I just wonder because I can't figured out. especially the search box. they use <input></input> is that XHTML 1.0 ? I also wonder that google didn't care about valid html because their website is invalid also.
I read this article a while back on valid HTML and thought the experiment relatively interesting: http://www.site-reference.com/articles/Search-Engines/Valid-HTML-Does-Google-Care.html It would seem, based on this, that valid HTML isn't really all that essential.