103 Early Hints

Contact: early-hints-experiment@chromium.org

103 Early Hints is the new HTTP status code used for preloading subresources earlier. In general, browsers cannot preload subresources until the main response is served, as resources to be preloaded are listed on headers or <meta> in the main response. Early Hints will enable browsers to start preloading before the main response is served. In addition, this can be used with other Resource Hints APIs like preconnect.

As of version 87, Chrome doesn't support Early Hints yet, but is going to run an open experiment to evaluate the effectiveness of Early Hints. This does NOT give you the real benefit yet, but will help us and the community to quantify the potential benefit of the feature.

How To Contribute to the Measurement

Chrome landed code to record the time between when the hints are received to when the real navigation responses are received as of version 85, and this will help us learn how much benefit this may bring.

Note that this timing information will be recorded only for the users who opted-in to help Chrome gather usage statistics. See the Google Chrome Privacy Whitepaper for details.

In order to gather this data, we will need sites to start sending Early Hints status code (103), so that Chrome can record the timing information for the hints for the navigation.

Note that not all browsers may handle Early Hints status code gracefully. We are collaborating with Fastly on the timing to run this measurement, and they collect breakage reports here: https://early-hints.fastlylabs.com/.

Once enough data is collected, we plan to publish our conclusions and the learnings from the experiments with the aggregated stats publicly. Upon requests we may also share the per-site metrics with the sites who have participated.

Metrics

This section is mainly written for Chromium developers.

Chrome will record the following metrics. These intervals indicate how much earlier we could start preloading with Early Hints. For example, we could calculate the ratio of “the interval between request start and response start” to “the interval between request start and Early Hints” to see the ratio of speed-up.

UMA

These are recorded under PageLoad.Experimental.EarlyHints UMA event.

• FirstRequestStartToEarlyHints: The interval between when the first HTTP request is sent and when the headers of the Early Hints response is received in reply to the request for the main resource of a main frame navigation.
• FinalRequestStartToEarlyHints: The interval between when the final HTTP request is sent and when the headers of the Early Hints response is received in reply to the request for the main resource of a main frame navigation.
• EarlyHintsToFinalResponseStart: The interval between when the headers of the Early Hints response is received in reply to the final HTTP request and when the headers of the final HTTP response is received for the main resource of a main frame navigation.

UKM

These are recorded under the NavigationTiming UKM event.

• EarlyHintsForFirstRequest: The time relative to navigation start that the headers of the Early Hints response are received in reply to the first HTTP request for the main resource of a main frame navigation.
• EarlyHintsForFinalRequest: The time relative to navigation start that the headers of the Early Hints response are received in reply to the final HTTP request for the main resource of a main frame navigation.
• FinalResponseStart: The time relative to navigation start that the headers of the final HTTP response are received for the main resource of a main frame navigation.