Definition CLS is a visual stability metric. Unexpected layout shifts (async banners, images without reserved space, web font swap) push the score up.
How it works At each new layout step, the browser multiplies the impacted area share by the movement distance. All shift values aggregate. Shifts within 500ms after a user interaction are excluded (expected shift). Window size, image width/height attributes, and font-display settings affect CLS.
Where you see it in Scope Trends The **Core Web Vitals** report gives CLS at the 75th percentile. **Site Crawl** reports the largest layout shift candidate element and cause per page.
Frequently asked questions **How do I reduce CLS?** Declare width/height on images, use font-display: optional or swap + size-adjust, reserve space for dynamic content.
**Does opening a modal count toward CLS?** If user-triggered, it is expected shift and does not affect score. Auto-opened modals do count.