Intent to extend experimentation: Permissions-Policy: unload

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Daisuke Enomoto

Jan 17, 2023, 3:54:35 AM1/17/23
to blink-dev, bfcache-dev, Fergal Daly, Ian Clelland

API owners,

We would like to extend the origin trial for 3 additional milestones, with the extension starting in 110 continuing through 112. The initial experiment was approved for the OT running from 107 through 109.

Contact emails




This feature allows pages to disable the running of unload event handlers. The goal is to :

- allow sites that have removed all unload handlers to ensure they do not accidentally add new ones

- allow sites to remove unload handlers when updating the code is infeasible

Unload event handlers are problematic for various reasons and prevent use of BFCache on Desktop (see

Blink component


TAG review

TAG review status




Interoperability and Compatibility

3rd-party frames that rely on unload may not work as expected when navigating away. This is solvable by the frame authors by use of alternatives to unload and is unlikely to impact users. See detailed discussion.

Gecko: Negative ( FF objects to this similar to sync-xhr and document-domain providing a way to cause cross-origin interference with script. Explainer addresses this ( At a recent TPAC meeting with Mozilla people present, no negative feedback was received. Request for formal position is here

WebKit: No signal

Web developers: Positive Private discussions with devs are positive. Sites that have made efforts to remove all unload handlers want to use this to prevent accidental returns. Also some providers of 3rd-party iframes which have content outside of their control (e.g. ad network) want to guarantee themselves to be unload-free. Also positive feedback about using this to deny unload as a source of security problems.

Other signals: TAG review is here but has no feedback on the API itself.

Goals for experimentation

  • Validate that this allows sites using it to improve their BFCache hit rate

Reason this experiment is being extended

We had requested this Origin Trial to be run for 3 milestones, specifically from 107 to 109 without realizing the Origin Trial guideline suggesting 6 milestones. We would like to extend this OT for another 3 milestones or to 112 inclusive by applying the 6 milestone guidelines we originally missed, to give sufficient time for partners to give us feedback.

Ongoing technical constraints


When this header is present, attempts to add an unload event handler will result in an error on the console (just as would happen for any other Permissions Policy violation).

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?


Is this feature fully tested by web-platform-tests?


Flag name


or via Origin Trial Token

Tracking bug

Launch bug

Estimated milestones

OriginTrial desktop last (new request)


OriginTrial desktop last


OriginTrial desktop first


OriginTrial desktop last (new request)


OriginTrial Android last


OriginTrial Android first


Link to entry on the Chrome Platform Status

Links to previous Intent discussions

Intent to prototype:

Ready for Trial:

Intent to Experiment:

Yoav Weiss

Jan 17, 2023, 4:05:44 AM1/17/23
to Daisuke Enomoto, blink-dev, bfcache-dev, Fergal Daly, Ian Clelland
LGTM to experiment M110-M112 (inclusive)

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