For ordinary websites none of the existing platforms support WebAuthn attestation AFAIK, because the spec for attestation was not designed with key synchronizat

For ordinary websites none of the existing platforms support WebAuthn attestation AFAIK, because the spec for attestation was not designed with key synchronization in mind, which is a key use case; otherwise, basic features like Keychain synchronization via iCloud would not work and the whole thing would be DOA. Synchronization is widely considered to be a core necessity to match the functionality of existing password managers. I believe you can enforce attestation using MDM and a custom app as far as I'm aware. But for general people using Safari or whatever, you can't request any kind of device attestation.