Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) measures visual stability: how much visible content unexpectedly moves while a page loads, expressed as a unitless score. Good is under 0.1 at the 75th percentile of real users. Classic causes: images and embeds without declared dimensions, late-loading web fonts swapping in at different sizes, and ads or banners injected above existing content. Reserving space in advance fixes most of it.