> RFC for a standard allowing websites to list other domains that should be considered as same origin
No, they allowed an origin to list other origins whose cookies would be sent back to the serving origin correctly even if they were iframes loaded in the parent origin DOM.
I.e. this is the expected behavior for iframes until Safari decided that there was such a thing as "third party" origins whose web semantics could be broken in their war against advertising.
Google is trying to (partially) restore the expected behavior of iframes so that named origins get their own cookies sent to them, which is how things worked for the first two decades of the web.
No, they allowed an origin to list other origins whose cookies would be sent back to the serving origin correctly even if they were iframes loaded in the parent origin DOM.
I.e. this is the expected behavior for iframes until Safari decided that there was such a thing as "third party" origins whose web semantics could be broken in their war against advertising.
Google is trying to (partially) restore the expected behavior of iframes so that named origins get their own cookies sent to them, which is how things worked for the first two decades of the web.