sanjuu - 11:56 am on Jul 14, 2011 (gmt 0)
I've always asserted that valid HTML might not help you rank higher, but it'll certainly help you avoid a lot of issues that could negatively impact on your rankings through errors that you don't see amongst the bad HTML.
Also, if it works for more people across different platforms and browsers, then the people you do attract will stay around longer and convert. So getting it right is a good idea. Can also help with usability and accessibility - and making as many of your visitors happy can only be a good thing, surely?