HTML validator dislikes PrimeNet
 

I fed [URL]http://www.mersenne.org/primenet/[/URL] to [URL]http://validator.w3.org/[/URL] and got quite a number of warnings; other mersenne.org pages give similar results. Now I fully understand if compatibility with HTML standards isn't exactly a high priority for GIMPS, but at least some issues should be easy to fix (adding a document type declaration and moving all CSS to the header, for starters).