A new mode for navigator.credentials.get() that causes browser sign-in UI to be displayed to the user if there is a passkey or password for the site that is immediately known to the browser, or else rejects the promise with NotAllowedError if there is no such credential available. This allows the site to avoid showing a sign-in page if the browser can offer a choice of sign-in credentials that are likely to succeed, while still allowing a traditional sign-in page flow for cases where there are no such credentials.
Most sign-in experiences on the web are through sign-in pages that offer multiple options for accessing an account, such as username/password input fields, federated sign-in buttons, and sometimes explicit WebAuthn or passkey buttons. In cases where the browser is aware of passkeys or passwords that the user has for the site, this API feature would make the sign-in page unnecessary, by instead showing simple browser account selection UI when the user begins a sign-in attempt. Signing in with this flow would have less friction, and avoid user confusion from having to remember which sign-in option they have used previously on a given site. The main difference between this and current modal WebAuthn sign-in UI is that for users without any such credentials, no browser UI will be shown, and their sign-in experience will be unchanged from what it is today (typically, a navigation to the site's sign-in page).
Docs: https://github.com/w3c/webauthn/wiki/Explainer:-WebAuthn-immediate-mediation
Samples: https://deephand.github.io/webauthn-immediate-demo
Explainers: https://github.com/w3c/webauthn/wiki/Explainer:-WebAuthn-immediate-mediation