The CTAP protocol used by the webauthn.io demo web program can normally perform external device authentication by invoking Android gms, but only fingerprint, pin, and pattern.
Can I apply the facial authentication function?
I can't find any related information from developer resources or other communities such as YouTube on the official Fido Alliance website. It was hard to find here, too. I wonder if Android OS doesn't support facial authentication on fido2
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Android FIDO2 (or passkey) supports secure screen lock feature which offers user verification with PIN, Pattern or Biometrics (such as fingerprint, face or possibly others). If you are saying about FIDO2 (or passkey) features provided by OS itself (GMS), then you have no way to add your own facial recognition as a user verification. If the device has facial recognition (which is strong enough), it should be used as an user verification for FIDO2 (or passkey).
WebAuthn Relying Parties do not have control over an authenticator's user verification method(s).
That being said, Android devices with a class 3 biometric offer it for user verification for FIDO2/WebAuthn/passkeys. I believe only the Google Pixel 4 XL, Google Pixel 8, and Pixel 8 Pro have face unlock at class 3.