diff --git a/wp-admin/edit-form-blocks.php b/wp-admin/edit-form-blocks.php
index 3aead40a7b..71328d0ad5 100644
--- a/wp-admin/edit-form-blocks.php
+++ b/wp-admin/edit-form-blocks.php
@@ -131,7 +131,8 @@ $available_templates = ! empty( $available_templates ) ? array_merge(
 // Editor Styles.
 $styles = array(
 	array(
-		'css' => 'body { font-family: -apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Oxygen-Sans,Ubuntu,Cantarell,"Helvetica Neue",sans-serif }',
+		'css'            => 'body { font-family: -apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Oxygen-Sans,Ubuntu,Cantarell,"Helvetica Neue",sans-serif }',
+		'__unstableType' => 'core',
 	),
 );
 if ( $editor_styles && current_theme_supports( 'editor-styles' ) ) {
@@ -140,15 +141,17 @@ if ( $editor_styles && current_theme_supports( 'editor-styles' ) ) {
 			$response = wp_remote_get( $style );
 			if ( ! is_wp_error( $response ) ) {
 				$styles[] = array(
-					'css' => wp_remote_retrieve_body( $response ),
+					'css'            => wp_remote_retrieve_body( $response ),
+					'__unstableType' => 'theme',
 				);
 			}
 		} else {
 			$file = get_theme_file_path( $style );
 			if ( is_file( $file ) ) {
 				$styles[] = array(
-					'css'     => file_get_contents( $file ),
-					'baseURL' => get_theme_file_uri( $style ),
+					'css'            => file_get_contents( $file ),
+					'baseURL'        => get_theme_file_uri( $style ),
+					'__unstableType' => 'theme',
 				);
 			}
 		}
diff --git a/wp-includes/block-editor.php b/wp-includes/block-editor.php
index 3234c4ad1f..101cc1ba71 100644
--- a/wp-includes/block-editor.php
+++ b/wp-includes/block-editor.php
@@ -283,10 +283,14 @@ function get_block_editor_settings( array $custom_settings, $block_editor_contex
 	$theme_json = WP_Theme_JSON_Resolver::get_merged_data( $editor_settings );
 
 	if ( WP_Theme_JSON_Resolver::theme_has_support() ) {
-		$editor_settings['styles'][] = array( 'css' => $theme_json->get_stylesheet( 'block_styles' ) );
+		$editor_settings['styles'][] = array(
+			'css'            => $theme_json->get_stylesheet( 'block_styles' ),
+			'__unstableType' => 'globalStyles',
+		);
 		$editor_settings['styles'][] = array(
 			'css'                     => $theme_json->get_stylesheet( 'css_variables' ),
 			'__experimentalNoWrapper' => true,
+			'__unstableType'          => 'globalStyles',
 		);
 	}
 
diff --git a/wp-includes/blocks/legacy-widget.php b/wp-includes/blocks/legacy-widget.php
index 6a49111b8e..f8b64ab5c1 100644
--- a/wp-includes/blocks/legacy-widget.php
+++ b/wp-includes/blocks/legacy-widget.php
@@ -50,21 +50,25 @@ function render_block_core_legacy_widget( $attributes ) {
 }
 
 /**
- * On application init this does two things:
- *
- * - Registers the 'core/legacy-widget' block.
- * - Intercepts any request with legacy-widget-preview in the query param and,
- *   if set, renders a page containing a preview of the requested Legacy Widget
- *   block.
+ * Registers the 'core/legacy-widget' block.
  */
-function init_legacy_widget_block() {
+function register_block_core_legacy_widget() {
 	register_block_type_from_metadata(
 		__DIR__ . '/legacy-widget',
 		array(
 			'render_callback' => 'render_block_core_legacy_widget',
 		)
 	);
+}
 
+add_action( 'init', 'register_block_core_legacy_widget' );
+
+/**
+ * Intercepts any request with legacy-widget-preview in the query param and, if
+ * set, renders a page containing a preview of the requested Legacy Widget
+ * block.
+ */
+function handle_legacy_widget_preview_iframe() {
 	if ( empty( $_GET['legacy-widget-preview'] ) ) {
 		return;
 	}
@@ -110,4 +114,7 @@ function init_legacy_widget_block() {
 	exit;
 }
 
-add_action( 'init', 'init_legacy_widget_block' );
+// Ensure handle_legacy_widget_preview_iframe() is called after Core's
+// register_block_core_legacy_widget() (priority = 10) and after Gutenberg's
+// register_block_core_legacy_widget() (priority = 20).
+add_action( 'init', 'handle_legacy_widget_preview_iframe', 21 );
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 658d14c5c4..9b5170df1f 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-51089';
+$wp_version = '5.8-alpha-51090';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.