Your browser can't just display any xml, like it wouldn't recognise:
<bigfont>Hello</bigfont>
<picture>img_5.gif</picture>
you have to parse the xml file first to convert it into it's html equivilent.
if you go to zend.com there are loads of scripts on there which parse xml files.
Hope this is of some use, if I have misunderstood the question please forgive me.