diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index ed33051c08..7a2b357d0a 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -4181,7 +4181,12 @@ function _add_default_theme_supports() {
 	add_theme_support( 'post-thumbnails' );
 	add_theme_support( 'responsive-embeds' );
 	add_theme_support( 'editor-styles' );
-	add_theme_support( 'html5', array( 'comment-form', 'comment-list', 'style', 'script' ) );
+	/*
+	 * Makes block themes support HTML5 by default for the comment block and search form
+	 * (which use default template functions) and `[caption]` and `[gallery]` shortcodes. 
+	 * Other blocks contain their own HTML5 markup.
+	 */
+	add_theme_support( 'html5', array( 'comment-form', 'comment-list', 'search-form', 'gallery', 'caption', 'style', 'script' ) );
 	add_theme_support( 'automatic-feed-links' );
 
 	add_filter( 'should_load_separate_core_block_assets', '__return_true' );
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 58b9777c83..f0d0b84ba9 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-beta4-52438';
+$wp_version = '5.9-beta4-52439';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.