append style tag to head

function writeCSS(css) {
  var head = document.head || document.getElementsByTagName('head')[0],
    style = document.createElement('style');

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