RFC: Site Affiliation

Hey all,

as Mike mentioned in the last phonecall, I wonder whether there's some
appetite to agree on a standardized way for multiple sites to declare
themselves to be affiliated with each other.

Android allows for associating an app with one or more sites[1], and so
does iOS[2]. Now it seems pretty straight forward to use this information
also for e.g. a password manager, however, I think it's a bit unfortunate
to have sites host potentially N different files.

Adding this information to the web manifest, or as part of an origin policy
comes to mind.

thoughts?

best
-jochen

[1]
https://developers.google.com/identity/smartlock-passwords/android/associate-apps-and-sites#example_associate_apps_with_multiple_websites
[2]
https://developer.apple.com/reference/security/shared_web_credentials#1654478