<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script>
function foo()
{
var div = document.getElementById("div");
div.appendChild(div.firstChild);
var td = document.getElementById("td");
td.setAttribute('rowspan', -2);
}
</script>
</head>
<body onload="foo()">
<div id="div"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mtable>
<mtr>
<mtd id="td"><mi>y</mi></mtd>
</mtr>
</mtable>
</math></div>
</body>
</html>