I believe that I have found a problem with your online CSS validator. When validating the CSS within an XHTML document which uses the 'Strict XHTML' doctype it comes back with the following error:

org.xml.sax.SAXException: Please, fix your system identifier (URI) in the DOCTYPE rule.

When the same document is using the 'Transitional XHTML' doctype it works fine.

I've provided two links for you below, both files are identical except the first is using Strict XHTML and the second is using Transitional XHTML.

http://jigsaw.w3.org/css-validator/validator?uri=http://www27.brinkster.com/stemgruppe/test/test.html
http://jigsaw.w3.org/css-validator/validator?uri=http://www27.brinkster.com/stemgruppe/test/test2.html
Cheers,
 
  | James Drake
  | jimdrake@bluesphraks.cjb.net
  |
  | http://smg.1444.net
  | http://bluesphraks.cjb.net
  |
  | 13/03/2002
  |
  | ¶