Contact: early-hints-experiment@chromium.org
As of version 95, Chrome experimentally supports Early Hints. Early Hints enable browsers to preload subresources or preconnect to servers before the main response is served. See the explainer how it works.
Currently Chrome is running A/B testing in the field to evaluate the performance impact of Early Hints. Chrome also provides some ways to opt-in Early Hints for web developers who want to try the feature. This document describes the status of the current implementation and how to enable Early Hints support.
Chrome supports preload and preconnect in Early Hints for top-level frame navigation.
To reduce security and privacy implications, the HTML and Fetch living standards have some restrictions on when Early Hints can be handled. Chrome ignores Early Hints sent in the following situations to comply these specifications.
Chrome ignores the second and following Early Hints responses. Chrome only handles the first Early Hints response so that Chrome doesn't apply inconsistent security policies (e.g. Content-Security-Policy).
Chrome doesn’t handle dns-prefetch and prefetch in Early Hints yet. We consider supporting them in the future.
Early Hints can be enabled by a command line flag, or via Origin Trial.
Passing the --enable-features=EarlyHintsPreloadForNavigation command line flag to Chrome enables Early Hints support.
Note: As of version 98 the origin trial has expired. The Chrome networking team is preparing to ship the feature and the origin trial may be available again in the near future until it's fully shipped.
You can opt any page on your origin into Early Hints by requesting a token for your origin. Include the token in both Early Hints and the final response so that Chrome can recognize your pages opted in Early Hints.
HTTP/1.1 103 Early Hints Origin-Trial: **your token** Link: </style.css>; rel="preload"; as="style" Link: <https://cdn.test>; rel="preconnect" HTTP/1.1 200 OK Origin-Trial: **your token** Link: </style.css>; rel="preload"; as="style" Link: <https://cdn.test>; rel="preconnect" Content-Type: text/html; charset=utf-8 <!DOCTYPE html> ...
<meta http-equiv="origin-trial" content="**your token**"> also works for the final response but doesn’t work for Early Hints since Early Hints cannot convey a response body.
If a resource is preloaded by Early Hints, the corresponding PerformanceResourceTiming object reports initiatorType is “early-hints”.
performance.getEntriesByName('https://a.test/style.css')[0].initiatorType
// => 'early-hints'