MathML JavaScript Question

Hi,
 
I created a simple example (attached) to display MathML on an xhtml page using the xsl style sheets from the w3C website. I then altered the example to display the MathML through simple javascript document.write commands. However, the page seems to refresh itself to apply the PresentationMathML at which point it removes the lines written by the javascript so they are no longer displayed.
 
Am I doing something wrong? Is this a limitation of the style sheets? Any help will be gratefully received!!!
 
Cheers
 
Matt