Cumulative Layout Shift (CLS)
A Core Web Vitals metric that measures how much a page's visible content unexpectedly shifts position while loading. It is scored as the sum of unexpected layout movements, and a good score is 0.1 or less. It captures visual stability, a key part of perceived page quality.
Why It Matters
Layout shifts frustrate users, causing misclicks when buttons jump, and CLS is a confirmed ranking signal within Core Web Vitals. A stable layout improves both user experience and search performance.
Common Mistake
Loading images and ads without reserved width and height, or injecting banners and fonts above existing content after render. Both push the page around and inflate the score. Always set explicit dimensions.