Nobody else?

Do you mean validity as per W3C? I wouldn't be too concerned. Try running big sites through it, they have errors, too, all of them.

W3C can be a tool to check for unclosed tags and things like that because browsers are very forgiving and will silently "fix" the errors while displaying the page.

One should look closely if the errors are really errors or things like "this tag isn't officially available in CSS version so and so".