Intent to Prototype: Same-site cross-origin prerendering triggered by the speculation rules API
Domenic Denicola
Contact emails
dom...@chromium.org, robe...@chromium.org
Explainer
https://github.com/WICG/nav-speculation/blob/main/opt-in.md
Specification
https://wicg.github.io/nav-speculation/prerendering.html#navigate-fetch-patch
Design docs
https://docs.google.com/document/d/1WsDYA8NMCSwsK8dXCKdajdAd3ZcQUu9w1eoe0hEB_nU/edit?usp=sharing
Summary
Previously we launched same-origin prerendering triggered by the speculation rules API. This expands coverage to also allow triggering same-site cross-origin pages. This prerendering will be done with credentials and storage access, but such prerender targets will need to opt in by using the Supports-Loading-Mode: credentialed-prerender header.
Blink component
Motivation
Prerendering is a useful technology for speeding up page loads, and ideally referrer pages would be able to prerender all target pages. By expanding the circle of prerenderable pages to include same-site cross-origin pages, we allow more navigations on the web to be instant.
Initial public proposal
https://github.com/WICG/nav-speculation/pull/181
TAG review
https://github.com/w3ctag/design-reviews/issues/721#issuecomment-1235043792
TAG review status
Pending. (Review for this expansion was requested as a comment on an existing issues-mostly-addressed TAG review.)
Risks
Interoperability and Compatibility
This feature does not have significant interoperability or compatibility risks on top of the already-shipped same-origin prerendering feature. This is mostly a straightforward extension of that.
The only potentially-interesting questions are around the design of the Supports-Loading-Mode header, which is the main new web-exposed "API". We've designed the header with an eye toward being easily implementable and future-extensible, using the structured headers infrastructure.
Gecko: No signal. No signal on previous requests for prerendering and prefetching. I added a comment to the existing issue about prerendering.
WebKit: No signal. No signal on previous requests for prerendering and prefetching. I took this opportunity to re-file on their new GitHub repository in the hopes of getting some feedback.
Web developers: Positive. We've heard from a few partners that they want to prerender among other same-site origins they own, but cannot yet do so.
Ergonomics
This feature is triggered by the speculation rules API.
Activation
Using this feature requires the target page to have some control over its HTTP headers. This is not possible on some free hosting sites, e.g. GitHub Pages. We have envisioned a future extension of allowing a <meta> version of Supports-Loading-Mode that could address this, but have not yet heard of a concrete case where this would be necessary, so it is not included in this Intent.
Security
This feature allows one origin to cause another origin to be rendered, including its JavaScript code. Because this can be dangerous, we require the target origin to opt in using the Supports-Loading-Mode header.
This feature respects the cross-origin-isolation process model, to prevent the referrer and target pages from attacking each other through side channels.
These issues are discussed further in the design doc and explainer.
WebView application risks
Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?
This feature is not available on WebView.
Debuggability
DevTools support for prerendering in general remains in the early stages; you can track that work in https://crbug.com/1217029, or see our general development guide.
However, this expansion to cross-origin same-site target pages does not have any special debuggability concerns.
Is this feature fully tested by web-platform-tests?
Not yet, but we'll be writing such tests before shipping.
Flag name
SameSiteCrossOriginForSpeculationRulesPrerender
Requires code in //chrome?
False
Tracking bug
https://bugs.chromium.org/p/chromium/issues/detail?id=1356449
Estimated milestones
Link to entry on the Chrome Platform Status
https://chromestatus.com/feature/4899735257743360
This intent message was generated by Chrome Platform Status and fixed up by hand.