This translation has not been editorially reviewed yet. The German version is authoritative. Deutsche Fassung →
Checking CLS: why content jumps around while loading
Cumulative Layout Shift measures how much visible content changes position while loading. It's the only core value with no time unit — and the only one whose effect everyone knows: you go to click, and the button slides away at that exact moment.
- The most common cause is images and embedded content with no specified dimensions. The browser doesn't know how much space to reserve, and pushes everything down once they arrive.
- The second most common cause is elements that appear afterwards: notice bars, consent banners, ad spaces. If they take up space instead of overlaying it, they shift all the content.
- The third case is fonts: if a fallback font is shown first and then swapped, line breaks change, and with them the position of everything that follows.
FAQ
From what value does CLS count as good?
The measurement tool classifies it based on published thresholds; our report states the measured value along with its classification. What matters in practice is the cause, not the decimal point.
Do shifts after a user action count?
Shifts immediately after an input are exempt — they're usually intentional, when expanding a section, say.
How do you prevent shifts caused by images?
By specifying width and height. The browser then reserves the space from the start, even if the image arrives later.