CLS (Cumulative Layout Shift)
Cumulative Layout Shift (CLS) measures unexpected visual movement of page elements after rendering – text jumping down because an image above it loads late, for example – and Google uses it as the Core Web Vital for visual stability.
In practice
A good CLS value is below 0.1. 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.