CLS quantifies the frustration of a page that moves under your finger: the button that slides away as you tap, the paragraph that drops mid-sentence when an ad loads above it. Each unexpected shift is scored by how much moved and how far, summed over the page's life; good is 0.1 or less, poor above 0.25. Deliberate moves — an accordion the user opened — do not count; only surprise shifts do.
The fixes are almost all reservations: width and height attributes on images and embeds so the slot exists before the content, font-display strategies and metric-matched fallbacks so text does not reflow when the real font lands, space held for ads and banners, and never injecting content above what the user is reading.
Related terms
Core Web Vitals
Google's three field metrics for loading, responsiveness and visual stability, measured on real visits rather than in a lab.
LCP (Largest Contentful Paint)
The moment the biggest piece of content in the viewport finishes rendering — in practice, when the page looks loaded.
INP
Interaction to Next Paint — the Core Web Vital that measures how fast a page responds to taps and clicks, replacing FID since 2024.
Lighthouse
Google's open-source audit tool — scores performance, accessibility, best practices and SEO in a lab environment.
The bench this belongs to
Web performanceA rebuild that loads on a phone on mobile data, in the country your customers are actually in. Core Web Vitals are the measurement, revenue is the point.
