When WordPress 4.7 was released, an option to use a header video instead of an image was added to the Customizer. Including the feature in a theme like Arcade seemed like a good idea but took me a while to finally get right.
Video Header Code
Including support for a video header is straightforward enough.add_theme_support( 'custom-header', array( 'video' => true, ) );If you already declare support for a custom header you can just add
'video' => true, to the argument array.
In order for the new header video to appear in your theme, you need to place the the_custom_header_markup() function in the right spot. To get it working in Arcade took some custom CSS and a few changes to the core PHP code but I managed to figure it all out.