Use same-origin instead of same-site since the latter should not be
supported.

(via web-platform-tests@bb1376b#commitcomment-36480961)

Definitely

As far as I can tell, it's possible to get duplicated headers with a
.headers file. I think that is nicer than .asis.

(via web-platform-tests@bb1376b#commitcomment-36481209)

And I think .asis is nicer :) We won't be able to move forward without more
substantive rationale, so I'll start.

The benefit of an .asis file is that all relevant information is expressed in
one place. Using .headers means that readers need to open two files to
understand the intent of a given file, and that assumes they even know to look.

.headers is more used in general, so people need to be aware of them anyway. Using .asis also means people need to learn how that works, too.

However, I think either is OK, so we should just submit this as-is (ahem) 😊

Cool. Here it is: web-platform-tests#20873