I think that a well written HTML and CSS code can make drastic changes in search engines results and we all know that the official W3C validator is a great place to check if your website’s HTML/XHTML code is well written. The problem is that the official W3C can only validate one URL at a time.
W3Clove is a free web service which crawls any given website and generates a sitemap of it (you can add a sitemap directly) and validates each URL through the W3C’s validator. The errors and warning are displayed as a list and a basic report is show containing the most common errors and warnings as a summary. W3Clove also remembers the recent checks you made and presents them to you quickly.
Markup validation for all your pages Example