Specifications should only use USVString for APIs that perform text processing and need a string of scalar values to operate on. Most APIs that use strings should instead be using DOMString, which does not make any interpretations of the code units in the string. When in doubt, use DOMString.
As recommended by the Web IDL spec:
Implementation commitment:
Documentation and checks
[ ] Pinged MDNcredProps.authenticatorDisplayName
not yet documented on MDN: WebAuthn ExtensionsPreview | Diff