Issue #1637 introduces possible experiences in a future WebAuthn, various aspects of which are enabled by "syncing platform credentials" via platform providers' sync fabrics. The spec will need updating to explicitly accommodate and explain the "synced credential" notion. This will accommodate other spec updates, e.g. PR #1663, which need to make reference to the "synced credential" notion.

Note that the spec mostly implies that public key credential sources are (presently) hardware-bound by default (though the Cred Private Key definition is explicit, see below). These various notions will all need to be made explicit.

[ Note: synced credentials are a crucial enabler of the "passkey" concept. ]