Cumulative Layout Shift (CLS)
A score for how much visible content moves unexpectedly during load.
SEO
Definition
CLS sums the impact of unexpected layout shifts across the page's lifespan, weighting the fraction of the viewport affected by the distance moved.
A score at or below 0.1 is considered good.
Why it matters
Layout shift causes misclicks and abandoned forms, and it is trivially preventable — which is why failing it signals wider engineering hygiene problems.
How to apply it
- Set explicit dimensions or aspect ratios on media.
- Reserve space for banners, consent bars and ad slots.
- Load webfonts with a size-matched fallback.