Intent to Extend Experiment: Prerendering cross-origin iframes

[Chromestatus]

Contact emails

yoi...@chromium.org

Explainer

https://github.com/WICG/nav-speculation/blob/main/prerendering-cross-origin-iframes.md

Specification

https://wicg.github.io/nav-speculation/prerendering.html

Summary

Prerenders cross-origin iframes with an opt-in response header. Browsers will now prerender all cross-origin frames if the top-level frame's HTTP response includes the Supports-Loading-Mode: prerender-cross-origin-frames.

Blink component

Blink

Web Feature ID

Missing feature

TAG review

https://github.com/w3ctag/design-reviews/issues/1207

TAG review status

Not applicable

Origin Trial Name

Prerendering cross-origin iframes

Goals for experimentation

Gather feedback from web authors who already are interested in this feature.

Chromium Trial Name

Prerender2CrossOriginIframes

Origin Trial documentation link

https://github.com/WICG/nav-speculation/blob/main/prerendering-cross-origin-iframes.md

WebFeature UseCounter name

kPrerender2CrossOriginIframes

Risks

Interoperability and Compatibility

The entire speculative loading feature is a progressive enhancement, so in general risks are low in this area.

Gecko: No signal (https://github.com/mozilla/standards-positions/issues/1376)

WebKit: No signal (https://github.com/WebKit/standards-positions/issues/636)

Web developers: Positive Our partners show interests in having this feature.

Other signals:

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?

No information provided

Reason this experiment is being extended

Partner requires additional implementation and assessment about the nested frames case.

Ongoing technical constraints

No information provided

Debuggability

The cross-origin iframe prerendering should be debuggable as well as top frame's one: https://developer.chrome.com/docs/devtools/application/debugging-speculation-rules#prerender

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?

No

Is this feature fully tested by web-platform-tests?

Yes

Flag name on about://flags

"prerender2-cross-origin-iframes" from M148

Finch feature name

Prerender2CrossOriginIframes

Requires code in //chrome?

False

Tracking bug

https://issues.chromium.org/issues/440387014

Estimated milestones

Origin trial desktop first	147
Origin trial desktop last	152
Origin trial extension 1 end milestone	158

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5112398709129216?gate=5151856880910336

Links to previous Intent discussions

Intent to Prototype: https://groups.google.com/a/chromium.org/d/msgid/blink-dev/68b670ba.050a0220.270bc4.0593.GAE%40google.com
Intent to Experiment: https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAAEV3p%3DzhNhfOpe8x%3DRKgxYFS9GCq_q-c8knZYRBRuijQ27o8w%40mail.gmail.com

This intent message was generated by Chrome Platform Status.

[Rick Byers]

LGTM to extend for 3 more months. Are you anticipating that after that you'll be ready to ship?

Is there a spec issue tracking the nested frames case? I didn't see one in a quick scan, and I notice TAG is asking about it as well.

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[Yoichi Osato]

Update on Specification and Behavior of Nested iframes in Prerendering

Hi blink-dev,

We would like to share an update regarding the specification and restriction model for nested iframes within this prerendering-cross-origin-iframes feature.

Motivation

The prerendering-cross-origin-iframes feature was designed to conditionally prerender cross-origin iframes under explicit opt-in controls.

However, we’ve found that the original specification did not fully account for nested iframes (iframes embedded within another iframe) based on feedback from Origin Trial and TAG reviews. Currently, nested iframes are prerendered without specific restriction checks regarding their parent frame's cross-origin relationship in both the spec and our implementation.

We believe nested iframes should not be prerendered automatically without explicit consent. We propose updating the model so that nested iframes also require the explicit opt-in header or must be same-origin relative to their parent frame.

Proposed Behavior Changes

Under the updated rules, a nested iframe will only be allowed to prerender if all of its ancestor frames have been prerendered and it meets either of the following conditions:

1. Same-Origin: The nested iframe is Same-Origin with respect to its immediate parent frame.

2. Explicit Opt-In: The immediate parent frame explicitly returns the following response header:
   Supports-Loading-Mode: prerender-cross-origin-frames

Note: This restriction model applies recursively down through the entire iframe nesting hierarchy, with no limit on depth.

Summary Comparison

Environment	Current Behavior	Proposed Behavior
Same-Origin Nested iframe	Prerendered	Prerendered
Cross-Origin Nested iframe (Default)	Prerendered	Deferred by default
Cross-Origin Nested iframe (with Opt-in Header)	Prerendered	Prerendered (requires Supports-Loading-Mode: prerender-cross-origin-frames on parent frame)

Technical Details & Specification Document

For a comprehensive breakdown of target scenarios, frame structures with diagrams, and interactions with other Supports-Loading-Mode headers, please review the detailed technical document:

Update on Specification and Behavior of Nested iframes in Prerendering

Please feel free to share any questions, feedback, or concerns regarding this update!

Best regards,
Yoichi Osato

2026年7月3日(金) 4:51 Rick Byers <rby...@chromium.org>: