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Pass 'Sec-Purpose: prefetch' header with <link rel=prefetch>

Category

Network / Connectivity

Type

New or changed feature

Status

Proposed (Chrome Proposed)

Intent stage

Start prototyping

Summary

Chrome currently passes both 'Purpose: prefetch' and 'Sec-Purpose: prefetch' headers as part of prefetch through speculation rules. However, only 'Purpose: prefetch' is passed with <link rel=prefetch>. We plan to standardize this header for both speculation rules and <link rel=prefetch> by adding 'Sec-Purpose: prefetch' header to the latter. Future work will involve stop sending 'Purpose: prefetch' for prefetches, prerenders, etc.

Motivation

SpeculationRules prefetch does send Sec-Purpose: "prefetch" header, but rel="prefetch" does not. The purpose headers allows servers to discern between regular requests and "resource hints" (requests that are only meant to populate caches).

Standards & signals

• Specification: https://fetch.spec.whatwg.org/#:~:text=If%20httpRequest%E2%80%99s%20initiator%20is%20%22prefetch%22%2C%20then%20set%20a%20structured%20field%20value%20given%20
• Firefox: Positive — Firefox has sent ‘X-moz: prefetch’.
• Safari: No signal — Safari has sent 'Purpose: prefetch' but changed to use 'X-Purpose: preview'.
• Web developers: No signals — rel=prefetch is not enabled, so no position needed.
• Tracking bug: https://issues.chromium.org/issues/40236973

Explainers: https://github.com/w3c/resource-hints/issues/74

View on chromestatus.com