CLS

CLS (Cumulative Layout Shift) measures how much a page's layout jumps unexpectedly while it loads - content sliding down when a late image or advert appears, buttons moving as you try to tap them. It is one of Google's .

A score of 0.1 or less is considered good; 0.25 or more is poor. The usual fixes are reserving space for images and embeds (width and height, or aspect-ratio boxes), avoiding inserting content above existing content, and being careful with web fonts that swap late.