An ad that pushes a paragraph down while someone reads is one possible source of layout shift. CLS measures qualifying unexpected movement; a screenshot of the finished page cannot show the movement that happened during loading. (web.dev: Optimize Cumulative Layout Shift ↗)
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Finding the movement a reader sees
Imagine a reader reaches a paragraph just as an ad slot expands from 250 to 600 pixels tall. The paragraph moves down 350 pixels, potentially losing the reader’s place. Reserving room can prevent that particular movement, but the correct reservation depends on the accepted ad sizes and page layout.
Why an ad slot needs a stable place
Reserve suitable space for ads before they load, then test small, large, late and absent creatives. A fixed container that works on desktop may waste space or break a phone layout. Use a performance trace or field report to measure the score, alongside watching the actual reading experience.
Separate ad movement from other movement
Watch the same portion of the article while an ad appears. If the headline moves before the ad loads, check other elements too, such as an image whose space was not reserved. The timing helps narrow down the cause.