Cumulative Layout Shift (CLS)

Cumulative Layout Shift: how much the page moves around while it loads. Under 0.1 is good, and the usual cause is images and ads with no space reserved for them.

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Cumulative Layout Shift measures visual stability: how far visible content jumps while the page is in use. A score of 0 means nothing moved; every unexpected shift — text pushed down by an image that just arrived, a button sliding away as a banner opens — adds to it. It is one of the three Core Web Vitals.

Thresholds

SiteScan uses Google's limits: 0.1 or less is good, up to 0.25 needs improvement, above that is poor. The lab figure comes from the Lighthouse run on the home page and covers only the load itself; the real-user figure, when the Chrome UX Report has one, is the 75th percentile over 28 days and includes shifts that happen later, while people scroll and click. That is why the two can differ noticeably.

Common causes and fixes

• Images, videos and iframes without dimensions — add width and height attributes or a CSS aspect-ratio so the browser reserves the space.
• Ads, embeds and cookie banners injected above existing content — reserve a fixed slot, or overlay instead of pushing.
• Web fonts swapping in with different metrics — use font-display: optional or tune the fallback with size-adjust.
• Animations of top or height — animate transform instead.

See Core Web Vitals for a deeper walkthrough.