Is HTML Validation Really Necessary for Ranking?
Matt Cutts from Google answers the question: Is HTML code validation really necessary for search engine ranking? The short answer is Google and other search engines are generally forgiving with non-validating code.
So although it is a good thing to strive for valid code, it isn’t really all that necessary in the grand scheme of things so long as the page renders well in the browser.
In that respect whenever you make changes to your blog/website just be sure to check how it looks in all of the major browsers like Firefox, Internet Explorer, Google Chrome, Safari and Opera. If it looks good then it’s probably good enough for the search engine spiders as well.
A good example is Google’s Blogger which uses non-conformist code. If you don’t believe me then just try to validate Blogger code for yourself and see the results you get. You will be shocked to find dozens, if not hundreds of errors. But Blogger blogs still look good in modern browsers and are indexed just fine by Google and the other search engines.
The same goes for 3rd party scripts and widgets you love to stuff in your sidebar. So don’t sweat the small stuff and concentrate on more important matters like writing great, original content according to your niche.