What is CLS? Definition and explanations

CLS (Cumulative Layout Shift) measures a page's visual instability: how much content moves unexpectedly during the entire page lifecycle, not only at load — for example an image without dimensions or a late banner. The metric keeps the largest session window. It is one of the three Core Web Vitals. The threshold considered good is 0.1 or less, assessed at the 75th percentile of real visits.