			}
+			static $nav_menu_item_number = 0;
+			$nav_menu_item_number++;
+
+			$expanded = in_array( 'current-menu-ancestor', $item->classes, true );
+
+			$expanded_state_id = 'navMenuItemExpanded' . $nav_menu_item_number;
+
+			// Create new state for managing storing the whether the sub-menu is expanded.
+			$item_output .= sprintf(
+				'<amp-state id="%s"><script type="application/json">%s</script></amp-state>',
+				esc_attr( $expanded_state_id ),
+				wp_json_encode( $expanded )
+			);
+
+			$dropdown_button  = '<button';
+			$dropdown_button .= sprintf(
+				' class="%s" [class]="%s"',
+				esc_attr( $args['sub_menu_button_class'] . ( $expanded ? ' ' . $args['sub_menu_button_toggle_class'] : '' ) ),
+				esc_attr( sprintf( "%s + ( $expanded_state_id ? %s : '' )", wp_json_encode( $args['sub_menu_button_class'] ), wp_json_encode( ' ' . $args['sub_menu_button_toggle_class'] ) ) )
+			);
+			$dropdown_button .= sprintf(
+				' aria-expanded="%s" [aria-expanded]="%s"',
+				esc_attr( wp_json_encode( $expanded ) ),
+				esc_attr( "$expanded_state_id ? 'true' : 'false'" )
+			);
+			$dropdown_button .= sprintf(
+				' on="%s"',
+				esc_attr( "tap:AMP.setState( { $expanded_state_id: ! $expanded_state_id } )" )
+			);
+			$dropdown_button .= '>';
+
+			if ( isset( $args['icon'] ) ) {
+				$dropdown_button .= $args['icon'];
+			}
+			if ( isset( $args['expand_text'] ) && isset( $args['collapse_text'] ) ) {
+				$dropdown_button .= sprintf(
+					'<span class="screen-reader-text" [text]="%s">%s</span>',
+					esc_attr( sprintf( "$expanded_state_id ? %s : %s", wp_json_encode( $args['collapse_text'] ), wp_json_encode( $args['expand_text'] ) ) ),
+					esc_html( $expanded ? $args['collapse_text'] : $args['expand_text'] )
+				);
+			}
+
+			$dropdown_button .= '</button>';
+
+			$item_output .= $dropdown_button;
+			return $item_output;
+		}, 10, 4 );
+	}
+}
diff --git a/includes/sanitizers/class-amp-video-sanitizer.php b/includes/sanitizers/class-amp-video-sanitizer.php
index b61c85343fb..668dfed9064 100644
--- a/includes/sanitizers/class-amp-video-sanitizer.php
+++ b/includes/sanitizers/class-amp-video-sanitizer.php
@@ -39,7 +39,7 @@ class AMP_Video_Sanitizer extends AMP_Base_Sanitizer {
 	 */
 	public function get_selector_conversion_mapping() {
 		return array(
-			'video' => array( 'amp-video' ),
+			'video' => array( 'amp-video', 'amp-youtube' ),
 		);
 	}
 
diff --git a/includes/validation/class-amp-invalid-url-post-type.php b/includes/validation/class-amp-invalid-url-post-type.php
index 8504a36ffce..aa9e9b71f1d 100644
--- a/includes/validation/class-amp-invalid-url-post-type.php
+++ b/includes/validation/class-amp-invalid-url-post-type.php
@@ -1192,7 +1192,7 @@ public static function print_url_as_title( $post ) {
 	 * @return string Title.
 	 */
 	public static function filter_the_title_in_post_list_table( $title, $post ) {
-		if ( get_current_screen()->base === 'edit' && get_current_screen()->post_type === self::POST_TYPE_SLUG && self::POST_TYPE_SLUG === get_post_type( $post ) ) {
+		if ( function_exists( 'get_current_screen' ) && get_current_screen()->base === 'edit' && get_current_screen()->post_type === self::POST_TYPE_SLUG && self::POST_TYPE_SLUG === get_post_type( $post ) ) {
 			$title = preg_replace( '#^(\w+:)?//[^/]+#', '', $title );
 		}
 		return $title;
diff --git a/includes/validation/class-amp-validation-error-taxonomy.php b/includes/validation/class-amp-validation-error-taxonomy.php
index 381e415ca4d..e63c36ee148 100644
--- a/includes/validation/class-amp-validation-error-taxonomy.php
+++ b/includes/validation/class-amp-validation-error-taxonomy.php
@@ -158,6 +158,8 @@ public static function register() {
 		if ( is_admin() ) {
 			self::add_admin_hooks();
 		}
+
+		self::accept_validation_errors( AMP_Core_Theme_Sanitizer::get_acceptable_errors( get_template() ) );
 	}
 
 	/**
@@ -246,6 +248,58 @@ public static function get_validation_error_sanitization( $error ) {
 		return compact( 'status', 'forced' );
 	}
 
