Google Webmaster Blog: #Validation: measuring and tracking #code #quality
http://googlewebmastercentral.blogspot.com/2011/07/validation-measuring-and-tracking-code.html
Why does validation matter? There are different perspectives on validation—at Google there are different approaches and priorities too—but the Webmaster Team considers validation a baseline quality attribute. It doesn’t guarantee accessibility, performance, or maintainability, but it reduces the number of possible issues that could arise and in many cases indicates appropriate use of technology.