Cumulative Layout Shift (CLS) measures unexpected movement of visible page content during load — images loading without reserved space, ads injecting, or fonts swapping.
Context
CLS is a Core Web Vital that correlates with user frustration. A page that jumps around as it loads causes mis-clicks, failed reads, and higher bounce rates.
The threshold for 'good' CLS is under 0.1 for 75% of real sessions. The fix is usually setting explicit width/height attributes on images and video, reserving space for ads, and avoiding font swaps that resize text.
A viewport-shifting banner ad that loads 500ms into a page render routinely contributes 0.15–0.30 CLS — enough on its own to fail the threshold and trigger a Core Web Vitals ranking penalty.
CLS is measured through the entire page lifecycle, not just initial load. Lazy-loaded content below the fold that shifts the viewport later also counts.
Related terms
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CTR (Click-Through Rate)
The percentage of people who clicked after seeing your link, ad, or snippet.
SERP (Search Engine Results Page)
The page Google shows after a search — far more than just ten blue links.
hreflang
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Canonical URL
The authoritative URL for a page when duplicate or near-duplicate versions exist.