Core Web Vitals
Google's set of three field-measured user experience metrics — LCP, INP and CLS — used as part of the page experience signals in ranking.
TechnicalCLS
Cumulative Layout Shift
A Core Web Vital scoring unexpected visual movement of page content, computed from the largest burst of layout shifts; good is 0.1 or less at p75.
In full
Each shift is scored as impact fraction times distance fraction, and CLS reports the worst session window of shifts within five seconds. Shifts caused within 500 ms of a user interaction are excluded. The usual causes are images and iframes without dimensions, injected banners and ads, late-loading web fonts causing reflow, and content inserted above existing content.
Example
A cookie banner injected 1.2 s after load pushes the article down by 90 px, producing a 0.28 CLS until it is reserved space in the layout.
Related terms
Web font loading
The strategy for downloading and applying custom fonts; poor handling causes invisible text (FOIT), layout shift (FOUT) and slow LCP for…
Third-party scripts
Externally hosted JavaScript such as analytics, tag managers, consent tools, chat widgets and ad code, which are a leading cause of poor…
Largest Contentful Paint
A Core Web Vital measuring the render time of the largest image, video poster or text block visible in the viewport; good is 2.5 s or less…
Chrome User Experience Report
Google's public dataset of real-user performance measurements from opted-in Chrome users, and the official source of Core Web Vitals…
Where to read more
- Cumulative Layout Shift (CLS) · Documentation · web.dev