This might be because your HTML is incorrect, or it might be some browser configuration issue. It's very unlikely to be anything to do with the XSLT that was used to generate the HTML (except of course that if you generated the wrong HTML then you need to change the XSLT to fix it).
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference