CLS Test
Lab Cumulative Layout Shift against the Core Web Vitals thresholds.
Runs a real Chromium page load — usually takes 5–15 seconds.
What this tool checks
Cumulative Layout Shift
Lab measurement of Cumulative Layout Shift against the Core Web Vitals thresholds (good ≤ 0.1, poor > 0.25).
Why it matters: CLS measures unexpected layout movement. High CLS causes mis-taps and a jarring experience. This is a lab measurement from a single automated load — it is not real-user (field) data and will differ from Chrome UX Report values.
If it fails: Set width/height on images and embeds, reserve space for ads, and avoid inserting content above existing content.
Source: web.dev/articles/cls