Let's fix HN first, then we'll worry about another site.
http://validator.w3.org/check?uri=http%3A%2F%2Fnews.ycombina...
http://validator.w3.org/check?uri=http%3A%2F%2Fnews.ycombina...
You can find some here: http://www.google.com/search?hl=en&q=site%3Anews.ycombin...
In particular, pg responds somewhere in this thread: http://news.ycombinator.com/item?id=100460). I might be carefully building a straw man here, as you are speaking to valid xhtml, not necessarily semantic xhtml. I hope you'll forgive me. I think the two topics are related, and that some of the arguments overlap.
The bottom line seems to be "Who cares? It works fine."
In any case, it doesn't really matter to me, so I apologize if I came off as snobbish. I was simply trying to point out the irony of asking HN if they could make a site better when HN itself has invalid xhtml.