How can I use iPhone as a roaming authenticator for webauthn? I would like to use my iPhone as a roaming authenticator using FaceID when authenticating to web sites using webauthn.
I have upgraded my iPhone iOS to version 14.0.1. I use webauthn.io as an example web site.
Use case
I want to register my iPhone as a roaming authenticator  on a web site, such that I can authenticate with it, both

when visiting a web site with my private laptop, but also
so that I can authenticate with it when visiting the same web site on a work laptop.

Registration
As I want to use the device as a roaming authenticator, I assume that I should choose Cross Platform as Authenticator Type on webauthn.io.
But it seems that I can only register the iPhone as authenticator when not chosing Cross Platform as Authenticator Type.
Now webauthn is new for me, is the problem on the web site I test with or is this not supported on iPhone, or how can I setup the iPhone as a roaming authenticator?
According to the project's issue system, this type of workflow is not supported by the standard (yet). It is not a specific problem with iPhone.
Please refer to this issue:
https://github.com/duo-labs/webauthn.io/issues/21
Note that a similar workflow recently became possible on some Android phones. However it is implemented via CaBLE, as mentioned in the above linked issue. CaBLE is cloud-assisted Bluetooth Low Energy - which is Google's own technology, and not part of the WebAuthn standard.