Yes, but if browsers were strict by default (or at least by default in dev mode), the bug in his compositing software likely would have been found and fixed muc

Yes, but if browsers were strict by default (or at least by default in dev mode), the bug in his compositing software likely would have been found and fixed much earlier, and it wouldn't have persisted into production. So again, I understand that it's nearly impossible to make the transition now, but that doesn't mean it wouldn't be preferable. And there's no reason the transition couldn't still be made, but much more slowly, and perhaps without ever making the big browsers reject by default. I don't know if there are other disadvantages to XHTML, but if the strictness issue is the only one, then it seems like there would still be value in slowly transitioning over.