W3c site is actually an authority we should always refer to when it comes to SEO. They are essential.
Seriously? Not very. It is good to validate the site in terms of designing but not that important when it comes to seo.
W3C is not counted in Google's algorithms as far as I know. It is more to do with the markup and appearance of the site, as well as making websites standardized for ease of coding and redevelopment.
This mopst important thing of your site html. google don't line any error or mistakes on html structure. so w3c show you this html errors and If you are completely okay it will show you that you are validated. If you rae not validated then as soon as possible fix your problems of html which is howed on w3c
w3 validation can help your website rank to break a tie if all things are equal... maybe it is included in 200 ranking factors!
w3c only provide your site have no coding error in your page and have perfect design with 0 error. w3c not related with any backlink factor.
W3C is offering a number of other tools to help you check other types of documents (CSS, RDF, P3P, ...), find broken links in your Web pages, and so on. Demand Generation