Cumulative Layout Shift (CLS)

Cumulative Layout Shift scores unexpected movement of visible elements. Each shift's value is its impact fraction (how much of the viewport moved) multiplied by its distance fraction (how far it moved), reported through Layout Instability API entries that exclude shifts within 500 ms of user input.

Since 2021 CLS is not a lifetime sum: it is the largest session window — shifts separated by under one second, capped at five seconds. The good threshold is 0.1. Images without declared dimensions and late-injected banners are the classic sources.