Sebastian: you have it backwards.  A paraphrase of Postel's recommendation (calling it a Law is wrong) is 'accept dirty data, emit clean data'.  This is the current behavior.  See https://en.wikipedia.org/wiki/Robustness_principle.  This article also explains the opposite viewpoint, that dirty data should be rejected, as SpaceOne is suggesting.

SpaceOne: unless it is your intention to discourage people from volunteering their time to respond to issues raised on the tracker, you should read what they write more carefully and think more carefully about how you express your opinions.  If you really want a ValueError here, open an new enhancement issue for 3.6.