diff --git a/amp.php b/amp.php
index 66b44f38762..0e59906c557 100644
--- a/amp.php
+++ b/amp.php
@@ -87,15 +87,7 @@ function amp_after_setup_theme() {
 		define( 'AMP_QUERY_VAR', apply_filters( 'amp_query_var', 'amp' ) );
 	}
 
-	add_action( 'init', 'amp_init' );
-	add_action( 'widgets_init', 'AMP_Theme_Support::register_widgets' ); // @todo Let this be called by AMP_Theme_Support::init().
-	add_action( 'init', 'AMP_Theme_Support::setup_commenting' ); // @todo Let this be called by AMP_Theme_Support::init().
-	add_action( 'admin_init', 'AMP_Options_Manager::register_settings' );
-	add_action( 'wp_loaded', 'amp_post_meta_box' );
-	add_action( 'wp_loaded', 'amp_add_options_menu' );
-	add_action( 'parse_query', 'amp_correct_query_when_is_front_page' );
-	AMP_Post_Type_Support::add_post_type_support();
-	AMP_Validation_Utils::init();
+	add_action( 'init', 'amp_init', 0 ); // Must be 0 because widgets_init happens at init priority 1.
 }
 add_action( 'after_setup_theme', 'amp_after_setup_theme', 5 );
 
@@ -103,7 +95,6 @@ function amp_after_setup_theme() {
  * Init AMP.
  *
  * @since 0.1
- * @global string $pagenow
  */
 function amp_init() {
 
@@ -118,8 +109,14 @@ function amp_init() {
 
 	add_rewrite_endpoint( AMP_QUERY_VAR, EP_PERMALINK );
 
+	AMP_Theme_Support::init();
+	AMP_Post_Type_Support::add_post_type_support();
+	AMP_Validation_Utils::init();
 	add_filter( 'request', 'amp_force_query_var_value' );
-	add_action( 'wp', 'amp_maybe_add_actions' );
+	add_action( 'admin_init', 'AMP_Options_Manager::register_settings' );
+	add_action( 'wp_loaded', 'amp_post_meta_box' );
+	add_action( 'wp_loaded', 'amp_add_options_menu' );
+	add_action( 'parse_query', 'amp_correct_query_when_is_front_page' );
 
 	// Redirect the old url of amp page to the updated url.
 	add_filter( 'old_slug_redirect_url', 'amp_redirect_old_slug_to_new_url' );
@@ -127,7 +124,9 @@ function amp_init() {
 	if ( class_exists( 'Jetpack' ) && ! ( defined( 'IS_WPCOM' ) && IS_WPCOM ) ) {
 		require_once AMP__DIR__ . '/jetpack-helper.php';
 	}
-	amp_handle_xhr_request();
+
+	// Add actions for legacy post templates.
+	add_action( 'wp', 'amp_maybe_add_actions' );
 }
 
 // Make sure the `amp` query var has an explicit value.
@@ -150,15 +149,9 @@ function amp_force_query_var_value( $query_vars ) {
  * @return void
  */
 function amp_maybe_add_actions() {
-	$is_amp_endpoint = is_amp_endpoint();
 
-	// Add hooks for when a themes that support AMP.
+	// Short-circuit when theme supports AMP, as everything is handled by AMP_Theme_Support.
 	if ( current_theme_supports( 'amp' ) ) {
-		if ( $is_amp_endpoint ) {
-			AMP_Theme_Support::init();
-		} else {
-			amp_add_frontend_actions();
-		}
 		return;
 	}
 
