Cumulative Layout Shift (CLS) is a Core Web Vital that measures the visual stability of a page by quantifying how much its layout unexpectedly shifts while loading — specifically, whether visible elements jump around as new content appears. A layout shift occurs when an element changes its start position between rendered frames without user interaction triggering it. CLS is scored on a scale from 0 (no shifts) upward; Google's "good" threshold is 0.1 or below at the 75th percentile across page loads. Common causes include images and video embeds without explicitly declared width and height attributes (so the browser doesn't reserve space), web fonts causing text to reflow when they load (FOUT/FOIT), ads or embeds inserted above existing content, and animations that move layout-affecting elements. Fixes include adding explicit dimension attributes to all media, using CSS font-display: optional for web fonts, and reserving space for dynamic content blocks with CSS aspect-ratio or min-height.
Glossary entry