> They want to share them across devices
I do this today on Bitwarden, Apple users do this today with Keychain. Who's the "they" here?
> sometimes even devices made by different vendors.
> they want to hand a passkey to a family member or friend
..... Why? What's the use case here?
Tying a credential to a single identity (and therefore, human) is another explicit design goal of webauthn. I seem to remember the original proposal was that locking a private key to a device in an unextractable, un-copyable way was an explicit benefit - if it can't be exported, then it can't be stolen/copied without the device also being stolen. This was softened with the purpose of allowing syncing amongst devices that already have a good story on sharing sensitive data, but this mechanism does not exist generically. There is no standard way, right now, that my iPad and Pixel device can share a private, sensitive piece of information without the help of a 3rd-party syncing provider. Without that, cross-platform credential sharing can't exist out of the box by default.