CLS

What is CLS?

CLS, or Cumulative Layout Shift, is a Core Web Vital metric that measures the visual stability of a webpage — specifically, how much the page's content unexpectedly shifts or jumps around as it loads.

CLS is scored on a scale from 0 upward. Lower is better. Google's thresholds: Good (under 0.1), Needs improvement (0.1 to 0.25), and Poor (over 0.25).

What Causes Layout Shifts?

A layout shift happens when a visible page element changes its position on screen after the page has begun rendering — without any user interaction causing it. Imagine reading a paragraph and suddenly the text jumps down because an image above it finished loading and pushed everything down.

Common causes of poor CLS include: images without defined dimensions (if the browser doesn't know how tall an image will be before it loads, it can't reserve space for it — always define width and height attributes on images), web fonts causing FOUT/FOIT (font swapping causes layout shifts), dynamically injected content (ads, embeds, or personalization content that inserts above existing content after load), and animations that change layout properties.

In Webflow, CLS is usually well-controlled because layouts are defined in clean CSS. The most common Webflow CLS issues come from images without set dimensions and third-party embed scripts. At Appsrow, we audit all third-party scripts before launch to ensure CLS scores stay in the "Good" range.