gistfile1.js

var css = '@keyframes pound {50% { transform: scale(1.4); }} .thechosenone {background-image: url("http://i.gyazo.com/8e5ca2b4889a1094fa1e71aa977eb4fe.png"); background-size:cover; animation: pound .25s ease}',
  iframe = document.querySelector('iframe'),
  head = iframe.contentDocument.head,
  style = iframe.contentDocument.createElement('style');

style.type = 'text/css';
if (style.styleSheet){
  style.styleSheet.cssText = css;
} else {
  style.appendChild(document.createTextNode(css));
}

head.appendChild(style);