In principle, none. In practice, many websites will check the user agent and flat out refuse service unless it's one of a pre-approved list of implementations, regardless of whether it would work or not.

This has been happening for a long, long time already, for many browser features, and such "standards" pushed forward by the major mobile vendors will only make the problem worse.

Well if we're making up things that websites haven't done yet then yeah I'm angry too.

Unless you're using one of the proprietary desktop OSes then you have to use one of the proprietary mobile OSes.

https://developers.google.com/identity/passkeys/supported-en...

No you do not need to use anything proprietary to use WebAuthn. There are open source software[0] and hardware keys[1].

[0]: https://github.com/psanford/tpm-fido

[1]: https://solokeys.com/

WebAuthn supports attestation of key storage and site owners might choose not to trust keys coming from an unknown DIY device.

Fortunately that feature isn't used outside of enterprise environments. Apple's passkeys don't support it so there will not be any mainstream adoption for requiring an attestation cert.

Assuming desktop Chrome doesn’t support third party password managers like 1Password and Lastpass that are similarly part of the FIDO alliance.

Passkeys aren’t a Chrome feature they’re a Webauthn feature. So if Chrome doesn’t provide the features you want on your platform you should be able to use another browser / password manager combo that does.