[burtonator2011]

My expectation is that you could only do this for the same domain.

[ChrisSD]

If that is true, how would that interact with signed exchanges[0]?

[0] https://developers.google.com/web/updates/2018/11/signed-exc...

[stevens32]

Looks like that's not the case based on the blog post linked in the article https://web.dev/hands-on-portals/