Definition
CLS (Cumulative Layout Shift) — CLS measures visual stability: how much page elements shift unexpectedly.
Quick answer
Target: ≤ 0.1. Set width/height on images and iframes, reserve space for banners, use font-display with adjusted fallback fonts.
Typical causes
- images without dimensions;
- ads and banners injected above content;
- web fonts changing text metrics.
Sources
- Cumulative Layout Shift (CLS) — web.dev