Define opaque-response blocking (updated)

[sefeng211]

This PR is based on what @annevk has proposed in #1442, with additional changes. It includes the validate a partial response and the Content-Range header parser algorithm, plus the additional changes that Firefox has made in its implementation.

#1442 had some discussions and references, and I wish I can keep them, but I don't have write access to annevk/orb, hence this PR. Please let me know if there's a better way to move this forward.

cc @zcorpan @annevk

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TODO:

• Define "validate a partial response" (and Content-Range header parser)
• "ParseText" vs "create a classic script" + mock data
• A PR against HTML is needed to ensure it passes the appropriate metadata for media element and classic script requests.
• Double check this operates on the internal response (and if not pass it)
• Also use response in encoding extraction, depends on Add algorithm to extract an encoding from a MIME type #1447
• https://github.com/annevk/orb/labels/mvp
• PR to make tests non-tentative

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• At least two implementers are interested (and none opposed):

  • Chrome
  • Firefox

• Tests are written and can be reviewed and commented upon at:

  • https://wpt.fyi/results/fetch/orb/tentative?label=experimental&label=master&aligned
  • Non-tentative: todo

• Implementation bugs are filed:

  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (not for CORS changes): …

• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

---

💥 Error: 422 Unprocessable Entity 💥

PR Preview failed to build. (Last tried on Jun 22, 2026, 1:15 PM UTC).

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Error output:

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[zcorpan]

From the build errors, it looks like mimesniff needs to export more terms, e.g. https://github.com/whatwg/mimesniff/blob/main/mimesniff.bs#L988