I'd argue your #3 is wide of the mark.

It's not that there's a well-defined open standard.

It's that browsers will eat any old crap that's thrown at them and turn it into something plausible, if not precisely what the author intended or reader really wants.

Yes, there's a standard, and yes it's open. It's observed far more in the breach, as a few minutes with a validator on well-known sites will demonstrate.

Your comment alone (prior to my response to it) returns:

    Tidy found 21 warnings and 0 errors!