<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script>
function boom()
{
var s = document.createElementNS("http://www.w3.org/1999/xhtml", "span");
var b = document.createElementNS("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul", "box");
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document.getElementById("a").appendChild(s);
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<body onload="boom();" style="-moz-column-width: 1px;"><span id="a"><box xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"/></span><div>Hello</div></body>
</html>