Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 27409)
+++ src/wp-includes/theme.php	(working copy)
@@ -1379,7 +1379,9 @@
  * The init hook may be too late for some features.
  *
  * @since 2.9.0
+ *
  * @param string $feature the feature being added
+ * @return void|bool False if trying to add html5 support and not passing an array as 2nd argument (empty 2nd arg works but is discouraged)
  */
 function add_theme_support( $feature ) {
 	global $_wp_theme_features;
@@ -1397,8 +1399,9 @@
 
 		case 'html5' :
 			// You can't just pass 'html5', you need to pass an array of types.
+			// Note: as of 3.9.0 core supports the types 'comment-list', 'comment-form', 'search-form' and 'gallery'
 			if ( empty( $args[0] ) ) {
-				$args = array( 0 => array( 'comment-list', 'comment-form', 'search-form' ) );
+				$args = array( 0 => array( 'comment-list', 'comment-form', 'search-form', 'gallery' ) );
 			} elseif ( ! is_array( $args[0] ) ) {
 				_doing_it_wrong( "add_theme_support( 'html5' )", 'You need to pass an array of types.', '3.6.1' );
 				return false;
Index: tests/phpunit/tests/theme/support.php
===================================================================
--- tests/phpunit/tests/theme/support.php	(revision 27409)
+++ tests/phpunit/tests/theme/support.php	(working copy)
@@ -90,6 +90,7 @@
 		$this->assertTrue( current_theme_supports( 'html5', 'comment-form' ) );
 		$this->assertTrue( current_theme_supports( 'html5', 'comment-list' ) );
 		$this->assertTrue( current_theme_supports( 'html5', 'search-form' ) );
+		$this->assertTrue( current_theme_supports( 'html5', 'gallery' ) );
 		$this->assertFalse( current_theme_supports( 'html5', 'something-else' ) );
 	}