What is Cumulative Layout Shift (CLS)?

Cumulative Layout Shift (CLS) measures how much your page visually jumps around while it loads — the annoying moment when you go to tap a button and an image pushes it out from under your finger:

• Good is under 0.1 — a low score means the layout settles and stays put as it loads.
• The usual culprit is images and ads with no reserved space — the browser does not know how tall they are until they arrive, so everything below them lurches down.
• Late-loading fonts and injected banners — swapping a font or dropping in a cookie bar after render also shoves content around.

CLS is the cheapest Core Web Vital to fix because the cause is almost always missing dimensions. Put width and height on every image and reserve space for anything that loads late, and CLS looks after itself.