[recursive]

There's no way it could have gained traction. Consider two browsers. One follows the spec explicitly, and one goes into "best-effort" mode on encountering invalid markup. End users aren't going to care about the philosophical reasoning for why Browser A doesn't show them their school dance recital schedule.

Consider JSON and CSV. Both have formal specs. But in the wild, most parsers are more lenient than the spec.

[WorldMaker]

Which is also largely what happened: HTML 5 is in some ways that "best-effort" mode, standardized by a different standards body to route around XHTML's philosophies.

[ifwinterco]

Yeah this is it. We can debate what would be nicer theoretically until the cows come home but there's a kind of real world game theory that leads to browsers doing their best to parse all kinds of slop as well as they can, and then subsequently removing the incentive for developers and tooling to produce byte perfect output