@@ -168,6 +161,8 @@ function amp_maybe_add_actions() {
 		return;
 	}
 
+	$is_amp_endpoint = is_amp_endpoint();
+
 	/**
 	 * Queried post object.
 	 *
diff --git a/includes/amp-helper-functions.php b/includes/amp-helper-functions.php
index d22d49cec8b..a7c2331487d 100644
--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -484,94 +484,3 @@ function amp_print_schemaorg_metadata() {
 	<script type="application/ld+json"><?php echo wp_json_encode( $metadata ); ?></script>
 	<?php
 }
-
-/**
- * Hook into a form submissions, such as comment the form or some other .
- *
- * @since 0.7.0
- * @global string $pagenow
- */
-function amp_handle_xhr_request() {
-	global $pagenow;
-	if ( ! isset( $_GET['__amp_source_origin'] ) ) { // WPCS: CSRF ok. Beware of AMP_Theme_Support::purge_amp_query_vars().
-		return;
-	}
-
-	if ( isset( $pagenow ) && 'wp-comments-post.php' === $pagenow ) {
-		// We don't need any data, so just send a success.
-		add_filter( 'comment_post_redirect', function() {
-			// We don't need any data, so just send a success.
-			wp_send_json_success();
-		}, PHP_INT_MAX, 2 );
-		amp_handle_xhr_headers_output();
-	} elseif ( isset( $_GET['_wp_amp_action_xhr_converted'] ) ) { // WPCS: CSRF ok.
-		add_filter( 'wp_redirect', 'amp_intercept_post_request_redirect', PHP_INT_MAX, 2 );
-		amp_handle_xhr_headers_output();
-	}
-}
-
-/**
- * Handle the AMP XHR headers and output errors.
- *
- * @since 0.7.0
- */
-function amp_handle_xhr_headers_output() {
-	// Add die handler for AMP error display.
-	add_filter( 'wp_die_handler', function() {
-		/**
-		 * New error handler for AMP form submission.
-		 *
-		 * @param WP_Error|string $error The error to handle.
-		 */
-		return function( $error ) {
-			status_header( 400 );
-			if ( is_wp_error( $error ) ) {
-				$error = $error->get_error_message();
-			}
-			$amp_mustache_allowed_html_tags = array( 'strong', 'b', 'em', 'i', 'u', 's', 'small', 'mark', 'del', 'ins', 'sup', 'sub' );
-			wp_send_json( array(
-				'error' => wp_kses( $error, array_fill_keys( $amp_mustache_allowed_html_tags, array() ) ),
-			) );
-		};
-	} );
-
-	// Send AMP header.
-	$origin = esc_url_raw( wp_unslash( $_GET['__amp_source_origin'] ) ); // WPCS: CSRF ok.
-	header( 'AMP-Access-Control-Allow-Source-Origin: ' . $origin, true );
-}
-
-/**
- * Intercept the response to a non-comment POST request.
- *
- * @since 0.7.0
- * @param string $location The location to redirect to.
- */
-function amp_intercept_post_request_redirect( $location ) {
-
-	// Make sure relative redirects get made absolute.
-	$parsed_location = array_merge(
-		array(
-			'scheme' => 'https',
-			'host'   => wp_parse_url( home_url(), PHP_URL_HOST ),
-			'path'   => strtok( wp_unslash( $_SERVER['REQUEST_URI'] ), '?' ),
-		),
-		wp_parse_url( $location )
-	);
-
-	$absolute_location = $parsed_location['scheme'] . '://' . $parsed_location['host'];
-	if ( isset( $parsed_location['port'] ) ) {
-		$absolute_location .= ':' . $parsed_location['port'];
-	}
-	$absolute_location .= $parsed_location['path'];
-	if ( isset( $parsed_location['query'] ) ) {
-		$absolute_location .= '?' . $parsed_location['query'];
-	}
-	if ( isset( $parsed_location['fragment'] ) ) {
-		$absolute_location .= '#' . $parsed_location['fragment'];
-	}
-
-	header( 'AMP-Redirect-To: ' . $absolute_location );
-	header( 'Access-Control-Expose-Headers: AMP-Redirect-To' );
-	// Send json success as no data is required.
-	wp_send_json_success();
-}
diff --git a/includes/class-amp-theme-support.php b/includes/class-amp-theme-support.php
index 48ef2f8367b..2bee8dc416e 100644
--- a/includes/class-amp-theme-support.php
+++ b/includes/class-amp-theme-support.php
@@ -71,6 +71,20 @@ class AMP_Theme_Support {
 	 * Initialize.
 	 */
 	public static function init() {
+		if ( ! current_theme_supports( 'amp' ) ) {
+			return;
+		}
+
+		self::purge_amp_query_vars();
+		self::handle_xhr_request();
+
+		if ( ! is_amp_endpoint() ) {
+			amp_add_frontend_actions();
+		} else {
+			self::setup_commenting();
+			add_action( 'widgets_init', array( __CLASS__, 'register_widgets' ) );
+		}
+
 		require_once AMP__DIR__ . '/includes/amp-post-template-actions.php';
 
 		// Validate theme support usage.
@@ -95,7 +109,6 @@ public static function init() {
 			self::register_paired_hooks();
 		}
 
-		self::purge_amp_query_vars(); // Note that amp_prepare_xhr_post() still looks at $_GET['__amp_source_origin'].
 		self::register_hooks();
 		self::$embed_handlers    = self::register_content_embed_handlers();
 		self::$sanitizer_classes = amp_get_content_sanitizers();
@@ -128,6 +141,18 @@ public static function is_paired_available() {
 		return true;
 	}
 
+	/**
+	 * Determine whether the user is in the Customizer preview iframe.
+	 *
+	 * @since 0.7
+	 *
+	 * @return bool Whether in Customizer preview iframe.
+	 */
+	public static function is_customize_preview_iframe() {
+		global $wp_customize;
+		return is_customize_preview() && $wp_customize->get_messenger_channel();
+	}
+
 	/**
 	 * Register hooks for paired mode.
 	 */
@@ -159,11 +184,16 @@ public static function register_hooks() {
 		 * install is not on utf-8 and we may need to do a encoding conversion.
 		 */
 		add_action( 'wp_head', array( __CLASS__, 'add_amp_component_scripts' ), 10 );
-		add_action( 'wp_head', array( __CLASS__, 'print_amp_styles' ) );
+		add_action( 'wp_print_styles', array( __CLASS__, 'print_amp_styles' ), 0 ); // Print boilerplate before theme and plugin stylesheets.
 		add_action( 'wp_enqueue_scripts', array( __CLASS__, 'enqueue_amp_default_styles' ), 9 );
 		add_action( 'wp_head', 'amp_add_generator_metadata', 20 );
 		add_action( 'wp_head', 'amp_print_schemaorg_metadata' );
 
+		if ( is_customize_preview() ) {
+			add_action( 'wp_enqueue_scripts', array( __CLASS__, 'dequeue_customize_preview_scripts' ), 1000 );
+		}
+		add_filter( 'customize_partial_render', array( __CLASS__, 'filter_customize_partial_render' ) );
+
 		add_action( 'wp_footer', 'amp_print_analytics' );
 
 		/*
@@ -178,6 +208,7 @@ public static function register_hooks() {
 		 */
 		add_action( 'template_redirect', array( __CLASS__, 'start_output_buffering' ), 0 );
 
+		// Commenting hooks.
 		add_filter( 'wp_list_comments_args', array( __CLASS__, 'amp_set_comments_walker' ), PHP_INT_MAX );
 		add_filter( 'comment_form_defaults', array( __CLASS__, 'filter_comment_form_defaults' ) );
 		add_filter( 'comment_reply_link', array( __CLASS__, 'filter_comment_reply_link' ), 10, 4 );
@@ -248,13 +279,100 @@ public static function purge_amp_query_vars() {
 	}
 
 	/**
-	 * Set up commenting.
+	 * Hook into a form submissions, such as comment the form or some other .
+	 *
+	 * @since 0.7.0
+	 * @global string $pagenow
 	 */
-	public static function setup_commenting() {
-		if ( ! current_theme_supports( AMP_QUERY_VAR ) ) {
+	public static function handle_xhr_request() {
+		global $pagenow;
+		if ( empty( self::$purged_amp_query_vars['__amp_source_origin'] ) ) {
 			return;
 		}
 
