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https://heycam.github.io/webidl/#es-DOMException-constructor-object [[ DOMException constructor object ]] Why is DOMException specified as prose instead of as IDL (with prose for things not expressable in IDL, if anything)? Having it as IDL has the advantage of being easier to test and maybe also implement.
Because there's no IDL to express Error objects properly.
I forgot that it's not an interface so I guess direct prose makes sense.