The current WebAuthn node provided by ForgeRock, requires the username to be input by the user before instigating the WebAuthn authentication. FIDO2 has the capability to store a UserID on the device along with the private key as part of the registration. It would be a much better user experience, if the user could opt to store their UserID on the credential device as part of the registration and that the WebAuthn authentication node would pick this up directly as an option when authemticating the user, e.g. "username from device".
This is possible in FIDO2 via the user Handle.
Please read the following article for more information: