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