What Is Cumulative Layout Shift?
Cumulative Layout Shift measures how much the visible content moves around while a page is loading. You've experienced it: you start reading an article, and then an ad loads above it and pushes everything down. Or you're about to click a button and it jumps because a font finished loading. CLS is measured on a scale where lower is better. A good score is under 0.1. The most common causes are images without dimensions, ads without reserved space, and fonts that swap after loading.