Can you clarify whether you mean only the `attestationObject` should be sent to the server or that everything should be encoded as a single CBOR object and then sent to the server?
There are two complementary parts participating in the registration and authentication ceremonies, WebAuthn and CTAP. The response from the authenticator, as per the CTAP protocol standard, is encoded as CBOR. We need the JSON parts from the WebAuthn browser API for the parts in the ceremony that talk to the relying party (particularly for relaying the challenge responses, and allowed authenticator algorithms and behaviors).