metrics
CLS (Cumulative Layout Shift)
Sum of unexpected layout shifts during page lifetime — under 0.1 is good.
Definition
Cumulative Layout Shift quantifies how much visible content jumps unexpectedly during page load. Each shift is scored by impact fraction × distance fraction; the cumulative score is the sum. Common causes: images without dimensions, late-loaded ads, font swap (FOUT/FOIT). Good < 0.1; needs improvement 0.1–0.25; poor > 0.25.
Example
A banner ad loading after 2s and pushing the article down by 200px = high CLS. Reserving the ad slot's height in CSS = zero CLS.How to optimize
Always set width + height on images/videos. Reserve ad slot dimensions. Use font-display: optional or swap with size-adjust. Test in Chrome DevTools Performance panel.