<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script>
function foo()
{
document.getElementById("mtr").appendChild(document.createTextNode("a"));
}
</script>
</head>
<body onload="foo()">
<div><math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>
<mfenced open="[" close="]">
<mtable>
<mtr id="mtr">
<mtd><mi>x</mi></mtd>
<mtd><mi>y</mi></mtd>
</mtr>
</mtable>
</mfenced>
</mrow>
</math></div>
</body>
</html>