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Explainer https://github.com/bashi/early-hints-explainer/blob/main/explainer.md Specification https://tools.ietf.org/html/rfc8297 https://html.spec.whatwg.org/#early-hints Design docs https://docs.google.com/document/d/1gCh_CnfrJq_VL7aGoq6skc7sn4yn5pKsM0gkHe5B9go/edit?usp=sharing Summary Support 103 Early Hints informational responses during top-level frame navigation. When a 103 response includes link headers Chromium tries to preload specified resources (including modulepreload) or preconnect to specified origins before the final response is received. This gives Web developers a way to optimize Core Web Vitals such as Largest Contentful Paint (LCP). Blink component Internals>Preload <https://bugs.chromium.org/p/chromium/issues/list?q=component:Internals%3EPreload> TAG review https://github.com/w3ctag/design-reviews/issues/638 TAG review status Issues addressed Risks Interoperability and Compatibility Browsers may not understand a 103 response but they are expected <https://www.ietf.org/archive/id/draft-ietf-httpbis-semantics-19.html#name-informational-1xx> to ignore 1xx responses if they don’t understand 1xx responses. Firefox is implementing 103 support. Proxies may not understand a 103 response. They may treat the 103 response as a part of the final response when the response is sent over HTTP/1.1. The problem is less likely to happen over HTTP/2 and HTTP/3 thanks to their frame format. Chromium only handles 103 responses over HTTP/2 and HTTP/3. The HTML and Fetch standards have been updated to handle preload link headers in 103 responses for navigations (whatwg/html#7675 <https://github.com/whatwg/html/pull/7675>, whatwg/fetch#1404 <https://github.com/whatwg/fetch/pull/1404>). For preconnect there are ongoing efforts to specify the behavior. See “Anticipated spec changes” section below for more information. Gecko: In development <https://bugzilla.mozilla.org/show_bug.cgi?id=1407355> . WebKit: No signal <https://lists.webkit.org/pipermail/webkit-dev/2021-May/031861.html>. Web developers: Positive. Positive interest and intent of support by popular CDNs (Fastly <https://www.fastly.com/blog/faster-websites-early-priority-hints>, Cloudflare <https://blog.cloudflare.com/early-hints/>). We've been collaborating with partners to evaluate the impact of Early Hints and we observed significant improvements on page loading metrics such as FCP. Ergonomics The primary usage of this feature is to send link headers to preload resources or preconnect to servers to improve page load performance. Due to potential security concerns Chromium only handles the first 103 response for the top-frame navigation. See this document <https://chromium.googlesource.com/chromium/src/+/master/docs/early-hints.md> for details of restrictions. These restrictions may be relaxed in the future. Activation Popular HTTP servers and CDNs provide configurations or options to send Early Hints responses. Security Request mode <https://fetch.spec.whatwg.org/#concept-request-mode> and credentials mode <https://fetch.spec.whatwg.org/#concept-request-credentials-mode> are calculated the same as normal preload and preconnect. Preloads triggered by a 103 response follow policies specified in the same 103 response. Chromium only handles the first 103 response and ignores following 103 responses to avoid contradiction policies between these responses and the final response. Chromium doesn’t use preloaded resources triggered by a 103 response when there are conflicting policies between a 103 response and the final response. See the security considerations section <https://docs.google.com/document/d/1gCh_CnfrJq_VL7aGoq6skc7sn4yn5pKsM0gkHe5B9go/edit#heading=h.fhq04xt0tah6> of the design doc for more information. WebView application risks We don’t expect WebView specific problems with this feature. Debuggability If a resource is preloaded by Early Hints, the corresponding PerformanceResourceTiming object reports that its initiatorType is "early-hints". When the document doesn’t use resources preloaded by Early Hints within a few seconds from the window’s load event Chromium will show warning messages (this is under review now). DevTools lack Early Hints support and we consider adding Early Hints support in DevTools in the future. In the meanwhile developers can use NetLog <https://www.chromium.org/for-testers/providing-network-details/>, WebPageTest, or network protocol analyzers such as Wireshark for debugging. Is this feature fully tested by web-platform-tests <https://chromium.googlesource.com/chromium/src/+/master/docs/testing/web_platform_tests.md> ? Yes <https://github.com/web-platform-tests/wpt/tree/master/loading/early-hints> Flag name EarlyHintsPreloadForNavigation Requires code in //chrome? False Tracking bug https://crbug.com/671310 Launch bug https://crbug.com/1197989 Sample links https://early-hints.fastlylabs.com/ Estimated milestones We would like to ship this feature in M103. Anticipated spec changes Historically preconnect hasn’t been specified well (whatwg/html#4224 <https://github.com/whatwg/html/issues/4224>). There are ongoing efforts to specify the behavior. For example whatwg/html#7814 <https://github.com/whatwg/html/pull/7814> specified the behavior of <link rel=preconnect>. whatwg/html#7835 <https://github.com/whatwg/html/pull/7835> will specify preconnect in link headers and it will cover 103 responses. Link to entry on the Chrome Platform Status https://chromestatus.com/feature/5207422375297024 Links to previous Intent discussions Intent to Experiment: https://groups.google.com/u/1/a/chromium.org/g/blink-dev/c/xb_NHDS3twY This intent message was generated by Chrome Platform Status <https://chromestatus.com/>. -- You received this message because you are subscribed to the Google Groups "blink-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAPLXX-9MUr5cdGw_x7Gy6B3dxcmpVp9tnXL%2BY%3DDeEXoABNy3bA%40mail.gmail.com.
