Don't we also need to error if a subtype is present?

We don't really need to, in my opinion. If you passed in any other non-applicable values in the dictionary (ie: { type: 'identity', foo: 'bar' }) then we just ignore it, so that feels like it could carry over to this as well? Then again, if you passed a subtype that wasn't valid the IDL would cause it to naturally error whether or not the type uses it, so maybe being more rigorous about it isn't a bad idea? IDK.

My general opinion is that we should do what's consistent with the rest of the web on this one. Got an API ergonomics person who wants to give us guidance one way or another?

I'll poke around and see.