#1207: Incubation: Prerendering cross-origin iframes

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Opened Mar 23, 2026

Explainer

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

The explainer

Where and by whom is the work is being done?

  • GitHub repo: https://wicg.github.io/nav-speculation/prerendering.html
  • Primary contacts:
  • Organization/project driving the design: Google
  • This work is being funded by: Google
  • Incubation and standards groups that have discussed the design:
  • Standards group(s) that you expect to discuss and/or adopt this work when it's ready: <unknown

Feedback so far

You should also know that...

In short,
We propose adding an opt-in for such pages, via a HTTP header

Supports-Loading-Mode: prerender-cross-origin-frames

which requests that the browser prerender cross-origin frames on pages containing the header. (The browser remains free to ignore this request, or to ignore the overall request to prerender, if it believes that would be better for user experience.)

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Discussions

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Discussed Apr 20, 2026 (See Github)

HF: updated brainstorming w/ comment. Please review.

Discussed Apr 27, 2026 (See Github)

jyasskin: Marcos, did you have any updates marcoscaceres: <looks>... I cannot recall where I was in this. I had opinions but I can't find them at themoment, I will try to come back to them

Discussed May 11, 2026 (See Github)

Heather: I asked a question about nested frames. What now?

Jeffrey: I think you can ask clarifying questions at any time.

Marcos: Will take a look.

Jeffrey: Ask the question now, in the public issue, as yourself, and then we can discuss whatever their answer is.

Comment by @hlflanagan May 13, 2026 (See Github)

Can the pre-rendered iFrame call on other cross-origin sites to nest n+1 iFrames and request pre-rendering for them as well?

Discussed Jun 1, 2026 (See Github)

Heather: We are waiting on Marcos to develop more of an opinion

Jeffrey: He had asked about standards positions and I looked and neither WebKit nor Mozilla had replied.

Discussed Jun 8, 2026 (See Github)

Heather: We're waiting for Marcos, who was going to check with some WebKit people so we can get multi-implementer feedback.

Marcos: I will follow up.

Comment by @hlflanagan Jun 24, 2026 (See Github)

Following up - @yoichio , has this been discussed further?

Comment by @yoichio Jul 1, 2026 (See Github)

Sorry, we're still discussing the nested case.

Discussed Jul 13, 2026 (See Github)

Heather: They are still thinking about my comment re: nested iFrames. Still waiting to hear back.

Discussed Jul 20, 2026 (See Github)

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Comment by @hlflanagan Aug 3, 2026 (See Github)

Hi @yoichio - checking in on this. Any further updates?

Comment by @yoichio Aug 4, 2026 (See Github)

Still in discussion, but we lean to allow the nesting with some restriction. Roughly:

  • Allow if the nested iframe's origin is same to parent frame
  • If it is cross, it is prohibited unless the parent frame has Supports-Loading-Mode: prerender-cross-origin-frames header.
Discussed Aug 10, 2026 (See Github)

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Comment by @marcoscaceres Aug 21, 2026 (See Github)

Ok, cool. Do you want to ping us once it’s settled. We can take another look then

Comment by @yoichio Sep 14, 2026 (See Github)

Hi. We've decided that formally and updated Prerendering cross-origin iframes explainer by https://github.com/WICG/nav-speculation/pull/438 and the spec update is already under review(https://github.com/WICG/nav-speculation/pull/440).

Details

  • Per-boundary check in create navigation params by fetching: Updates the child navigable navigation check to compare the navigation URL origin against its immediate parent frame's active document origin (navigable's parent's active document's origin). If cross-origin, it checks the parent document's allow cross origin iframes navigation while prerendering flag rather than the top-level traversable.
  • Recursive rule note: Adds an explanatory note clarifying that nested iframe prerendering is evaluated at each parent-child boundary: all ancestor frames must have prerendered, and each child must be same-origin with its immediate parent or opted in via the parent's Supports-Loading-Mode: prerender-cross-origin-frames header.
Comment by @hlflanagan Sep 23, 2026 (See Github)

Great. Thanks for the update! Closing as Satisfied, no concerns.