Upstream Document PiP changes to "fullscreen is supported"

[vinhill]

I think it makes sense to merge this after whatwg/html#12397 because Is Document Picture-in-Picture is currently not exported.

• At least two implementers are interested (and none opposed):
  • Gecko
  • ...
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/document-picture-in-picture/pip-fullscreen.https.html
• Implementation bugs are filed:
  • Chromium: already shipped
  • Gecko: already shipped
  • WebKit: open standard position Document Picture in Picture API WebKit/standards-positions#41
• MDN issue is filed: ...
• The top of this comment includes a clear commit message to use.

Fixes #258

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💥 Error: 422 Unprocessable Entity 💥

PR Preview failed to build. (Last tried on Apr 22, 2026, 12:05 PM UTC).

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PR Preview relies on a number of web services to run. There seems to be an issue with the following one:

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

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Error output:

[
    {
        "lineNum": "114:35",
        "messageType": "link",
        "text": "Multiple possible 'removing steps' dfn refs.\nArbitrarily chose https://dom.spec.whatwg.org/#concept-node-remove-ext\nTo auto-select one of the following refs, insert one of these lines into a <pre class=link-defaults> block:\nspec:dom; type:dfn; text:removing steps\nspec:geolocation-element; type:dfn; for:ActivationBlockersMixin; text:removing steps\nspec:geolocation-element; type:dfn; for:HTMLGeolocationElement; text:removing steps\nspec:geolocation-element; type:dfn; for:PowerfulFeatureObserver; text:removing steps\n<a bs-line-number=\"114:35\" data-link-type=\"dfn\" data-lt=\"removing steps\">removing steps</a>"
    },
    {
        "lineNum": "157:31",
        "messageType": "link",
        "text": "No 'dfn' refs found for 'is document picture-in-picture' that are marked for export.\n  (Possible specs this could be from: document-picture-in-picture)\n<a bs-line-number=\"157:31\" data-link-type=\"dfn\" data-lt=\"Is Document Picture-in-Picture\">Is Document Picture-in-Picture</a>"
    },
    {
        "lineNum": null,
        "messageType": "failure",
        "text": "Did not generate, due to errors exceeding the allowed error level."
    }
]

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