<svg xmlns="http://www.w3.org/2000/svg">
  <style type="text/css">
    .fail {fill: red;}
    .pass {fill: green;}
  </style>
  <rect width="100" height="100" id="test" class="fail"/>
  <script>
	document.getElementById('test').className.baseVal = 'pass';
  </script>
</svg>