Glossaire · SEO

CLS (Cumulative Layout Shift)

CLS (Cumulative Layout Shift) is one of Google's three Core Web Vitals. It measures the visual stability of a page by quantifying how much visible elements move unexpectedly during loading and navigation. Concretely, CLS produces a unitless score by multiplying the share of the screen affected by the shift (impact fraction) by the distance the elements travel (distance fraction). A score of 0.1 or below is considered good, between 0.1 and 0.25 it needs improvement, and above 0.25 it is judged poor. Common causes include images and iframes without declared dimensions, web fonts that trigger a reflow, dynamically injected banners or ads, and content inserted above the visible area. A high CLS degrades the user experience and acts as a ranking signal that Google factors into its assessment of page quality. Reducing it means delivering a stable, predictable layout from the first paint.

CLS is one of the metrics Google watches to assess how a page actually feels to a user. Unlike raw speed, it measures a concrete frustration: elements that move just as you are about to click.

How it works

The CLS score is the sum of every individual layout shift score that occurs over the full lifespan of the page. Each individual shift is the product of two values: the impact fraction (the portion of the viewport affected by the movement) and the distance fraction (the maximum distance an element travels, relative to the screen dimension). Google only counts shifts that are not triggered by a user interaction within 500 milliseconds: a change caused by a deliberate click is therefore not penalized.

Key takeaway

A CLS ≤ 0.1 is the goal. Above 0.25, the page is considered unstable and SEO suffers.

The most common causes

Four culprits show up almost every time: images and iframes without dimension attributes, which force the browser to recalculate the layout once the media has loaded; late web fonts, which trigger a FOIT or a FOUT; dynamically injected content (cookie banners, ads, recommendations); and animations that change properties triggering a reflow instead of using transform.

Why it matters for SEO

CLS is an official ranking factor within the Core Web Vitals. An unstable page increases bounce rate and hurts conversions, two indirect signals that search engines read. At LUWIZ, we address CLS alongside LCP and INP, because these three metrics are measured in the field through real-user data (CrUX) and shape the overall assessment of a URL. Stabilizing the layout means delivering a predictable experience and protecting rankings over the long term.

FAQ

Questions fréquentes

A good CLS score is 0.1 or below. Between 0.1 and 0.25 the page needs improvement, and above 0.25 Google considers the score poor.

Declare the width and height of images and videos, reserve space for ads and embeds, preload web fonts and avoid inserting content above elements that are already displayed. These steps keep the layout stable during loading.

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