Add onInformation interim response support

[jasnell]

Adds an optional fetch call for receiving 1xx responses in a fetch.

e.g.

fetch('...', {
  onInformation(status, headers) {
    // status is the 1xx code
    // headers is the Headers object
  },
});

Support is entirely optional. Implementations that currently would ignore onInformation are fully compliant.

---

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (not for CORS changes): …
• MDN issue is filed: …
• 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 Jul 11, 2026, 7:14 PM UTC).

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🚨 Spec Generator - Spec Generator is the web service used to build bikeshed/ReSpec specs

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Error output:

[
    {
        "lineNum": "9769:11",
        "messageType": "link",
        "text": "No 'dfn' refs found for 'invoke a callback function'."
    },
    {
        "lineNum": "6768:10",
        "messageType": "lint",
        "text": "RFC2119 keyword in non-normative section (use: might, can, has to, or override with <span class=allow-2119>): \n         callback is invoked for all informational responses, including 100 (Continue),\n         101 (Switching Protocols), 102 (Processing), and 103 (Early Hints).\n         Support for invoking this callback is entirely optional; implementations that\n         never invoke it are fully compliant with this specification.\n\n       "
    },
    {
        "lineNum": "8764:4",
        "messageType": "lint",
        "text": "RFC2119 keyword in non-normative section (use: might, can, has to, or override with <span class=allow-2119>):  object containing the informational\n   response’s headers, appropriately filtered. Any return value is ignored. Implementations are\n   not required to support informational responses and may ignore this option entirely.\n  "
    },
    {
        "lineNum": null,
        "messageType": "failure",
        "text": "Did not generate, due to errors exceeding the allowed error level."
    }
]

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