load a CSS file asynchronously.

loadCSS.js

/*!
loadCSS: load a CSS file asynchronously.
[c]2014 @scottjehl, Filament Group, Inc.
Licensed MIT
*/
function loadCSS( href, before, media ){
	"use strict";
	// Arguments explained:
	// `href` is the URL for your CSS file.
	// `before` optionally defines the element we'll use as a reference for injecting our <link>
	// By default, `before` uses the first <script> element in the page.
	// However, since the order in which stylesheets are referenced matters, you might need a more specific location in your document.
	// If so, pass a different reference element to the `before` argument and it'll insert before that instead
	// note: `insertBefore` is used instead of `appendChild`, for safety re: http://www.paulirish.com/2011/surefire-dom-element-insertion/
	var ss = window.document.createElement( "link" );
	var ref = before || window.document.getElementsByTagName( "script" )[ 0 ];
	ss.rel = "stylesheet";
	ss.href = href;
	// temporarily, set media to something non-matching to ensure it'll fetch without blocking render
	ss.media = "only x";
	// inject link
	ref.parentNode.insertBefore( ss, ref );
	// set media back to `all` so that the styleshet applies once it loads
	setTimeout( function(){
		ss.media = media || "all";
	} );
 }