Core Web Vitals: CLS / Layout Shift

[iankduffy]

Description

Last year Core Web Vitals: LCP, INP, CLS was a part of Interop 2025, focusing on both LCP and INP; however, CLS was omitted at the time, which is missing in Firefox and Safari. Adding a way to track CLS/Layout shift across all browsers can allow for greater visibility on user experience when collected, and help address different browser behaviours.

For example, currently, only Chromium browsers capture CLS / layout shifts and support Speculation rules, which means users on browsers that don't support Speculation rules could have a varied CLS score from what browsers that support it have, but we are unable to capture the data to see if there is a difference.

Specification

W3C - Layout Instability API - The API which CLS depends on.

web-feature

No response

Test Links

No response

Additional Signals

Mozilla/Firefox Cumulative Layout Shift
Mozill/Firefox Layout Instability API

[github-actions]

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Below is additional information about the web feature (from the web-features project) which is referenced in your proposal.
If this doesn't accurately correspond to your proposal, please update your initial comment to include web-features: <feature-id>.
To find feature IDs, use the web-features explorer.

Feature Layout instability

• ID: layout-instability
• Name: Layout instability
• Description: The layout-shift performance entry and LayoutShift API measures the layout stability of web pages based on movements of the elements on the page.
• Baseline status: Limited Availability
• Docs: LayoutShift
• Standard positions: Mozilla
• Chrome use counter: chromestatus.com
• WPT tests: wpt.fyi/results/?q=feature:layout-instability
• More information: See the web-features explorer