English 
I’ve read quite a few articles on the impact of the W3C (World Wide Web Consortium) validation on Search Engine Optimisation.
- SEO and importance of valid source code, Search Engine Journal
- W3C Code Validation is NOT Important in Search Engine Ranking, SEO contest 2008
- Developing a Web Accessibility Business Case for Your Organization: Overview, W3C
The source code should be valid for accessibility reasons, which have a direct impact on your online visibility.
The W3C provides a Markup Validator that helps check the validity of Web documents. A W3C website loads faster, works on major browsers and screen resolutions and is easier to manage. As a result, the W3C guidelines help you to reach more users (who may enjoy your website, write about it on the Internet, include a link to your website, … and finally improve your SEO). 
Tags: source code, w3c
