Theme Markup is a theme feature, first introduced in Version 3.6. This feature allows themes to explicitly choose to apply HTML5 markup for search forms, comment forms, comment lists, gallery and caption.
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' and 'comment-list' were added in Version 3.6. Support for 'gallery' and 'caption' were added in Version 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.
<figcaption>elements, instead of the generic definition list markup to output galleries. More on this in the Make WordPress Core article.
Theme Features: sidebar, menus, post-formats, title-tag, custom-background, custom-header, custom-logo, post-thumbnails, automatic-feed-links, html5, editor-style, content_width