Reserve space for images, ads, and dynamically injected content so the page does not jump while loading.
CLS is a Core Web Vital (target < 0.1). Unexpected shifts cause misclicks and a janky feel that hurts conversion and rankings.
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Cumulative Layout Shift measures unexpected movement of visible content. The target is < 0.1. Most shift comes from media without dimensions and content injected above existing content.
<!-- bad: no dimensions -> reflows when image loads -->
<img src="/photo.jpg" />
<!-- good: explicit size lets the browser reserve space -->
<img src="/photo.jpg" width="800" height="600" />Set width/height or aspect-ratio on media, reserve space for embeds/ads, and avoid inserting banners above content already in view.