Cumulative Layout Shift (CLS) is one of three Core Web Vitals metrics, measuring the visual stability of a page during load. It quantifies how much visible content unexpectedly shifts position — the frustrating experience of trying to click a button just as an ad loads above it and pushes everything down.
The 2026 thresholds are: under 0.1 = good, 0.1-0.25 = needs improvement, over 0.25 = poor. CLS is calculated as the sum of impact-fraction × distance-fraction for every unexpected shift in the page's lifetime, with shifts from user interactions (clicking a button that expands a menu) explicitly excluded.
The most common CLS culprits are images without explicit width/height attributes (the browser does not know how much space to reserve until the image loads), ads and embedded iframes that inject above the fold, web fonts that swap mid-render (FOUT), and dynamic content that injects above existing content.
For SEO, Google uses CLS as a ranking signal. For AEO, CLS affects user-experience signals that AI engines indirectly weight via their broader quality assessment. The fix in all cases is to reserve space for every dynamic element — set width/height on images, reserve container dimensions for ads and embeds, use font-display: swap with size-matched fallbacks.