CLS (Cumulative Layout Shift) is a Core Web Vitals metric measuring the visual stability of a page through the sum of unexpected layout shifts that occur during the lifespan of the page.
A "layout shift" happens when a visible element changes position from one rendered frame to the next without a user-initiated cause. Each shift is scored by impact (how much of the viewport moved) and distance (how far things moved); CLS is the sum of all such shifts. 2026 thresholds: ≤ 0.1 good, ≤ 0.25 needs improvement, > 0.25 poor.
Common CLS fixes for ecommerce: always set width and height attributes on images so browsers reserve space; reserve space for embedded content (iframes, ads, video) with explicit dimensions or aspect-ratio CSS; avoid font-swap reflows by using size-adjust on @font-face or font-display: optional; don't inject banners or popups above existing content; ensure skeleton loaders match the dimensions of the loaded content they're replacing.
CLS is often the easiest Core Web Vital to fix and the most impactful for perceived quality — even small layout shifts feel unprofessional.