+	/**
+	 * Automatically (forcibly) accept validation errors that arise.
+	 *
+	 * @since 1.0
+	 * @see AMP_Core_Theme_Sanitizer::get_acceptable_errors()
+	 *
+	 * @param array $acceptable_errors Acceptable validation errors, where keys are codes and values are either `true` or sparse array to check as subset.
+	 */
+	public static function accept_validation_errors( $acceptable_errors ) {
+		if ( empty( $acceptable_errors ) ) {
+			return;
+		}
+
+		/**
+		 * Check if one array is a sparse subset of another array.
+		 *
+		 * @param array $superset Superset array.
+		 * @param array $subset   Subset array.
+		 *
+		 * @return bool Whether subset is contained in superset.
+		 */
+		$is_array_subset = function( $superset, $subset ) use ( &$is_array_subset ) {
+			foreach ( $subset as $key => $subset_value ) {
+				if ( ! isset( $superset[ $key ] ) || gettype( $subset_value ) !== gettype( $superset[ $key ] ) ) {
+					return false;
+				}
+				if ( is_array( $subset_value ) ) {
+					if ( ! $is_array_subset( $superset[ $key ], $subset_value ) ) {
+						return false;
+					}
+				} elseif ( $superset[ $key ] !== $subset_value ) {
+					return false;
+				}
+			}
+			return true;
+		};
+
+		add_filter( 'amp_validation_error_sanitized', function( $sanitized, $error ) use ( $is_array_subset, $acceptable_errors ) {
+			if ( isset( $acceptable_errors[ $error['code'] ] ) ) {
+				if ( true === $acceptable_errors[ $error['code'] ] ) {
+					return true;
+				}
+				foreach ( $acceptable_errors[ $error['code'] ] as $acceptable_error_props ) {
+					if ( $is_array_subset( $error, $acceptable_error_props ) ) {
+						return true;
+					}
+				}
+			}
+			return $sanitized;
+		}, 10, 2 );
+	}
+
 	/**
 	 * Get the count of validation error terms, optionally restricted by term group (e.g. accepted or rejected).
 	 *
diff --git a/phpcs.xml b/phpcs.xml
index fb1137e950a..ef3a5c5b010 100644
--- a/phpcs.xml
+++ b/phpcs.xml
@@ -28,7 +28,7 @@
 	</rule>
 	<rule ref="WordPress.NamingConventions.ValidVariableName">
 		<properties>
-			<property name="customPropertiesWhitelist" value="childNodes,tagName,textContent,parentNode,nodeType,nodeName,nextSibling,firstChild,nodeValue,DEFAULT_ARGS,documentElement,removeChild,ownerDocument,DEFAULT_WIDTH,DEFAULT_HEIGHT" />
+			<property name="customPropertiesWhitelist" value="childNodes,tagName,textContent,parentNode,nodeType,nodeName,nextSibling,firstChild,lastChild,nodeValue,DEFAULT_ARGS,documentElement,removeChild,ownerDocument,DEFAULT_WIDTH,DEFAULT_HEIGHT" />
 		</properties>
 	</rule>
 
diff --git a/tests/test-class-amp-theme-support.php b/tests/test-class-amp-theme-support.php
index 049d1012473..6a7d088fb34 100644
--- a/tests/test-class-amp-theme-support.php
+++ b/tests/test-class-amp-theme-support.php
@@ -41,6 +41,7 @@ public function tearDown() {
 		parent::tearDown();
 		AMP_Validation_Manager::reset_validation_results();
 		remove_theme_support( 'amp' );
+		remove_theme_support( 'custom-header' );
 		$_REQUEST                = array(); // phpcs:ignore WordPress.CSRF.NonceVerification.NoNonceVerification
 		$_SERVER['QUERY_STRING'] = '';
 		unset( $_SERVER['REQUEST_URI'] );
@@ -299,6 +300,7 @@ public function test_add_hooks() {
 		$this->assertEquals( 10, has_filter( 'cancel_comment_reply_link', array( self::TESTED_CLASS, 'filter_cancel_comment_reply_link' ) ) );
 		$this->assertEquals( 100, has_action( 'comment_form', array( self::TESTED_CLASS, 'amend_comment_form' ) ) );
 		$this->assertFalse( has_action( 'comment_form', 'wp_comment_form_unfiltered_html_nonce' ) );
+		$this->assertEquals( 10, has_filter( 'get_header_image_tag', array( self::TESTED_CLASS, 'conditionally_output_header' ) ) );
 	}
 
 	/**
@@ -1246,4 +1248,31 @@ public function test_whitelist_layout_in_wp_kses_allowed_html() {
 		$image = '<img data-amp-layout="fill">';
 		$this->assertEquals( $image, wp_kses_post( $image ) );
 	}
+
+	/**
+	 * Test AMP_Theme_Support::conditionally_output_header().
+	 *
+	 * @see AMP_Theme_Support::conditionally_output_header()
+	 */
+	public function conditionally_output_header() {
+		$mock_image = '<img src="https://example.com/flower.jpeg">';
+
+		// If there's no theme support for 'custom-header', the callback should simply return the image.
+		$this->assertEquals(
+			$mock_image,
+			AMP_Theme_Support::conditionally_output_header( $mock_image )
+		);
+
+		// If theme support is present, but there isn't a header video selected, the callback should again return the image.
+		add_theme_support( 'custom-header', array(
+			'video' => true,
+		) );
+
+		// There's a YouTube URL as the header video.
+		set_theme_mod( 'external_header_video', 'https://www.youtube.com/watch?v=a8NScvBhVnc' );
+		$this->assertEquals(
+			'<amp-youtube width="0" height="0" layout="responsive" autoplay id="wp-custom-header-video" data-videoid="a8NScvBhVnc" data-param-rel="0" data-param-showinfo="0"></amp-youtube>',
+			AMP_Theme_Support::conditionally_output_header( $mock_image )
+		);
+	}
 }