After cleaning up the XML sources in order to be XHTML 1.0 Strict compliant, I realized the step towards XHTML 1.1 was minimal.
I hope I didn't miss a page while revalidating the whole site. If you stumble over an invalid document, please let me know, so I can fix it.
I'm aware of the fact that Netscape 4 cannot cope with certain parts of XHTML 1.1. However, it's still able to display the site and it wasn't able to display te XHTML 1.0 versions correctly either.