@heycam can probably confirm, but I think it will at least have an impact on Mozilla's build system - in that the binding might break if you return the wrong type. I know it's a bit of a far-fetched scenario, but I'm just saying. My point is, that WebIDL is used in unexpected places so it's an expectation that it be accurate - so I don't know if it's correct to say that it's "simply a form of documentation".