[GoMonad]

The fingerprint reader on my Thinkpad X1 Carbon (gen 7) can work as a FIDO device. I just re-tested it on https://webauthn.io/ with Firefox in Windows 10. I'm guessing other fingerprint readers will too. You need to know that it will be considered a "Platform" device rather than a "Cross platform" device, which is what YubiKeys are considered.

Related but not answering your question: I haven't found any major website that support Platform FIDO devices. I'm guessing they only want 2FA devices which can roam between computers. I think that's unfortunate. Perhaps a good policy would be to allow Platform devices to be used after a Cross Platform device has been registered first. But there are few websites that support multiple FIDO keys to begin with.

How nice would it be to log into websites with your builtin fingerprint reader? The client side stuff seems ready to go.