[johncolanduoni]

As I said, the attestation structurally does NOT attest to your OS or your browser that are displaying the website performing the authentication. It attests to the device that holds the passkey's key material, which is usually not your desktop computer.

[Borealid]

The attestation is in fact readable by the FIDO Platform (the browser/OS). It is not encrypted to be readable only by the RP (web site).

It talks about whatever you used to authenticate and the platform can manipulate (or omit) it.

[johncolanduoni]

Yes, but the attestation does not tell the RP anything about the browser. The whole point of the nightmare scenario above was for Google to sneak browser attestation in via passkey attestation. The browser being able to see the attestation doesn’t matter for that.