From Googles PageSpeed Insight - loading css files

raf.js

 
  var cb = function() {
    var l = document.createElement('link'); l.rel = 'stylesheet';
    l.href = 'small.css';
    var h = document.getElementsByTagName('head')[0]; h.parentNode.insertBefore(l, h);
  };
  var raf = requestAnimationFrame || mozRequestAnimationFrame ||
    webkitRequestAnimationFrame || msRequestAnimationFrame;
  if (raf) raf(cb);
  else window.addEventListener('load', cb);