Add .trailers to Response/Request

[jasnell]

Adds Promise<Headers> trailers properties to Request and Response

When sending... trailers resolves to an empty, immutable Headers since we're not dealing with sending trailers in this change.

When receiving... trailers resolves to an immutable, populated/filtered Headers resolved after the body is completed. This means the trailers promise will not resolve if the body is not consumed.

const trailers = await response.trailers;

The minimal conformance requirement here is the presence of the .trailers property. An implementation which chooses not to actually support trailers would need only return a resolved promise with an immutable empty Headers. Nothing in this should be interpreted as requiring any implementation to actually implement trailers support.

Refs: #981

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• At least two implementers are interested (and none opposed):
  • Node.js
  • Cloudflare Workers
  • …
• 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.)

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💥 Error: 422 Unprocessable Entity 💥

PR Preview failed to build. (Last tried on Jul 11, 2026, 5:40 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": "1675:17",
        "messageType": "link",
        "text": "Multiple possible 'dfn' local refs for 'trailer state'.\nRandomly chose one of them; other instances might get a different random choice."
    },
    {
        "lineNum": "1679:51",
        "messageType": "link",
        "text": "Multiple possible 'dfn' local refs for 'trailer state'.\nRandomly chose one of them; other instances might get a different random choice."
    },
    {
        "lineNum": "1699:48",
        "messageType": "link",
        "text": "Multiple possible 'dfn' local refs for 'trailer state'.\nRandomly chose one of them; other instances might get a different random choice."
    },
    {
        "lineNum": "2652:4",
        "messageType": "link",
        "text": "Multiple possible 'dfn' local refs for 'trailer state'.\nRandomly chose one of them; other instances might get a different random choice."
    },
    {
        "lineNum": "2652:63",
        "messageType": "link",
        "text": "Multiple possible 'dfn' local refs for 'trailer state'.\nRandomly chose one of them; other instances might get a different random choice."
    },
    {
        "lineNum": null,
        "messageType": "failure",
        "text": "Did not generate, due to errors exceeding the allowed error level."
    }
]

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[jasnell]

The struct here is really only to deal with cloning... wasn't sure if it would be better to go more specific / concrete or to describe things in a more generic / hand wavy way... went with more specific.