Clean HTML is GOOD - the w3.org validator is quite useful, and I've read a number of the HTML editor tools have this capability - I happen to use the Tidy plug-in for vim (highly recommended), but I'm a bit of a dinosaur for my HTML! ;-)
One thing that it has helped me with MANY time is mistakes in the HTML - i.e. forgetting to close tags, matching 'em up, etc. Browsers tend to be fairly forgiving, but sometimes you end up with something that looks decent on Netscape/Mozilla, but not so good on IE ... or whatever.
Having good HTML gets rid of most of these issues ... plus (similar to the browsers), it means the search engine should interpret your HTML just as you meant it.
alek
P.S. BTW,
http://validator.w3.org/ is (and has been) a PR10 site - boy, wouldn't you like a link from that page, eh?!? ;-)