i dont have a copy of the original style sheet, but you might want to run it thru the validator and see what comes out.

So I am not sure that Zen Cart is XHTML Transitional.
For example use your Developer Tool Kit to search for "<br>" and you'll find plenty of examples that are not xhtml transitional. The code *should* be <br /> for XHTML Transitional and Strict.

In order to get my site (and some pages are still not working) to be validated I had to copy my site to my hard drive and use dreamweavers site wide find and replace on the <br> code.

If you have a a defualt zen site see if you can validate it.

My style sheet now passes and my home page is XHTML Transitional. But my inner pages need some work.
But that might be my problem ;)
http://www.zulurose.com