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Codex

Semantic Markup

Semantic 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, and comment lists.

Adding Theme Support

Since Version 3.6, themes should use add_theme_support() in the functions.php file in order to support semantic markup, like so:

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.

Resources

Related

Theme Support: add_theme_support(), remove_theme_support(), current_theme_supports()
Features: sidebar, menus, post-formats, post-thumbnails, custom-background, custom-header, automatic-feed-links, content_width, editor-style, html5