WebCrypto is being updated to allow unknown values as key usages. We should preserve them in persistent storage, and not freak out when JS code uses one. https://www.w3.org/Bugs/Public/show_bug.cgi?id=23729
https://www.w3.org/TR/WebCryptoAPI/ The spec has been updated a lot. It looks like that this bug is no longer valid for the new spec.