background size cover polyfill for IE (6, 7, 8)

gistfile1.js

  if ("backgroundSize" in document.body.style) {
    return;
  }
  var body = $$('body').first(), 
  img = body.getStyle('background-image').sub(/url\((.+?)\)/,'#{1}') || 'default.jpeg',
  fill = new Element('img', {src: img,alt: ''}).
    setStyle('position:absolute; top:0; left:0; width:100%; height:100%; z-index:-1');
  body.setStyle('background:none');
  body.insert(fill);