[w3ctag/design-reviews] Incubation: Declarative Performance Observer (Issue #1285)

sisidovski created an issue (w3ctag/design-reviews#1285)

### Explainer

https://github.com/explainers-by-googlers/declarative-performance-observer

### 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://github.com/explainers-by-googlers/declarative-performance-observer
- Primary contacts:
  - Shunya Shishido (@sisidovski), Google, Proposer / Implementer 
- Organization/project driving the design: Google
- This work is being funded by: Google
- Incubation and standards groups that have discussed the design:
  - WICG: https://github.com/WICG/proposals/issues/280
  - Web Performance Working Group: Presented and discussed on June 18, 2026 (Minutes: https://w3c.github.io/web-performance/meetings/2026/2026-06-18/index.html)
- Standards group(s) that you expect to discuss and/or adopt this work when it's
  ready: W3C Web Performance Working Group

### Feedback so far

- Multi-stakeholder feedback:
  - Chromium comments:  Positive (proposer). The Chrome Loading team is actively implementing and experimenting with this feature in chromium (Origin Trial in M151–M155, tracking bug: https://crbug.com/505208781, ChromeStatus: https://chromestatus.com/feature/6594955352080384).
  - 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 -->
- Major unresolved issues with or opposition to this design:
  - No formal opposition has been expressed.
  - Active design discussions on GitHub include:
    - **Custom metadata / dimensions** (Issue #8): Exploring how developers can safely attach custom application dimensions.
    - **Long-lived SPA documents** (Issue #7): How to best support same-document transitions / Soft Navigations alongside terminal lifecycle report emission.

### You should also know that...

_No response_

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