[w3ctag/design-reviews] Layout Instability Shifted Element Surfacing (#485)

Hello TAG!

I'm requesting a TAG review of Layout Instability Shifted Element Surfacing.

Today it is difficult for web developers to understand the cause of a high [CLS score](https://github.com/WICG/layout-instability/blob/master/README.md) using the [Layout Instability API](https://wicg.github.io/layout-instability/), because nothing in the score or the PerformanceEntry connects back to the specific DOM elements that were affected by the layout shift.

CLS Shifted Element Surfacing (SES) is an effort to surface a subset of the shifted DOM elements in the [LayoutShift interface](https://wicg.github.io/layout-instability/#sec-layout-shift). This will improve the "actionability" of CLS for developers.

  - Explainer¹ (minimally containing user needs and example code): https://github.com/WICG/layout-instability/blob/master/sources.md

  - Specification URL: https://github.com/WICG/layout-instability/pull/32/files (will update when landed)
  - Tests: coming soon
  - Security and Privacy self-review²: this extension does not alter the security and privacy considerations of the overall Layout Instability API which were submitted with https://github.com/w3ctag/design-reviews/issues/393

  - GitHub repo (if you prefer feedback filed there): https://github.com/WICG/layout-instability

  - Primary contacts (and their relationship to the specification): skobes
  - Organization(s)/project(s) driving the specification: Google 
  - Key pieces of existing multi-stakeholder review or discussion of this specification: https://github.com/WICG/layout-instability/issues/11

  - External status/issue trackers for this specification (publicly visible, e.g. Chrome Status): https://github.com/WICG/layout-instability/issues/11


Further details:

  - [X] I have reviewed the TAG's [API Design Principles](https://w3ctag.github.io/design-principles/)
  - Relevant time constraints or deadlines: n/a
  - The group where the work on this specification is currently being done: WICG
  - The group where standardization of this work is intended to be done (if current group is a community group or other incubation venue): W3C
  - Major unresolved issues with or opposition to this specification: n/a
  - This work is being funded by: Google

You should also know that...

This feature adds a new attribute ("sources") to an interface defined in an existing specification (Layout Instability).  The PR to amend the spec with this feature is not yet landed but we are proactively initiating TAG review to optimize the time available to review.  There is some discussion on the PR review thread which may alter some aspects of the API.

We'd prefer the TAG provide feedback as (please delete all but the desired option):
  💬 leave review feedback as a **comment in this issue** and @-notify [github usernames]


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