Re: [heycam/webidl] Sort out when dictionaries and records should have default values (#76)

I agree with @jan-ivar that defaulting optional dictionary members to the empty dictionary is a violation of the principle of least astonishment. As noted in https://github.com/w3c/webauthn/issues/750#issuecomment-401752095, as an API author I expect to be able to express parameter structures where "this thing is optional, but if it's present then there's a required thing inside it".

Reading through this thread, though, I note that the case of nested dictionaries seems very different from the case of dictionaries as function arguments. I'm willing to agree that as _function arguments_, it's reasonable that optional dictionary arguments default to the empty dictionary - so that "no bag of arguments" is equivalent to "empty bag of arguments" as noted by @bzbarsky. However, dictionaries as optional _dictionary members_ should default to `undefined` (unless an explicit default is given), because having those default to `{}` would unnecessarily make WebIDL much less expressive.

[tbm]: https://w3c.github.io/webauthn/#dom-collectedclientdata-tokenbinding
[ccd]: https://w3c.github.io/webauthn/#dictdef-collectedclientdata

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