CLS is a unitless score that sums the impact of every unexpected layout shift between page load and user interaction. Only shifts that occur without user input contribute to the score.
Common causes
- Images without width and height attributes.
- Web fonts that cause a flash of unstyled text (FOUT) or unstyled content (FOUC).
- Ads or embeds inserted above existing content after load.
- Dynamically injected banners or cookie notices.
Example
An e-commerce PDP with CLS 0.34 drops to 0.05 after adding explicit dimensions to product images and reserving space for the review widget.
How SEM Optimiser reports it
CLS is reported alongside a shift attribution list so the specific elements responsible for the score are identified.
Related terms
- Core Web Vitals
Core Web Vitals is a set of three page-experience metrics Google uses to measure loading performance, interactivity, and visual stability: LCP, INP, and CLS.
- LCP (Largest Contentful Paint)
Largest Contentful Paint is the time from navigation start until the largest visible element in the viewport has been rendered, and should be under 2.5 seconds.
- INP (Interaction to Next Paint)
Interaction to Next Paint measures how quickly a page responds to a user's interactions across the whole session, and should be under 200 milliseconds.