https://codex.WordPress.org/Theme_Markup
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, caption and widgets.
WordPress 主題標記是一個主題特色，主題標記允許在 search forms, comment forms, comment lists, gallery, caption and widgets 等裏，使用 html5 標記。
在 function.php:
$args = array(
'search-form',
'comment-form',
'comment-list',
'gallery',
'caption',
'widgets'
);
add_theme_support( 'html5', $args );
-----------------