The CM spec says that "Some Credential objects contain data which aims to give users a human-readable disambiguation mechanism". Prior to WebAuthN merging with the CM API, all the credential objects used CredentialUserData, but PublicKeyCredentials do not. CredentialUserData should be refactored out so PublicKeyCredential can extend Credential.
Comment 1 by kpaulhamus@chromium.org
, Jun 14 2017