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Field guide

Cumulative Layout Shift (CLS)

Cumulative Layout Shift, or CLS, is a measure of unexpected visual movement during a page’s lifetime. It uses the largest burst of layout-shift scores, rather than simply adding every movement on the page. It is one of Google’s Core Web Vitals.

Identify content that moves after loading

CLS describes stability, not how quickly the page first appears. A page can load its initial content promptly and still move a control when a banner arrives later. Movement that the metric excludes can also be awkward for a person, so inspect the actual buying task alongside the score.

Illustrative checkout: the order summary and Continue button appear, then an unreserved delivery banner loads above them. The button moves while the shopper is aiming at it. The team reserves the banner’s space before it loads and repeats the sequence with a slow connection. The test records the visible movement and the layout trace, rather than assuming that changing one image solved every shift.

Protect selection and checkout controls from unexpected movement

One laboratory run does not represent every device, connection or later interaction. Personalization, consent notices and third-party content can produce different movement. A low CLS score is also not evidence that the checkout is clear, accessible or free of errors.

Next, reproduce the late-loading state and identify which element displaced the control. Compare field measurements with an inspected trace, then retest the buyer’s selection and checkout steps after the change.

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Reference: web.dev: Cumulative Layout Shift. Checked November 4, 2026.