CLS
Cumulative Layout Shift: a measure of how much a page's content unexpectedly moves around while it loads.
Cumulative Layout Shift, or CLS, is one of Google’s three Core Web Vitals. It measures visual stability: how much things on the page jump around unexpectedly while it loads. You’ve felt it when you go to tap a button and the page shifts, so you tap something else.
According to web.dev’s CLS guide, a good CLS score is 0.1 or less.
Why it matters to a small business
Layout shifts are irritating and can cause mis-taps, such as tapping an ad or the wrong menu item. On a phone, where space is tight, they’re more noticeable. A stable page feels professional and trustworthy.
Common causes:
- Images or videos without a set width and height, so the space isn’t reserved.
- Banners, cookie notices or chat widgets that push content down after loading.
- Ads or embeds that load late.
- Web fonts that swap in and change text size.
Example
A restaurant’s menu page jumps down when a promotional banner loads above it. Visitors scrolling to the menu lose their place. Reserving space for the banner, or showing it as an overlay that doesn’t push content, brings CLS back to near zero.
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