The problem is, when www.somecraphpytmlsite.com doesn't display in the web browser, users blame the browser, not the crap HTML writers.
It's like the good old days of writing device drivers for Windows, where we had to reproduce bugs from the default OS drivers in our custom drivers becasue there were so many crappy apps which only worked because of those bugs, and people would complain to us that CrappyWord Xtreme didn't work after they installed our driver which didn't have those bugs.
We should have tossed the whole thing long ago, and told people to fix their crap. Backward compatiblity sounds good, but it kills you in the long run.