Missing dns-prefetch link when using "nested" wp_register_style

Question

Given this script:

function register_styles(){
    wp_register_style('google-fonts', '//fonts.googleapis.com/css?family=Oswald|Open+Sans');
    wp_register_style('bootstrap', get_template_directory_uri() . '/css/bootstrap.min.css', array('google-fonts'));
    wp_register_style('font-awesome', '//maxcdn.bootstrapcdn.com/font-awesome/4.6.3/css/font-awesome.min.css', array('bootstrap'));
}
add_action('init', 'register_styles');

function enqueue_styles(){
    wp_enqueue_style('font-awesome');
}
add_action('wp_enqueue_scripts', 'enqueue_styles');

Styles are rendered correctly, but I get only one dns-prefetch link:

<link rel='dns-prefetch' href='//maxcdn.bootstrapcdn.com' />

Instead, since font-awesome depends on bootstrap that in turn depends on google-fonts, I'd expect a dns-prefetch for Google Fonts too, like this:

<link rel='dns-prefetch' href='//fonts.googleapis.com' />
<link rel='dns-prefetch' href='//maxcdn.bootstrapcdn.com' />

Is this a Wordpress bug? I'm using version 4.6.1

Answer 1

Since WP 4.6 prefetch dependencies are only generated when styles are enqueued, not when they are only registered. Since you are only enqueueing Font Awesome, that is the only prefetch link you get. If you enqueue all style files properly, your problem should be solved.