Ahh, the UI Events spec supplements the Event spec and specifically saves the modifier keys which is not originally in there. However, I don't think they should be exposed like that, at least I can't find the IDL for it. They should only be retrievable via getModifierState.

Re webidl2js: I'm not sure what the best choice would be there. I think it'd make sense for the impl class to provide defaults, I think it makes sense to require other impl class to know about the required impl args, and it also makes sense to just use the public constructur if you want to use these convenience features. I think there's a point to make for all of these choices.

We ended up with a very similar problem in the specs, resulting in https://dom.spec.whatwg.org/#concept-event-create. So I should implement that :)