One Size Does Not Fit All
Tim Bray: Micah Dubinko asks “Is HTML on the Web a special case?”, and the answer is obviously “yes”. Note that the HTML language being developed by the WhatWG is not XML at all, and I’m not brave enough to predict whether that is a good idea.
I assert that HTML isn’t the only special case. Take a look at the pinkish rows in this table. Some merely violate RFC 3023. In others, the errors are somewhat more... troublesome.
Tim continues: At the other end, if someone sends a blog post from their cellphone with a picture of a cute kitten, you don’t want to reject it because there’s an “&” in the wrong spot.
I would assert that solutions to that use case would include APP.
I will also note in passing that while Tim’s post is exceedingly well formed (albeit served with the text/html MIME type), it does contain a significant error: at the present time the link doesn’t work. Well-formedness may be a cluefulness indicator, but it certainly isn’t sufficient to guarantee correctness. Update: now corrected.
The world is complicated
One size does not fit all. Some use cases require a bit of tolerance.