CLS (Cumulative Layout Shift) is a Core Web Vital that measures how much a page's visible content unexpectedly moves around while it loads.
When images, ads, fonts, or injected elements arrive without reserved space, the content below them jumps, and users tap the wrong thing or lose their place. CLS scores that instability by combining how much of the viewport shifts with how far it moves. Lower is better: a score of 0.1 or less is good, and above 0.25 is poor.
Most layout shift comes from a few habits: images and embeds without width and height attributes, ads or banners inserted after load, web fonts that swap and reflow text, and content injected above what the user is already reading. The fixes are to reserve space for media and dynamic elements, set explicit dimensions, and load fonts in a way that avoids reflow. For the full diagnosis and remediation workflow, see the CLS complete guide.
Related: CLS Complete Guide, Field vs Lab Data, Above the Fold
Claude Vincent is a technical SEO consultant focused on crawlability, rendering, and AI-search visibility. He writes the field guides and case studies at SEO ProCheck, with a bias toward the durable, unglamorous work that decides whether search engines and AI answer engines can actually read and cite a site.
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