The idea here is that the "browser's address bar origin" is an additional key for its HTTP cache, to prevent certain classes of attacks.

Safari ships a variant of this (uses registrable domain, not origin), but seems willing to adjust to origin. Other implementers are interested in shipping and are at various stages of experimentation.

This will require making all accesses of "the HTTP cache" more contextual, by accessing the HTTP cache of X whereby X is some defined origin. (Other ideas welcome, @mnot?)

I'm not sure where to store the defined origin. We could do a browsing context ancestor walk and that might be okay as I think all fetches always require a fully active document, but would be nice to have that confirmed.

(I'm also assuming that auxiliary browsing contexts are not special here and behave like other top-level browsing contexts for the purposes of this.)

cc @youennf @whatwg/security