Cumulative Layout Shift (CLS) is the Core Web Vital that measures visual stability: whether the page jumps under the user while they are trying to read or tap. A bad CLS score rarely means “CSS is slow” in the abstract. It usually means something changed size or position after the browser first painted, without the user doing something that should move the layout. Google’s own guidance defines CLS as a score that combines how much unstable content shifts and how far it moves, with good experiences targeting 0.1 or lower at the 75th percentile of page loads in the field ( Cumulative Layout Shift (CLS) , Optimize CLS ). What matters is that you are optimising for real navigations, not a single Lighthouse run. We go deeper here than the CLS section inside LCP, INP, CLS: What Each Core Web Vital Means and How to Fix It . That piece sets thresholds and a short fix list.…