add_theme_support( 'html5', array( 'gallery' ) );
The second parameter accepts one or more values indicating which HTML5 features to support. Support for 'search-form,' 'comment-form,' 'comment-list' were added in 3.6. Support for 'gallery,' and 'caption' were added in 3.9:
$args = array( 'search-form', 'comment-form', 'comment-list', 'gallery', 'caption' ); add_theme_support( 'html5', $args );
The second argument is a required parameter for forward compatibility reasons. Namely, so a theme won't break in newer versions of WordPress that have HTML5 features not supported by an older theme.
Features: sidebar, menus, post-formats, post-thumbnails, custom-background, custom-header, automatic-feed-links, content_width, editor-style, html5