Intent to Experiment: Back/forward cache NotRestoredReason API

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Yuzu Saijo

21 Eki 2022 03:34:4121.10.2022
alıcı, bfcache-dev,,

Contact emails


Design docs


NotRestoredReason API reports the reasons why a page is not served from BFCache after a history navigation. The report is accessible via the PerformanceNavigationTiming API. The report attributes reasons to specific frames as much as possible, while respecting cross-origin security concerns.

Blink component


TAG review

TAG review status

Issues addressed


Interoperability and Compatibility

Gecko: No signal

WebKit: No signal

Web developers: Positive (


We do not report detailed information about cross-origin iframes. For cross-origin iframes, we only report if they blocked bfcache or not, and we do not report the reasons why.

Goals for experimentation

We would like developers to use the new field notRestoredReasons in performance navigation entry, e.g. performance.getEntriesByType('navigation')[0].notRestoredReasons; The goal of the experiment is to gather feedback from developers about the API shape.


In DevTools console, try: performance.getEntriesByType('navigation')[0].notRestoredReasons;

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?


DevTrial instructions

Flag name


Requires code in //chrome?


Tracking bug

Launch bug

Estimated milestones

OriginTrial desktop last110
OriginTrial desktop first108
DevTrial on desktop108
OriginTrial Android last110
OriginTrial Android first108
DevTrial on Android108
OriginTrial webView last110
OriginTrial webView first108

Link to entry on the Chrome Platform Status

Links to previous Intent discussions

Intent to prototype:

This intent message was generated by Chrome Platform Status.

Mike Taylor

21 Eki 2022 10:09:3721.10.2022
alıcı Yuzu Saijo,, bfcache-dev,,
LGTM to experiment from M108 to M110 inclusive.

Out of curiosity, where did we land on extensions?

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Yuzu Saijo

24 Eki 2022 02:09:5524.10.2022
alıcı Mike Taylor,, bfcache-dev,,
Thanks Mike!
I update the description in the explainer. We talked to the privacy team and decided to say "Extensions" for all the extension related reasons (we decided that it's okay to expose the presence of extensions, but will avoid giving any further information).


Mike Taylor

24 Eki 2022 10:28:4224.10.2022
alıcı Yuzu Saijo,, bfcache-dev,,
Cool - thanks for the info and for updating the explainer.

Yuzu Saijo

17 Kas 2022 02:46:3717.11.2022
alıcı Mike Taylor,, bfcache-dev,,
There's an update on the experiment.

The Privacy team has spotted a potential information leak in this API design, and we're working to land the fix in M109.
Hence changing the OT period to M109-M111. (Updated ChromeStatusEntry)

Please let us know if the Milestone change is okay.


Yoav Weiss

17 Kas 2022 06:32:4217.11.2022
alıcı Yuzu Saijo, Mike Taylor,, bfcache-dev,
Sounds great! Thanks for addressing the issues :)
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