The dubious <http://url.spec.whatwg.org/#percent-encoded-bytes> suggests behavior equivalent to the now-implemented behavior.

This leniency does destroy some value in a general-purpose library, however, as you would often like to be alerted to an error of this kind rather than having your intent corrupted. If we supported two functions, one which threw and exception and this one, a user could attempt it in strict-mode and then fall-back to lenient mode for an application interacting with intelligent life.