QWebElement and MathMl tags

I have a HTML page with MathMl. I use QWebElement to parse it. I don't want to render the MathMl with QtWebKit, I just to go through the DOM and look the tags and values. So, I do it, I get MathMl tag names but I cannot get values of them. For example:
@<p>Formula <b>begin</b><math>
<mrow>
<mfrac>
<mrow>
<mn>15</mn>
<mo>+</mo>
<mn>789</mn>
</mrow>
<mrow>
<mn>234</mn>
</mrow>
</mfrac>
</mrow>
</math>end</p>"@
I take the tags, but their elements don't have any attributes to do
@QString value = element.attribute("value")@
How can I get the elements' values?