A topic that has been discussed extensively by some people in the SEO community is the notion that having a W3C compliant web site is either critical to SEO or it’s not. If the goal is to ensure that a client site has the best possible chance at it’s highest organic rankings, then we need to acknowledge that even just factoring in Google, there are about 200 indicators to consider.
http://www.searchenginejournal.com/w3c-validation-for-seo-myth-and-reality/18566/