+		if ( isset( $pagenow ) && 'wp-comments-post.php' === $pagenow ) {
+			// We don't need any data, so just send a success.
+			add_filter( 'comment_post_redirect', function() {
+				// We don't need any data, so just send a success.
+				wp_send_json_success();
+			}, PHP_INT_MAX );
+			self::handle_xhr_headers_output();
+		} elseif ( ! empty( self::$purged_amp_query_vars['_wp_amp_action_xhr_converted'] ) ) {
+			add_filter( 'wp_redirect', array( __CLASS__, 'intercept_post_request_redirect' ), PHP_INT_MAX );
+			self::handle_xhr_headers_output();
+		}
+	}
+
+	/**
+	 * Handle the AMP XHR headers and output errors.
+	 *
+	 * @since 0.7.0
+	 */
+	public static function handle_xhr_headers_output() {
+		// Add die handler for AMP error display.
+		add_filter( 'wp_die_handler', function() {
+			/**
+			 * New error handler for AMP form submission.
+			 *
+			 * @param WP_Error|string $error The error to handle.
+			 */
+			return function( $error ) {
+				status_header( 400 );
+				if ( is_wp_error( $error ) ) {
+					$error = $error->get_error_message();
+				}
+				$amp_mustache_allowed_html_tags = array( 'strong', 'b', 'em', 'i', 'u', 's', 'small', 'mark', 'del', 'ins', 'sup', 'sub' );
+				wp_send_json( array(
+					'error' => wp_kses( $error, array_fill_keys( $amp_mustache_allowed_html_tags, array() ) ),
+				) );
+			};
+		} );
+
+		// Send AMP header.
+		$origin = esc_url_raw( self::$purged_amp_query_vars['__amp_source_origin'] );
+		header( 'AMP-Access-Control-Allow-Source-Origin: ' . $origin, true );
+	}
+
+	/**
+	 * Intercept the response to a non-comment POST request.
+	 *
+	 * @since 0.7.0
+	 * @param string $location The location to redirect to.
+	 */
+	public static function intercept_post_request_redirect( $location ) {
+
+		// Make sure relative redirects get made absolute.
+		$parsed_location = array_merge(
+			array(
+				'scheme' => 'https',
+				'host'   => wp_parse_url( home_url(), PHP_URL_HOST ),
+				'path'   => strtok( wp_unslash( $_SERVER['REQUEST_URI'] ), '?' ),
+			),
+			wp_parse_url( $location )
+		);
+
+		$absolute_location = $parsed_location['scheme'] . '://' . $parsed_location['host'];
+		if ( isset( $parsed_location['port'] ) ) {
+			$absolute_location .= ':' . $parsed_location['port'];
+		}
+		$absolute_location .= $parsed_location['path'];
+		if ( isset( $parsed_location['query'] ) ) {
+			$absolute_location .= '?' . $parsed_location['query'];
+		}
+		if ( isset( $parsed_location['fragment'] ) ) {
+			$absolute_location .= '#' . $parsed_location['fragment'];
+		}
+
+		header( 'AMP-Redirect-To: ' . $absolute_location );
+		header( 'Access-Control-Expose-Headers: AMP-Redirect-To' );
+		// Send json success as no data is required.
+		wp_send_json_success();
+	}
+
+	/**
+	 * Set up commenting.
+	 */
+	public static function setup_commenting() {
 		/*
 		 * Temporarily force comments to be listed in descending order.
 		 *
@@ -729,6 +847,27 @@ protected static function ensure_required_markup( DOMDocument $dom ) {
 		}
 	}
 
+	/**
+	 * Dequeue Customizer assets which are not necessary outside the preview iframe.
+	 *
+	 * Prevent enqueueing customize-preview styles if not in customizer preview iframe.
+	 * These are only needed for when there is live editing of content, such as selective refresh.
+	 *
+	 * @since 0.7
+	 */
+	public static function dequeue_customize_preview_scripts() {
+
+		// Dequeue styles unnecessary unless in customizer preview iframe when editing (such as for edit shortcuts).
+		if ( ! self::is_customize_preview_iframe() ) {
+			wp_dequeue_style( 'customize-preview' );
+			foreach ( wp_styles()->registered as $handle => $dependency ) {
+				if ( in_array( 'customize-preview', $dependency->deps, true ) ) {
+					wp_dequeue_style( $handle );
+				}
+			}
+		}
+	}
+
 	/**
 	 * Start output buffering.
 	 *
@@ -763,6 +902,31 @@ public static function finish_output_buffering() {
 		echo self::prepare_response( ob_get_clean() ); // WPCS: xss ok.
 	}
 
