<html xmlns="http://www.w3.org/1999/xhtml" xmlns:m="http://www.w3.org/1998/Math/MathML">
<head>
<script>
function boom()
{
document.getElementById("iframe").appendChild(document.createElement("span"));
document.getElementById("mrow").appendChild(document.createElement("span"));
}
</script>
</head>
<body onload="boom();"><m:mrow id="mrow"><iframe id="iframe"/></m:mrow></body>
</html>