I would like to suggest checking the markup of SO to uncover bugs.
No, not for accessibility or philosophical reasons, and definitely not for the sake of validation itself.
And forget the goal of producing 100% valid HTML.
I'm just talking about a sensible sanity check to catch simple mistakes before they multiply.
For example, many pages have multiple elements with the same ID value. This is likely to lead to undesired behaviour of CSS or JavaScript in at least some browsers.