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Now that the spec PR is about to land, this PR should be reopened.

Hey, aside from web-platform-tests/wpt#31442, I also opened web-platform-tests/wpt#38437 with some extra tests to address some of the review feedback from the Webkit implementation. Note that the "Set-Cookie is a forbidden response header" test is about browser behavior and doesn't apply to Deno.

Tried to use this today, was missing in Deno

For the record, this implementation is correct according to the spec as it is now. The only changes in the spec text since were editorial. There is one failing test in fetch/api/headers/header-setcookie.any.js, that for Set-Cookie being a forbidden response header, that Deno fails intentionally (and which will eventually be standardized by the fetch work for server-side runtimes in WinterCG). So the only thing that's blocking this PR from landing is resolving the merge conflicts.