This applies both to normal responses as synthetic responses from redirects. The Fetch specification has required this for quite a while, though only Safari implemented it thus far.

Chrome is aligning with this in https://bugs.chromium.org/p/chromium/issues/detail?id=1170379 and adding tests: https://github.com/web-platform-tests/wpt/pull/27444.

See https://github.com/whatwg/fetch/issues/1146 for context. See https://github.com/whatwg/fetch/pull/1149 for the clarification made to Fetch.

(Exact details of parsing are still not defined yet, e.g., how to map bytes to code points. We can keep our parsing code as-is for now.)

One more special case that came up with synthetic redirects in service workers and fragments: https://github.com/web-platform-tests/wpt/pull/12680 (https://github.com/whatwg/fetch/pull/696 has context).