animationStartTime: shim layer with Date.now() fallback

animStartTime.js

window.animStartTime = (function(w) {
  return  w.animationStartTime       || 
          w.webkitAnimationStartTime || 
          w.mozAnimationStartTime    || 
          w.oAnimationStartTime      || 
          w.msAnimationStartTime     || 
          +new Date();
})(window);