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You cannot make normative requirements with regards to the message field of the DOMException class. This should be clear from reading http://dom.spec.whatwg.org/
To confirm, you're talking about "Section 14.4: Exceptions" I agree, there's an inconsistency in the spec in that. In the tables, Message is clearly noted as optional (for the reasons you discuss). However, it appears the language "return an error" imposes a normative requirement to include the message, when there shouldn't be.
Confirmed.
https://dvcs.w3.org/hg/webcrypto-api/rev/52cea848d2e6