+	/**
+	 * Filter rendered partial to convert to AMP.
+	 *
+	 * @see WP_Customize_Partial::render()
+	 *
+	 * @param string|mixed $partial Rendered partial.
+	 * @return string|mixed Filtered partial.
+	 * @global int $content_width
+	 */
+	public static function filter_customize_partial_render( $partial ) {
+		global $content_width;
+		if ( is_string( $partial ) && preg_match( '/<\w/', $partial ) ) {
+			$dom  = AMP_DOM_Utils::get_dom_from_content( $partial );
+			$args = array(
+				'content_max_width'    => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
+				'use_document_element' => false,
+				'allow_dirty_styles'   => true,
+				'allow_dirty_scripts'  => false,
+			);
+			AMP_Content_Sanitizer::sanitize_document( $dom, self::$sanitizer_classes, $args ); // @todo Include script assets in response?
+			$partial = AMP_DOM_Utils::get_content_from_dom( $dom );
+		}
+		return $partial;
+	}
+
 	/**
 	 * Process response to ensure AMP validity.
 	 *
@@ -794,6 +958,8 @@ public static function prepare_response( $response, $args = array() ) {
 				'content_max_width'       => ! empty( $content_width ) ? $content_width : AMP_Post_Template::CONTENT_MAX_WIDTH, // Back-compat.
 				'use_document_element'    => true,
 				'remove_invalid_callback' => null,
+				'allow_dirty_styles'      => self::is_customize_preview_iframe(), // Dirty styles only needed when editing (e.g. for edit shortcodes).
+				'allow_dirty_scripts'     => is_customize_preview(), // Scripts are always needed to inject changeset UUID.
 			),
 			$args
 		);
diff --git a/includes/sanitizers/class-amp-base-sanitizer.php b/includes/sanitizers/class-amp-base-sanitizer.php
index 4b96a62ff6e..de5c08c8881 100644
--- a/includes/sanitizers/class-amp-base-sanitizer.php
+++ b/includes/sanitizers/class-amp-base-sanitizer.php
@@ -48,6 +48,13 @@ abstract class AMP_Base_Sanitizer {
 	 *      @type string[] $amp_allowed_tags
 	 *      @type string[] $amp_globally_allowed_attributes
 	 *      @type string[] $amp_layout_allowed_attributes
+	 *      @type array $amp_allowed_tags
+	 *      @type array $amp_globally_allowed_attributes
+	 *      @type array $amp_layout_allowed_attributes
+	 *      @type array $amp_bind_placeholder_prefix
+	 *      @type bool $allow_dirty_styles
+	 *      @type bool $allow_dirty_scripts
+	 *      @type callable $remove_invalid_callback
 	 * }
 	 */
 	protected $args;
diff --git a/includes/sanitizers/class-amp-style-sanitizer.php b/includes/sanitizers/class-amp-style-sanitizer.php
index 92e9831d454..f085144cf87 100644
--- a/includes/sanitizers/class-amp-style-sanitizer.php
+++ b/includes/sanitizers/class-amp-style-sanitizer.php
@@ -152,6 +152,11 @@ public function get_stylesheets() {
 	public function sanitize() {
 		$elements = array();
 
+		// Do nothing if inline styles are allowed.
+		if ( ! empty( $this->args['allow_dirty_styles'] ) ) {
+			return;
+		}
+
 		/*
 		 * 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.
diff --git a/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php b/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php
index c9a13367ab9..18f4412882e 100644
--- a/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php
+++ b/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php
@@ -103,6 +103,65 @@ public function __construct( $dom, $args = array() ) {
 
 		parent::__construct( $dom, $args );
 
