CLS (Cumulative Layout Shift)

Cumulative Layout Shift is the Core Web Vital for visual stability. It scores unexpected movement of visible elements throughout a page visit, not only during initial loading, while excluding shifts that occur shortly after qualifying user input. The score uses the largest burst of layout shifts within a session window: shifts less than one second apart are grouped, up to five seconds. A lower score means a more stable layout.

Why it matters: Aim for a CLS of 0.1 or less at the 75th percentile of page views, assessed separately for mobile and desktop; above 0.25 is considered poor. Reserve space for images, video, embeds, ads, banners, and asynchronously inserted content. Preload necessary fonts or use compatible fallbacks, and prefer transform-based motion when animation should not move surrounding content. Compare real-user field data with lab traces: a short lab run can miss cookie notices, late ads, interactions, and shifts on long-lived pages. Fix the elements that contribute most rather than treating the aggregate score as a diagnosis.