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Glossary
CLS (Cumulative Layout Shift)
CLS measures how much the page’s content jumps around while it is loading.
Also known as: cumulative layout shift · layout stability
An image that arrives and pushes the text down, an ad that appears above a button, a font swap that changes line heights — every one of these moves something the visitor was about to read or click.
The fix is mostly reservation: give images and embeds explicit dimensions so the space is held before the content arrives.
Common questions
What causes it most often?
Images without width and height attributes, and content injected at the top of the page after load.
Do fonts cause layout shift?
They can. A fallback font with different metrics changes line lengths when the real font arrives. Matching the fallback’s metrics removes most of it.
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