The main page validates OK.
This one does not:
http://www.x-cart.com/demo/product.php?productid=12&cat=24&page=1
This page is not Valid XHTML 1.0 Transitional!
Below are the results of checking this document for XML well-formedness and validity.
1. Error Line 345 column 3: document type does not allow element "ul" here; missing one of "object", "applet", "map", "iframe", "button", "ins", "del" start-tag.
<ul>
The mentioned element is not allowed to appear in the context in which you've placed it; the other mentioned elements are the only ones that are both allowed there and can contain the element mentioned. This might mean that you need a containing element, or possibly that you've forgotten to close a previous element.
One possible cause for this message is that you have attempted to put a block-level element (such as "<p>" or "<table>") inside an inline element (such as "<a>", "<span>", or "<font>").