A minimal relying party that loads include.js from browserid.me and signs in via
navigator.id.request(). FedCM is force-enabled here, so on a supporting browser it should show the
native account chooser instead of the popup.
Before you click: sign into browserid.me/account first, with an account that has an SMTP-verified (Secondary) email — that's what FedCM surfaces.
On load this page attempts a silent FedCM login (no UI) — that only works once a grant exists. To create the grant the intended way: click "Sign in (popup)", and in the browserid dialog tick "Automatically sign in to this site next time" before signing in. The dialog closes, then the FedCM chooser appears (click Continue) — that banks the grant. Reload and you'll be logged in silently. (The middle button is the manual way to bank the grant; the normal dialog no longer uses FedCM on click.)