Intent to Experiment: ElementTiming for Images

[n...@chromium.org]

Contact emails

n...@chromium.org, tdre...@chromium.org

Spec

Explainer https://github.com/npm1/Element-Timing

More detailed doc: https://docs.google.com/document/d/1blFeMVdqxB0V3BAJh60ptOBFY7cJSXnf7VyW3wspbZ8/

Summary

The Element Timing API enables monitoring when large or developer-specified elements are displayed on screen. It allows developers to monitor the first time specific important elements are displayed on the screen. It also allows analytics providers to measure display time of key images without explicit opt-in from developers. For this Origin Trial, we are restricting ourselves to image elements because of the complications arising from hierarchical elements and from web fonts (when do we consider a text element to first render?)

Link to “Intent to Implement” blink-dev discussion

https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/2twP4Xdd1VI

Goals for experimentation

We’d like to gain feedback on the following aspects of the API:

• Whether the timings being reported correspond to user-observed behavior.

• Ease of usage: is it easy to register important images? Is the threshold for ‘large’ images good?

• Which types of elements should be added next to the collection of observable elements of Element Timing.

Experimental timeline

M73 to M76 inclusive.

Any risks when the experiment finishes?

No, this is a performance API.

Ongoing technical constraints

None

Debuggability

Entries before onload are accessible from the performance timeline (performance.getEntriesByType(‘element’)). Other entries can be observed by registering a PerformanceObserver. It’s worth noting that when shipping we plan to not buffer any entries and instead have support for the PerformanceObserver buffered flag (to see entries computed before registration of the observer).

Will this feature be supported on all five Blink platforms supported by Origin Trials (Windows, Mac, Linux, Chrome OS, and Android)?

Yes.

Link to entry on the feature dashboard

https://www.chromestatus.com/features/6727079454310400

[n...@chromium.org]

Friendly ping

[Chris Harrelson]

Sorry for the delay. LGTM. Happy experimenting.

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