Google believes that pages that validate do not rank high. Having valid HTML to boost up ranking in Google and other search engine may not have much impact on your ranking and this is what most SEOs believe.
In a recent You Tube Video, Google's Matt Cutts explained why Google.com does not validate pages. He also said that Google doesn't "give any sort of boost to web pages that validate." He pointed out because "the vast majority of pages on the web don't validate," including Google.com.
Here is the video to give more details: