Looks incredible, valid CSS, nearly valid XHTML
I am the w3c nazi, so I sorta have to do this
XHTML Validation Check
tips:
<a> tags dont have alt, its title
<table> height is more something that is autosized, but if you want to specify it then use CSS
<img> alt is required even if its only alt=""
<td> no background, use the style attribute or put it in the CSS file
<img> is a self closing element so needs to finish with />
for hrefs with a query string, dont use & in the string, use & it will be replaced by the browser when the page is rendered so all your links will still work fine.
that should take care of most of your errors, the only remaining ones are caused by Javascript. Looks like you are using it for the rollovers though, this type of effect can be more compliantly performed using CSS and should work with a wider audience as people still have the option to disable Javascript
PVII CSS Uberlinks
Mo' Betta Rollovers
CSS Image Rollovers
also 'Butterfly Hell' amazing, very nice work