[w3c/manifest] Add `installed` CSS media feature (PR #1218)

Closes #1092

This change:

* Adds new normative requirements

Implementation commitment:

* [ ] WebKit (https://bugs.webkit.org)
* [ ] Chromium (https://bugs.chromium.org/) — crbug.com/382114911
* [ ] Gecko (http://bugzilla.mozilla.org)

If change is normative, and it adds or changes a member:

* [ ] [updated JSON schema](https://github.com/SchemaStore/schemastore/blob/master/src/schemas/json/web-manifest.json)

Commit message:

Adds the `installed` CSS media feature, a boolean discrete feature that
reflects whether the current browsing context is running as an installed
web application. Values: `none` (not installed) and `installed`.

Same-origin iframes inherit the parent value; cross-origin iframes
evaluate to `none`. The value can transition from `none` to `installed`
if the UA promotes a browsing context (e.g., Chrome tab reparenting).

Agreed at TPAC 2025 (w3c/manifest#1092).


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<a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/pull/1218.html" title="Last updated on May 11, 2026, 12:19 PM UTC (2941d69)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/1218/d5e2eeb...2941d69.html" title="Last updated on May 11, 2026, 12:19 PM UTC (2941d69)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/1218

-- Commit Summary --

  * Add `installed` CSS media feature

-- File Changes --

    M index.html (178)

-- Patch Links --

https://github.com/w3c/manifest/pull/1218.patchhttps://github.com/w3c/manifest/pull/1218.diff
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