Arguably, if there's ambivalence about numbers or other core structures or doubt about the intent, the correct thing to do is to return an error and not try to guess. The point of an error bubbling up is asking for clarity from the end-user, while guessing will lead to random results in ways that can not be detected or corrected anymore later on in the process.

I think Postel's law was intended to apply to alternative implementations of machine-level protocols.

That's not to say that I don't agree that it might be better if JSON implementations would allow trailing commas, which is unlikely to lead to semantic ambiguities. That's too late now though, unless a new JSON to rule them all would appear and we would all agree on that new spec.

Postel's Law was a bad idea.

https://datatracker.ietf.org/doc/html/draft-thomson-postel-w...