Cumulative Layout Shift (CLS)
Core Web Vital measuring how much visible content unexpectedly moves during page load. Target: under 0.1.
In long form.
CLS captures the frustrating experience of clicking the wrong button because an ad loaded above your tap target. The metric sums layout-shift scores across the page lifecycle, weighted by impact area and movement distance. Common causes: images without explicit width/height, ads or embeds without reserved space, web fonts swapping in (FOIT/FOUT), dynamically-injected content above the fold. Fixes are usually straightforward: reserve space, set explicit dimensions, use `font-display: optional` or proper font preload.
CLS wins are often a five-minute fix that lifts page experience scores significantly — adding `width` and `height` attributes to images, or `aspect-ratio` containers around third-party embeds.
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