Request for position: Layout Instability API CSS pixel attribution

[anediaz]

Information about the proposal

• Explainer URL: Clarify that attribution rectangles use CSS pixels WICG/layout-instability#125
• Specification URL: https://wicg.github.io/layout-instability/
• TAG Review: Not applicable (compatibility fix for existing API)
• Chromium bug: https://issues.chromium.org/issues/399058544
• ChromeStatus: https://chromestatus.com/feature/5155103518228480

Summary

The Layout Instability API currently reports attribution rectangles (prevRect and currentRect) in device pixels. This proposal changes them to CSS pixels for consistency with other Web Platform APIs (e.g., getBoundingClientRect, IntersectionObserver, ResizeObserver).

Key changes:

• prevRect and currentRect now use CSS pixels instead of device pixels
• Aligns with standard coordinate space used across the web platform
• Improves consistency for developers measuring layout stability
• Minimal breaking change (primarily affects telemetry/monitoring tools)

Motivation:

Device pixels create inconsistencies across different device pixel ratios and make it harder for developers to correlate layout shift data with other measurements. CSS pixels are the standard coordinate system for web content.

Compatibility:

This is behind a flag in Chrome. We're seeking feedback from other vendors before enabling by default.

We'd appreciate WebKit's position on this compatibility improvement.