How do I async loaded CSS

gistfile1.txt

//Stolen from http://stackoverflow.com/questions/24020254/async-true-for-css-link-tag

  var resource = document.createElement('link'); 
  resource.setAttribute("rel", "stylesheet");
  resource.setAttribute("href","path/to/cssfile.css");
  resource.setAttribute("type","text/css");      
  var head = document.getElementsByTagName('head')[0];
  head.appendChild(resource);