CLS
Cumulative Layout Shift: how much a page's layout jumps around while it loads.
What CLS measures
Cumulative Layout Shift measures how much the content of a page moves around unexpectedly while it loads and while someone uses it. A good CLS score is 0.1 or less, according to web.dev's CLS guidance. Shifts caused straight after a user's own action, such as opening a menu, don't count.
How to measure it
PageSpeed Insights highlights the elements that shift. You can also load the page on a phone with a slow connection and watch: anything that jumps after you've started reading is a candidate. Search Console's Core Web Vitals report shows which page groups have poor CLS for real users.
Common causes of a poor result
- Images and embeds with no width and height set.
- Banners or pop-ups inserted above existing content.
- Web fonts that swap in with very different sizes.
- Ads or widgets injected into the middle of text without reserved space.
What a fix looks like
A restaurant's menu page loads text first, then a cookie banner pushes everything down, then images pop in without reserved space. Diners trying to tap "Book a table" hit the wrong link. Setting width and height on images and placing the banner as an overlay brings CLS close to zero.
Why speed and stability matter
Layout shift is the reason people tap the wrong button, lose their place in a paragraph or accidentally click an ad. On a booking or contact page it costs enquiries. It's usually easy to fix once you know what's moving.
Small sites often cause shifts without realising it, through cookie banners, late-loading fonts or embedded reviews widgets. Each one is usually simple to fix once identified.
Other speed and experience terms
- Core Web Vitals: Google user-experience metrics: LCP, INP and CLS.
- LCP: Largest Contentful Paint: how quickly the main content of a page loads.
- UX and SEO: Overlap where usability, speed and clarity support search performance.
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