I would argue that if you're strictly defining the accepted variations in the grammar, then all you've done is define a new grammar that you're conservatively a

I would argue that if you're strictly defining the accepted variations in the grammar, then all you've done is define a new grammar that you're conservatively adhering to. And that's fine! There's no rule that says that standards cannot evolve to match new use cases. However, I would argue that having a strictly defined (yet evolving) standard is much different than the common interpretation of Postel's Law, which is that undefined behavior should "do the right thing", with the specific interpretation of "the right thing" being left to the implementers. Predictably, different implementers interpret that in different ways, which leads to security holes. But if the specification defines what the right thing to do is, then the behavior is no longer undefined, and therefore Postel's Law no longer applies.