The funny thing is that as an HTML page gets increasingly large and complicated, I frequently see developers add HTML comments demarcating precisely which element was just closed, so that in the future it's easier to insert elements at the right level of nesting. Example:

  </div><!-- End of #section-nav -->

Would it be a horrible thing if HTML supported an optional id or class attribute in the closing tag?

  </div id="section-nav">

Of course it would have to match to validate. Probably not realistic in that my guess is this wouldn't be SGML-backwards compatible.