Some MathML? examples from http://www.w3.org/Math/XSL/pmathml2.xml :

<math xmlns="http://www.w3.org/1998/Math/MathML">
<mrow>
<msup>
<mfenced open="[" close="]?">
<mrow>

<mi>a</mi> <mo>+</mo>

<mi>b</mi>

</mrow>

</mfenced> <mn>260</mn>

</msup> <mo>+</mo> <msub>

<mfenced open="{" close="}">
<mrow>
<mi>a</mi> <mo>+</mo> <mi>b</mi>

</mrow>

</mfenced>

<mi>i</mi>

</msub>

</mrow>

</math>

<math xmlns="http://www.w3.org/1998/Math/MathML">
<msup>
<msqrt>
<mrow>

<mi>a</mi> <mo>+</mo>

<mi>b</mi>

</mrow>

</msqrt> <mn>27</mn>

</msup>

</math>

</html>

Hmm, I think we need declarations in the skin like:

<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="pmathml.xsl"?>
<html xmlns="http://www.w3.org/1999/xhtml">