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In section 2.3 "A filtered response is a limited view on a response, referred to as the filtered response's associated internal response." takes a few passes to understand. How about: "A filtered response is a limited view on a response. A filtered response's original response is its associated internal response."
I tried to clarify it without introducing a new word ("original"): https://github.com/whatwg/fetch/commit/b6e7d7c81095d50d0728911c08c2a7f2b231723c