Having seen so many failure getting worse because people did not paid attention to it in the first place, I would go for the error page.

That said, even if we choose a defense in depth, it still is not a valid reason to throw away XHTML as it was done.

The same way you will not remove your firewall because it does not block dormant viruses, but add an antivirus, the browsers could have still continue putting effort in rendering the correct, and powerful, XHTML, while setting down some decent fallbacks to recover. Being we a be red warning to raise awareness that something need to be fixed.