Safari has "implemented" the `preventSilentAccess` function from the Credential Management API (https://w3c.github.io/webappsec-credential-management/#dom-credentialscontainer-preventsilentaccess). However, it unconditionally throws a NotSupportedError whenever it's called. This happens on both macOS and iOS from my testing. The specification doesn't mention this as an expected behaviour and I was surprised to see that the function is implemented but also not implemented, meaning my feature detection looking for the existence of the function fails. The below code can be pasted into Web Inspector on a secure domain to demonstrate the issue. Both Chromium and Firefox work correctly but Safari fails. if (navigator.credentials.preventSilentAccess) { console.log('preventSilentAccess is implemented'); navigator.credentials.preventSilentAccess() .then(() => { console.log('preventSilentAccess succeeded'); }) .catch((err) => { console.log('preventSilentAccess failed'); console.error(err); }); } else { console.log('preventSilentAccess is NOT implemented'); }