Would it be possible to write a proxy which would convert IE-only (or IE6-only) webpages into equivalent standards-compliant webpages on the fly?
(I don't know CSS, and especially IE's mangling of it, well enough to know if this problem is AI-complete or not.)
Edit: In case it wasn't obvious, the point would be to make the app work just as before. Simply running an anti-htmltidy would be pointless.