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Your front page validates when I check it - its a full 100% valid as far as I am concerned. :)
I also set up my XOOPS site
(Webguys.com) to be XHTML. It validates on the front page and most of the main pages but I found some issues with certain modules and core sections. But I believe the developers have said that future versions of XOOPS will imporve compatibility with XHTML.
Also if you will notice, my site layout is entirely CSS based. No tables (except where the XOOPS forms module is used).
I may release my theme later on but it requires major modifications to most module templates used. Since most modules are laid out with the default table based layout in mind.
Hopefully in the future XOOPS will have more (XHTML/CSS) web standards friendliness out of the box (changing all those module templates is a pain).