[w3ctag/design-reviews] Incubation: IdP-Initiated FedCM (Issue #1184)

samuelgoto created an issue (w3ctag/design-reviews#1184)

### Explainer

https://github.com/w3c-fedid/FedCM/blob/main/explorations/drafts/interception.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/).
- [ ] Includes or links to answers to the [Security/Privacy Questionnaire](https://www.w3.org/TR/security-privacy-questionnaire/).
- [ ] 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://github.com/w3c-fedid/FedCM/blob/main/explorations/drafts/interception.md
- Primary contacts:
  - Sam Goto (@samuelgoto), Google, Author
- Organization/project driving the design: Google Chrome
- This work is being funded by: Google
- Incubation and standards groups that have discussed the design:
  - FedID CG
- Standards group(s) that you expect to discuss and/or adopt this work when it's
  ready: FedID WG

### Feedback so far

- Multi-stakeholder feedback:
  - Chromium comments: https://groups.google.com/a/chromium.org/g/blink-dev/c/_5P00uaafKM
  - Mozilla comments: not yet
  - WebKit comments: not yet
- Major unresolved issues with or opposition to this design:
  - We ran a session at TPAC in Kobe ([notes](https://github.com/w3c-fedid/meetings/blob/main/2025/2025-11-13-FedCM-TPAC-notes.md#google-chrome-browser--google-idp-irene-chang-nick-watson---oauth))

### You should also know that...

We think there are two things that this can help:

- First, it makes it a lot more scalable to deploy FedCM by redeploying IdPs-only, rather than requiring changing all RPs. This can meaningfully remove one of the main blockers to combat bounce tracking. Outside of tracking, we are finding that the built-in native FedCM UX often outperforms top-level navigations (specially on mobile), so we'd expect that this will be useful to IdPs/RPs and users in and on itself.
- Second, we think that we can use this to turn federation into a structured/programmable tool, rather than unstructured computer-use actuation, whenever FedCM is used in agentic browsers, making federation a meaningful way to help agentic browsers, so finding a way to deploy it at scale (i.e. redeploying as fewer binaries as we can) meaningfully helps agentic browsers

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