<html>
<head>
<style>
.green {
  background-color: lime;
}
</style>
</head>
<body>
<div id="foo">The background of this div should be green.</div>
<script>
document.getElementById('foo').id = 'goo';
document.getElementById('goo').className = 'green';
</script>
</body>
</html>