Re: [whatwg/fetch] Define opaque-response blocking (updated) (PR #1755)

zcorpan left a comment (whatwg/fetch#1755)

From reviewing WICG/background-fetch#181, which switches Background Fetch to the content-range definitions exported here:

**D1**: "validate a partial response" declared _previousResponse_ as an optional response, but Background Fetch passes null when nothing has been stored yet. Fixed in dacc8c9.

**Q1**: "parse a single byte content-range value" only accepts lowercase `bytes ` followed directly by digits, so `Content-Range: BYTES 0-15/16` or `bytes  0-15/16` fails "validate a partial response" (ORB's 206 check, HTML's media check, Background Fetch). RFC 9110 says range units are case-insensitive. Should it be stricter than HTTP here?

- a) Keep as is. Matches WebKit; the note already says it's a subset of HTTP.
- b) Match `bytes` ASCII case-insensitively. Follows RFC 9110.
- c) b, plus allow whitespace around the numbers. Matches Chromium; more lenient than HTTP's grammar.

<details>
<summary>Evidence</summary>

- WebKit requires exactly `bytes ` ([ParsedContentRange.cpp](https://github.com/WebKit/WebKit/blob/15cb3560da6c863025957fce3ee065cf61902e75/Source/WebCore/platform/network/ParsedContentRange.cpp#L69-L73)).
- Chromium's ORB ([orb_impl.cc](https://github.com/chromium/chromium/blob/6ca4e60b94475fa916dfb688608e6cc72693a1bf/services/network/public/cpp/orb/orb_impl.cc#L170)) and Background Fetch use `ParseContentRangeHeaderFor206`, which compares the unit case-insensitively and trims whitespace around the numbers ([http_util.cc](https://github.com/chromium/chromium/blob/6ca4e60b94475fa916dfb688608e6cc72693a1bf/net/http/http_util.cc#L321-L367)). Chrome Canary's Background Fetch succeeds with `BYTES 0-15/16` and `bytes  0-15/16`.
- Gecko's ORB doesn't check the unit ([OpaqueResponseUtils.cpp](https://github.com/mozilla-firefox/firefox/blob/cac25a6fa321b0051932359b84c4e2fe3caabdc5/netwerk/protocol/http/OpaqueResponseUtils.cpp#L196-L240)).

</details>


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