>> I don't think there could be a compromise, 1.9.x API should be the same as in 2.0, right?
> The documented API, I would say. We're free to have the two versions behave differently for invalid input; we just need to be careful about exercising that freedom. And although my preference is liberal acceptance in traversing (historically true but never guaranteed), it's just as reasonable to take an "elements only" stance—as long as we give enough notice.

I truly think having the same API with the same behaviour is non-negotiable. If we have differences, they should be treated as bugs or else we'll get a hella lot of fire.