Technical Definition
CLS measures unexpected layout shifts during page load. It calculates impact fraction (viewport affected) times distance fraction (how far elements moved). Good CLS is under 0.1. Common causes: images without dimensions, ads/embeds without reserved space, dynamically injected content, web fonts causing FOIT/FOUT. Fix by specifying dimensions and reserving space for dynamic content.
Simple Explanation (ELI13)
CLS measures how much stuff jumps around while your page loads. You know when you're about to click something and suddenly an ad loads and pushes it down? That's layout shift. Google hates this because it's annoying for users. Fix it by telling the browser how big images and ads will be before they load.
Related Terms
Core Web Vitals, Page Experience, Above the Fold
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