"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.
Also visit: http://googlewebmastercentral.blogspot.com/2011/07/validation-measuring-and-tracking-code.htmlWhile paying a lot of attention to validation, we've developed a system to use it as a quality metric to measure how we're doing on our own pages. Here's what we do: we give each of our pages a score from 0-10 points, where 0 is worst (pages with 10 or more HTML and CSS validation errors) and 10 is best (0 validation errors). We started doing this more than two years ago, first by taking samples, now monitoring all our pages."