[w3ctag/design-reviews] Incubation: Prerendering cross-origin iframes (Issue #1207)

yoichio created an issue (w3ctag/design-reviews#1207)

### Explainer

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

### The explainer

- [x] Includes the information requested by the [Explainer Explainer](https://w3ctag.github.io/explainer-explainer/#introduction).
- [x] Follows the [Web Platform Design Principles](https://www.w3.org/TR/design-principles/).
- [x] Includes or links to answers to the [Security/Privacy Questionnaire](https://www.w3.org/TR/security-privacy-questionnaire/).
- [x] Describes user research you did to validate the problem and/or design.

### Where and by whom is the work is being done?

- GitHub repo: https://wicg.github.io/nav-speculation/prerendering.html
- Primary contacts:
  - Yoichi Osato ([@yoichio](https://github.com/yoichio)), Google
  - Hiroki Nakagawa ([@nhiroki](https://github.com/nhiroki)), Google
- 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

- Multi-stakeholder feedback:
  - Chromium comments:
  - Mozilla comments: https://github.com/mozilla/standards-positions/issues/NNN <!-- And/or other places they've given feedback -->
  - WebKit comments: https://github.com/WebKit/standards-positions/issues/NNN <!-- And/or other places they've given feedback -->
  - {{...include feedback/review from developers, implementers, civil society, and others}}
- Major unresolved issues with or opposition to this design:


### 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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