Performance
Cumulative Layout Shift (CLS)
A Core Web Vital measuring how much visible content moves unexpectedly while someone uses a page, such as text jumping when an image loads above it.
Layout shifts are the moments when a button moves just as you tap it, or the paragraph you are reading drops out of view. CLS scores the largest burst of unexpected movement during a visit. Movement that follows straight after a click or tap, such as an accordion opening, does not count.
A good score is 0.1 or less at the 75th percentile of visits (web.dev). The usual causes are images and embeds without set dimensions, ads, banners or notices inserted above existing content, and web fonts that change the size of text when they load.
To fix it, give every image and video a width and height (or a CSS aspect ratio), reserve space for anything that loads later, show late notices where they cannot push content down, and choose fallback fonts with similar proportions. Check field data as well as lab tests, because shifts that happen after scrolling often do not appear in a Lighthouse run.
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