Well, Google likes W3c validated website and helps in ranking well on SERP. But many times webmaster use Grey Hat and Black hat (Against to S.E. policies) Even that sites are ranking on top. So just to do or not to do will not lead to any final decision of its significance.
The pages built according to the validation given by W3C is considered more. seo traffic the seo traffic
Having valid HTML / CSS is NOT a factor in ranking a site. How could it be? Having a site that is semantically correct which uses XHTML tags such as p, ul, li, strong, h1, h2, etc IS a very important ranking factor. http://validator.w3.org/check?uri=w...(detect+automatically)&doctype=Inline&group=0
Valid HTML / XHTML is good for SEO and importantly - good for site visitors. IMHO it's about time that the SEs penalised sites that do not meet basic standards compliance and accessibility guidelines.
Mmm, I have a feeling that everyone is just going to come in here and say that 'Yes, valid HTML is important'... So this is a quote from Matt Cutts about valid HTML and it confirms that valid HTML is NOT IMPORTANT FOR RANKING.