+		if ( ! empty( $this->args['allow_dirty_styles'] ) ) {
+
+			// Allow style attribute on all elements.
+			$this->args['amp_globally_allowed_attributes']['style'] = array();
+
+			// Allow style elements.
+			$this->args['amp_allowed_tags']['style'][] = array(
+				'attr_spec_list' => array(
+					'type' => array(
+						'value_casei' => 'text/css',
+					),
+				),
+				'cdata'          => array(),
+				'tag_spec'       => array(
+					'spec_name' => 'style for Customizer preview',
+				),
+			);
+
+			// Allow stylesheet links.
+			$this->args['amp_allowed_tags']['link'][] = array(
+				'attr_spec_list' => array(
+					'async'       => array(),
+					'crossorigin' => array(),
+					'href'        => array(
+						'mandatory' => true,
+					),
+					'integrity'   => array(),
+					'media'       => array(),
+					'rel'         => array(
+						'dispatch_key' => 2,
+						'mandatory'    => true,
+						'value_casei'  => 'stylesheet',
+					),
+					'type'        => array(
+						'value_casei' => 'text/css',
+					),
+				),
+				'tag_spec'       => array(
+					'spec_name' => 'link rel=stylesheet for Customizer preview', // phpcs:ignore WordPress.WP.EnqueuedResources.NonEnqueuedStylesheet
+				),
+			);
+		}
+
+		// Allow scripts if requested.
+		if ( ! empty( $this->args['allow_dirty_scripts'] ) ) {
+			$this->args['amp_allowed_tags']['script'][] = array(
+				'attr_spec_list' => array(
+					'type'  => array(),
+					'src'   => array(),
+					'async' => array(),
+					'defer' => array(),
+				),
+				'cdata'          => array(),
+				'tag_spec'       => array(
+					'spec_name' => 'scripts for Customizer preview',
+				),
+			);
+		}
+
 		// Prepare whitelists.
 		$this->allowed_tags = $this->args['amp_allowed_tags'];
 		foreach ( AMP_Rule_Spec::$additional_allowed_tags as $tag_name => $tag_rule_spec ) {
diff --git a/tests/test-amp-helper-functions.php b/tests/test-amp-helper-functions.php
index 26a4963e27d..d6846c33708 100644
--- a/tests/test-amp-helper-functions.php
+++ b/tests/test-amp-helper-functions.php
@@ -372,69 +372,4 @@ public function test_amp_get_schemaorg_metadata() {
 		$this->assertArrayHasKey( 'did_amp_schemaorg_metadata', $metadata );
 		$this->assertEquals( 'George', $metadata['author']['name'] );
 	}
-
-	/**
-	 * Test amp_intercept_post_request_redirect().
-	 *
-	 * @runInSeparateProcess
-	 * @preserveGlobalState disabled
-	 * @covers amp_intercept_post_request_redirect()
-	 */
-	public function test_amp_intercept_post_request_redirect() {
-		if ( ! function_exists( 'xdebug_get_headers' ) ) {
-			$this->markTestSkipped( 'xdebug is required for this test' );
-		}
-
-		add_theme_support( 'amp' );
-		$url = get_home_url();
-
-		add_filter( 'wp_doing_ajax', '__return_true' );
-		add_filter( 'wp_die_ajax_handler', function () {
-			return '__return_false';
-		} );
-
-		ob_start();
-		amp_intercept_post_request_redirect( $url );
-		$this->assertEquals( '{"success":true}', ob_get_clean() );
-
-		$this->assertContains( 'AMP-Redirect-To: ' . $url, xdebug_get_headers() );
-		$this->assertContains( 'Access-Control-Expose-Headers: AMP-Redirect-To', xdebug_get_headers() );
-
-		ob_start();
-		amp_intercept_post_request_redirect( '/new-location/' );
-		$this->assertEquals( '{"success":true}', ob_get_clean() );
-		$this->assertContains( 'AMP-Redirect-To: https://example.org/new-location/', xdebug_get_headers() );
-
-		ob_start();
-		amp_intercept_post_request_redirect( '//example.com/new-location/' );
-		$this->assertEquals( '{"success":true}', ob_get_clean() );
-		$headers = xdebug_get_headers();
-		$this->assertContains( 'AMP-Redirect-To: https://example.com/new-location/', $headers );
-
-		ob_start();
-		amp_intercept_post_request_redirect( '' );
-		$this->assertEquals( '{"success":true}', ob_get_clean() );
