Cumulative Layout Shift (CLS)
Cumulative Layout Shift (CLS) measures the total amount of unexpected layout movement that occurs during the lifespan of a page load. It quantifies visual instability — the degree to which page elements shift position while the page is loading. A CLS score of 0.1 or lower is considered "Good," 0.1–0.25 "Needs Improvement," and above 0.25 "Poor."
Layout shifts are caused by images or ads without explicit dimensions, dynamically injected content above existing content, web fonts that swap and cause text reflow, and third-party embeds that load asynchronously. CLS is particularly frustrating for users because it causes unintended clicks — a button that shifts position right as a user taps can trigger the wrong action. Always defining size attributes for images and reserving space for dynamic content are the primary fixes.
Why it matters for SEO
CLS directly measures user experience frustration from unstable page layouts. High CLS leads to accidental clicks, broken interactions, and user distrust — all of which increase bounce rates. As a Core Web Vital and ranking factor, poor CLS scores can measurably hurt search performance.
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