fontfaceObserver.js

//  Detect if web fonts are available

function add_custom_script_font_face_observer_js() {

			wp_register_script( 'fontface_observer', 'https://cdnjs.cloudflare.com/ajax/libs/fontfaceobserver/2.3.0/fontfaceobserver.standalone.js', array(), '2.3.0', true );

			wp_enqueue_script( 'fontface_observer' );

}

add_action( 'wp_enqueue_scripts', 'add_custom_script_font_face_observer_js' );