Diego, SVG, XUL and MathML are just examples, furthermore, ItsNat knows nothing about MathML
You're needing something like this:
<g:plusone id="googlePlusId"
xmlns:g="http://mygoogle/mygoogle">To be replaced</g:plusone>
<script>
//window.onload = function ()
//{
var elem = document.getElementById("googlePlusId");
var elem2 = document.createElement("button");
elem2.appendChild(document.createTextNode("Google+"));
elem.parentNode.replaceChild(elem2,elem);
//}
</script>
Declaring an invented namespace is needed in ItsNat (server side).
It works in W3C browsers (Chrome, FireFox, IE 9+ etc) and near-W3C like MSIE 6-8
Of you can convert this especial element to HTML in server side, with some care no <g:plusone> will be sent to client, only converted HTML by you.
Regards