CLS (Cumulative Layout Shift)
Cumulative Layout Shift measures how much visible content moves unexpectedly while a page loads. A score of 0.1 or below is good. It is the metric behind the experience of reaching for a button just as the page jumps and hitting something else.
Why it matters for your rankings
CLS is the Core Web Vital that most directly annoys people. Everyone has tapped the wrong thing because an advert loaded late and pushed the page down. On ecommerce and forms that mis-tap can mean an accidental purchase or an abandoned checkout.
It is also the easiest of the three to fix, because the causes are few and well understood: images without dimensions, ads and embeds without reserved space, web fonts swapping in late, and content injected above existing content. Each has a standard remedy. Because it is measured across the entire page lifespan rather than just initial load, shifts caused by lazy-loaded content far down the page still count against you, which surprises teams who only test the first screen.
How to check it on your site
Get your field score
Check the Core Web Vitals report in Search Console for real-user CLS, then PageSpeed Insights for element-level detail.
Set image dimensions
Every img tag needs width and height attributes, or a CSS aspect-ratio, so the browser reserves the correct space before loading.
Reserve space for injected content
Give ad slots, embeds and cookie banners fixed minimum heights so they cannot push content when they appear.
Fix font swap shift
Use font-display: optional or preload your primary fonts to avoid a visible reflow when the web font replaces the fallback.