I don't mean to toot my horn, but I've gotta spread the gospel that HTML::Lint,and its corresponding weblint wrapper are pretty darn useful. Every so often someone will ask me "Hey, can you look at my site, and make sure that it's OK?" The first thing I do is run weblint on it to check that the HTML is reasonably clean.
As an example, I ran it on Randal's website: at (292:6) is never closed
Nothing very serious, since most browsers will handle the unclosed TD tag, and the IMG HEIGHT & WIDTH are just rendering helpers. Still, they're worth fixing.
Here's another example from someone still fixing up the pages for his upcoming book: