code: function loadCSS( cssUrl, currentLink ) {
  	return new Promise( resolve => {
  		const link = document.createElement( 'link' );
  		link.rel = 'stylesheet';
  		link.type = 'text/css';
  		link.href = cssUrl;
  
  		if ( 'onload' in link ) {
  			link.onload = () => {
  				link.onload = null;
  				resolve( link );
  			};
  		} else {
  			// just wait 500ms if the browser doesn't support link.onload
  			// https://pie.gd/test/script-link-events/
  			// https://github.com/ariya/phantomjs/issues/12332
  			setTimeout( () => resolve( link ), 500 );
  		}
  
  		document.head.insertBefore( link, currentLink ? currentLink.nextSibling : null );
  	} );
  }
docs: Loads a CSS stylesheet into the page.
  
  @param {string} cssUrl URL of a CSS stylesheet to be loaded into the page
  @param {Element} currentLink an existing <link> DOM element before which we want to insert the new one
  @returns {Promise<String>} the new <link> DOM element after the CSS has been loaded