Searching through youTube, you know how good they are suggesting videos, I ran across this short and helpful snippet about the logic google uses to handle invalid HTML markup on the web and weather or not the google bot penalizes your site for the mistakes.
The short answer is “No”. Google-bot does not penalize your site if there are mistakes or improperly nested HTML tags. Matt Cutts explains that there’s so much invalid html on the web out there that in might actually hurt the quality of search results if google started knocking off points for problems with HTML.
The biggest take away from this video was when Matt says that they have to code for the web we have and not the web we wish we had. fact of the matter is that there are way too many potential problems out there and they code their Google-bots to do the best they can to handle improperly written HTML.