diff --git a/wp-includes/blocks/categories.php b/wp-includes/blocks/categories.php
index bd1175733b..83bf1c9db2 100644
--- a/wp-includes/blocks/categories.php
+++ b/wp-includes/blocks/categories.php
@@ -16,11 +16,6 @@ function render_block_core_categories( $attributes ) {
 	static $block_id = 0;
 	$block_id++;
 
-	$align = 'center';
-	if ( isset( $attributes['align'] ) && in_array( $attributes['align'], array( 'left', 'right', 'full' ), true ) ) {
-		$align = $attributes['align'];
-	}
-
 	$args = array(
 		'echo'         => false,
 		'hierarchical' => ! empty( $attributes['showHierarchy'] ),
@@ -46,10 +41,14 @@ function render_block_core_categories( $attributes ) {
 		$type           = 'list';
 	}
 
-	$class = "wp-block-categories wp-block-categories-{$type} align{$align}";
+	$class = "wp-block-categories wp-block-categories-{$type}";
+
+	if ( isset( $attributes['align'] ) ) {
+		$class .= " align{$attributes['align']}";
+	}
 
 	if ( isset( $attributes['className'] ) ) {
-		$class .= ' ' . $attributes['className'];
+		$class .= " {$attributes['className']}";
 	}
 
 	$block_content = sprintf(
diff --git a/wp-includes/blocks/latest-comments.php b/wp-includes/blocks/latest-comments.php
index dd310028ce..199a426a93 100644
--- a/wp-includes/blocks/latest-comments.php
+++ b/wp-includes/blocks/latest-comments.php
@@ -5,32 +5,34 @@
  * @package gutenberg
  */
 
-/**
- * Get the post title.
- *
- * The post title is fetched and if it is blank then a default string is
- * returned.
- *
- * Copied from `wp-admin/includes/template.php`, but we can't include that
- * file because:
- *
- * 1. It causes bugs with test fixture generation and strange Docker 255 error
- *    codes.
- * 2. It's in the admin; ideally we *shouldn't* be including files from the
- *    admin for a block's output. It's a very small/simple function as well,
- *    so duplicating it isn't too terrible.
- *
- * @since 3.3.0
- *
- * @param int|WP_Post $post Optional. Post ID or WP_Post object. Default is global $post.
- * @return string The post title if set; "(no title)" if no title is set.
- */
-function gutenberg_draft_or_post_title( $post = 0 ) {
-	$title = get_the_title( $post );
-	if ( empty( $title ) ) {
-		$title = __( '(no title)', 'gutenberg' );
+if ( ! function_exists( 'gutenberg_draft_or_post_title' ) ) {
+	/**
+	 * Get the post title.
+	 *
+	 * The post title is fetched and if it is blank then a default string is
+	 * returned.
+	 *
+	 * Copied from `wp-admin/includes/template.php`, but we can't include that
+	 * file because:
+	 *
+	 * 1. It causes bugs with test fixture generation and strange Docker 255 error
+	 *    codes.
+	 * 2. It's in the admin; ideally we *shouldn't* be including files from the
+	 *    admin for a block's output. It's a very small/simple function as well,
+	 *    so duplicating it isn't too terrible.
+	 *
+	 * @since 3.3.0
+	 *
+	 * @param int|WP_Post $post Optional. Post ID or WP_Post object. Default is global $post.
+	 * @return string The post title if set; "(no title)" if no title is set.
+	 */
+	function gutenberg_draft_or_post_title( $post = 0 ) {
+		$title = get_the_title( $post );
+		if ( empty( $title ) ) {
+			$title = __( '(no title)', 'gutenberg' );
+		}
+		return esc_html( $title );
 	}
-	return esc_html( $title );
 }
 
 /**
diff --git a/wp-includes/post-template.php b/wp-includes/post-template.php
index b80ac4d4e4..9ff1d08ea4 100644
--- a/wp-includes/post-template.php
+++ b/wp-includes/post-template.php
@@ -722,6 +722,10 @@ function get_body_class( $class = '' ) {
 		$classes[] = 'wp-custom-logo';
 	}
 
+	if ( current_theme_supports( 'responsive-embeds' ) ) {
+		$classes[] = 'wp-embed-responsive';
+	}
+
 	$page = $wp_query->get( 'page' );
 
 	if ( ! $page || $page < 2 )
diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
index 4aa7db4f6f..4c958f8f67 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
@@ -111,8 +111,9 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 			$formats                           = array_merge( array( 'standard' ), $formats );
 			$data['theme_supports']['formats'] = $formats;
 
-			$data['theme_supports']['post-thumbnails'] = false;
-			$post_thumbnails                           = get_theme_support( 'post-thumbnails' );
+			$data['theme_supports']['post-thumbnails']   = false;
+			$data['theme_supports']['responsive-embeds'] = (bool) get_theme_support( 'responsive-embeds' );
+			$post_thumbnails                             = get_theme_support( 'post-thumbnails' );
 
 			if ( $post_thumbnails ) {
 				// $post_thumbnails can contain a nested array of post types.
@@ -156,16 +157,21 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 					'type'        => 'array',
 					'readonly'    => true,
 					'properties'  => array(
-						'formats'         => array(
+						'formats'           => array(
 							'description' => __( 'Post formats supported.' ),
 							'type'        => 'array',
 							'readonly'    => true,
 						),
-						'post-thumbnails' => array(
+						'post-thumbnails'   => array(
 							'description' => __( 'Whether the theme supports post thumbnails.' ),
 							'type'        => array( 'array', 'bool' ),
 							'readonly'    => true,
 						),
+						'responsive-embeds' => array(
+							'description' => __( 'Whether the theme supports responsive embedded content.', 'gutenberg' ),
+							'type'        => 'bool',
+							'readonly'    => true,
+						),
 					),
 				),
 			),
diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index 947226c4d4..3a7a9e871d 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -2210,12 +2210,14 @@ function get_theme_starter_content() {
  * @since 4.1.0 The `title-tag` feature was added
  * @since 4.5.0 The `customize-selective-refresh-widgets` feature was added
  * @since 4.7.0 The `starter-content` feature was added
+ * @since 5.0.0 The `responsive-embeds` feature was added.
  *
  * @global array $_wp_theme_features
  *
  * @param string $feature  The feature being added. Likely core values include 'post-formats',
  *                         'post-thumbnails', 'html5', 'custom-logo', 'custom-header-uploads',
- *                         'custom-header', 'custom-background', 'title-tag', 'starter-content', etc.
+ *                         'custom-header', 'custom-background', 'title-tag', 'starter-content',
+ *                         'responsive-embeds/', etc.
  * @param mixed  $args,... Optional extra arguments to pass along with certain features.
  * @return void|bool False on failure, void otherwise.
  */
diff --git a/wp-includes/version.php b/wp-includes/version.php
index bf85ac755b..d5c6edee29 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-43789';
+$wp_version = '5.0-alpha-43790';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.