<html xmlns="http://www.w3.org/1999/xhtml" xmlns:math="http://www.w3.org/1998/Math/MathML">
<head>
<script>
function boom()
{
var mtd1 = document.getElementById("mtd1");
var mtd2 = document.getElementById("mtd2");
var newSpan = document.createElementNS("http://www.w3.org/1999/xhtml", "span");
mtd1.appendChild(newSpan);
mtd1.removeAttribute("columnspan");
mtd2.setAttribute("columnspan", 0);
}
</script>
</head>
<body onload="boom();">
<math:mtd id="mtd1" columnspan="5" /><math:mtd id="mtd2" />
</body>
</html>