-		$this->assertContains( 'AMP-Redirect-To: https://example.org', xdebug_get_headers() );
-	}
-
-	/**
-	 * Test amp_handle_xhr_request().
-	 *
-	 * @runInSeparateProcess
-	 * @preserveGlobalState disabled
-	 * @covers amp_handle_xhr_headers_output()
-	 */
-	public function test_amp_handle_xhr_request() {
-		global $pagenow;
-		if ( ! function_exists( 'xdebug_get_headers' ) ) {
-			$this->markTestSkipped( 'xdebug is required for this test' );
-		}
-
-		$_GET['__amp_source_origin'] = 'https://example.org';
-		$pagenow                     = 'wp-comments-post.php';
-
-		amp_handle_xhr_request();
-		$this->assertContains( 'AMP-Access-Control-Allow-Source-Origin: https://example.org', xdebug_get_headers() );
-
-	}
 }
diff --git a/tests/test-class-amp-theme-support.php b/tests/test-class-amp-theme-support.php
index cab00e90668..3555afca149 100644
--- a/tests/test-class-amp-theme-support.php
+++ b/tests/test-class-amp-theme-support.php
@@ -90,11 +90,15 @@ public function test_register_widgets() {
 	/**
 	 * Test prepare_response.
 	 *
+	 * @global WP_Widget_Factory $wp_widget_factory
 	 * @covers AMP_Theme_Support::prepare_response()
 	 */
 	public function test_prepare_response() {
 		add_theme_support( 'amp' );
 		AMP_Theme_Support::init();
+		$wp_widget_factory = new WP_Widget_Factory();
+		wp_widgets_init();
+
 		ob_start();
 		?>
 		<!DOCTYPE html>
@@ -261,4 +265,69 @@ public function test_get_amp_scripts() {
 			$scripts
 		);
 	}
+
+	/**
+	 * Test intercept_post_request_redirect().
+	 *
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
+	 * @covers AMP_Theme_Support::intercept_post_request_redirect()
+	 */
+	public function test_intercept_post_request_redirect() {
+		if ( ! function_exists( 'xdebug_get_headers' ) ) {
+			$this->markTestSkipped( 'xdebug is required for this test' );
+		}
+
+		add_theme_support( 'amp' );
+		$url = get_home_url();
+
+		add_filter( 'wp_doing_ajax', '__return_true' );
+		add_filter( 'wp_die_ajax_handler', function () {
+			return '__return_false';
+		} );
+
+		ob_start();
+		AMP_Theme_Support::intercept_post_request_redirect( $url );
+		$this->assertEquals( '{"success":true}', ob_get_clean() );
+
+		$this->assertContains( 'AMP-Redirect-To: ' . $url, xdebug_get_headers() );
+		$this->assertContains( 'Access-Control-Expose-Headers: AMP-Redirect-To', xdebug_get_headers() );
+
+		ob_start();
+		AMP_Theme_Support::intercept_post_request_redirect( '/new-location/' );
+		$this->assertEquals( '{"success":true}', ob_get_clean() );
+		$this->assertContains( 'AMP-Redirect-To: https://example.org/new-location/', xdebug_get_headers() );
+
+		ob_start();
+		AMP_Theme_Support::intercept_post_request_redirect( '//example.com/new-location/' );
+		$this->assertEquals( '{"success":true}', ob_get_clean() );
+		$headers = xdebug_get_headers();
+		$this->assertContains( 'AMP-Redirect-To: https://example.com/new-location/', $headers );
+
+		ob_start();
+		AMP_Theme_Support::intercept_post_request_redirect( '' );
+		$this->assertEquals( '{"success":true}', ob_get_clean() );
+		$this->assertContains( 'AMP-Redirect-To: https://example.org', xdebug_get_headers() );
+	}
+
+	/**
+	 * Test handle_xhr_request().
+	 *
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
+	 * @covers AMP_Theme_Support::handle_xhr_request()
+	 */
+	public function test_handle_xhr_request() {
+		global $pagenow;
+		if ( ! function_exists( 'xdebug_get_headers' ) ) {
+			$this->markTestSkipped( 'xdebug is required for this test' );
+		}
+
+		$_GET['__amp_source_origin'] = 'https://example.org';
+		$pagenow                     = 'wp-comments-post.php';
+		AMP_Theme_Support::purge_amp_query_vars();
+
+		AMP_Theme_Support::handle_xhr_request();
+		$this->assertContains( 'AMP-Access-Control-Allow-Source-Origin: https://example.org', xdebug_get_headers() );
+	}
 }