Is it safe to enqueue a font style without putting http or https?

I have seen a few WordPress sites and tutorials enqueuing fonts like this. Does this work with http and https sites? Is it safe to do this? I see these exact URL’s in the source code when testing this function on an http site.

add_action('wp_enqueue_scripts', 'theme_external_styles');
function theme_external_styles(){
    wp_enqueue_style( 'main_css', '//maxcdn.bootstrapcdn.com/font-awesome/4.2.0/css/font-awesome.min.css' );
    wp_enqueue_style( 'roboto', '//fonts.googleapis.com/css?family=Roboto:400,300,100,500,700,500italic,400italic,300italic,100italic,700italic' );
}

Yes this is fine and is a good practice, especially if your site may switch between HTTPS/HTTP (like for a shopping cart etc) that way you are not loading mixed content.

As long as the CDN you are pulling from offers both HTTPS and HTTP you’re fine, which they do.