Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) scores how much visible content unexpectedly moves during a page's lifetime, weighted by how much of the viewport shifted and how far. A score under 0.1 is good; 0.1-0.25 is graded medium here, above 0.25 is high.

It's the Core Web Vital most directly tied to a visitor's own experience rather than a network condition: it happens on a fast connection just as easily as a slow one, whenever something loads in above content that's already rendered.

The canonical failure mode is an image or ad with no reserved dimensions: the browser doesn't know its final size until it loads, so everything below it jumps down the moment it does — often exactly when a visitor is tapping a button that a moment later is a different element entirely.

Reserving space with explicit width/height (or CSS aspect-ratio) on anything that loads asynchronously — images, embeds, ads, injected banners — fixes the large majority of CLS findings without touching anything else about the page.