Intent to Experiment: Viewport-height client hint

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Max Curran

Apr 6, 2022, 5:25:57 PMApr 6

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Currently, Responsive Image Client Hints provide a way for origins to obtain the viewport’s width. However, no such attribute exists for viewport height. We’ve observed that to optimize the loading of content that appears in viewport, it is essential for the origins to adapt HTML response based on viewport height.

Blink component


TAG review

TAG review status

Issues addressed

Link to origin trial feedback summary


Interoperability and Compatibility

Gecko: No signal (

WebKit: Negative ( Likely to be neutral based on discussion in

Web developers: No signals

Other signals:



WebView Application Risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?

Goals for experimentation

Client hints opt-ins from origins are session-sticky which makes it difficult to experiment with using origin trials since origin trials operate at document-level. We plan to run an A/B experiment using Chrome’s field trial infrastructure. Our plan is to enable the feature for at most 1% users to collect metrics, and ship this feature only if it shows performance improvement for origins that choose to use this feature.

Reason this experiment is being extended

Due to Chrome approvals taking longer than expected, and partners still needing to implement how to use the client hint, we have not been able to experiment yet. We would like to extend the origin trial from M101 to M104.

Ongoing technical constraints



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


Requires code in //chrome?


Tracking bug

Launch bug

Estimated milestones

OriginTrial desktop last

101 → 104

OriginTrial desktop first


OriginTrial android last

101 → 104

OriginTrial android first


Link to entry on the Chrome Platform Status

Links to previous Intent discussions

Intent to Experiment:

This intent message was generated by Chrome Platform Status.

Yoav Weiss

Apr 7, 2022, 5:18:15 AMApr 7
to Max Curran, blink-dev
Has the experiment started in the M99 timeframe? Or are you planning to start it later? Have any partners signed up? 

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Max Curran

Apr 7, 2022, 4:52:50 PMApr 7
to Yoav Weiss, blink-dev
We are running a coordinated experiment with Search. The Chrome side was started in M99, but they were only able to start their side of the experiment this week. Other than Search, no other partners have signed up.

Yoav Weiss

Apr 8, 2022, 2:01:42 AMApr 8
to Max Curran, blink-dev
In that case, I'm inclined to say that the actual experimentation started in M100.
Regardless, extending the experimentation till M104 LGTM.
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