<html xmlns='http://www.w3.org/1999/xhtml'> 
<head> 
<title>msup javascript base change test</title> 
</head> 
<body> 
<p> 
<math xmlns='http://www.w3.org/1998/Math/MathML'> 
<msup id="sup"><mi>x</mi><mn>1</mn></msup> 
</math> 
</p> 
<script type='text/javascript'>
var msup = document.getElementById("sup");
msup.removeChild(msup.firstChild);
var base = document.createElementNS("http://www.w3.org/1998/Math/MathML","mi");
base.appendChild(document.createTextNode("y"));
msup.insertBefore(base,msup.firstChild);
</script>
</body>
</html>