Cumulative Layout Shift [CLS]

Cumulative Layout Shift
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Cumulative Layout Shift (CLS) calculates how many unique and unexpected elements on the page shift as it is loaded in the browser. The amount of shift is important because page movement while a user is trying to interact with it can be a frustrating user experience.

A Cumulative Layout Shift happens when you’re about to click on a link or a button, and then the layout shifts just as you’re about to click, and you end up clicking on a different element instead.

Another example is when you’re reading a blog post and an ad or a video loads, and you lose your place. Both of these are frustrating for the user and result in a bad experience. This metric is designed to help resolve bad UX.

Measuring Cumulative Layout Shift

Unlike other Core Web Vitals metrics, CLS is measured as a calculated score rather than a timing. The amount of layout shift score is the outcome of two measures of movement: the impact fraction and the distance fraction.

A good Cumulative Layout Shift is zero, which means no shifting on the page, and a bad score is anything above 0.25. The larger the CLS score, the more movement there is.

layout shift
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