Summary: Early Hints is an informational status code allowing servers to send headers likely present in the final response. Early Hints is enabled in nightly and beta since version 111. We're now experimenting with Early Hints preconnect in release version 117 since 2023-09-18 and expect to roll out afterwards. Early Hints preload will be experimented and rolled out separately afterwards.

Bug: https://bugzil.la/1407355

Standard:

 - https://html.spec.whatwg.org/multipage/semantics.html#early-hints
 - https://www.rfc-editor.org/rfc/rfc8297

Platform coverage: all

Preference:

 - network.early-hints.preconnect.enabled true - enabling Early Hints preconnect
 - network.early-hints.enabled true - enabling Early Hints preload

Other Browsers:

 - Webkit: testing preconnect in 17.1 technical preview
 - Blink: Shipped preload and preconnect since 103

Demo:

These test websites artificially add delay between 103 response and 200 responses:

 - preconnect (IPv6 only):
   https://lab.neon.rocks/1000/eh/images-preconnect-100.html
 - preconnect (less visible impact):
   https://lab.neon.rocks/1000/eh/images-preconnect.html
 - preload: https://lab.neon.rocks/1000/eh/images-100.html

Web-platform-tests: https://wpt.fyi/results/loading/early-hints

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