CLS (Cumulative Layout Shift)
Cumulative Layout Shift (CLS) measures unexpected movement of page elements across the entire lifespan of a page – text jumping down because an image above it loads late, for example – and Google uses it as the Core Web Vital for visual stability. What counts is the largest connected burst of shifts (the session window), not the sum of all of them.
In practice
A good CLS value is 0.1 or less, assessed at the 75th percentile of real page views, separately for mobile and desktop. The most common fix for high CLS values is to reserve space for images and adverts before they load, with fixed width and height attributes or an aspect-ratio. Dynamically loaded content such as banners or cookie notices should also plan for space in the layout from the start, instead of pushing existing content aside.