CLS (Cumulative Layout Shift)
Definition
Cumulative Layout Shift (CLS) measures how much visible content moves around while a page loads, for example when an image or banner appears and pushes text down. It is a unitless score, and Google's good threshold is 0.1 or less at the 75th percentile of visits. Shifts frustrate visitors and cause mis-taps on buttons.
Why it matters for your store
A visitor reaches for "Add to cart," the page shifts, and they tap "Compare" instead. Or they start reading a description and it jumps below a late-arriving banner. Each shift is small; together they make a store feel unreliable. Google counts them and uses the total as a ranking signal.
The causes are almost always technical and almost always fixable at the template level. Images need declared dimensions so the browser reserves space. Fonts need a fallback of a similar size so text does not reflow when the real font arrives. Anything injected after load, such as an announcement bar or a chat widget, needs a placeholder of the same height.
How Kambloo handles it
Kambloo's storefront budget sets CLS under 0.1, and the build pipeline measures it on every change. Every image carries width and height or an aspect ratio, and product images are pre-generated at fixed sizes, so the browser reserves the right space before the file arrives.
Loading placeholders (skeletons) have fixed dimensions. Themes that use custom fonts must ship a fallback with size adjustment and use font-display swap, so the swap does not reflow the page. Themes are checked against these rules before acceptance.
Real-user CLS is collected by a lightweight measurement snippet and shown in the store's SEO health report.
Questions about CLS (Cumulative Layout Shift)
What causes layout shift on a store page?
Images without width and height, banners or notices injected after load, web fonts that swap in at a different size, and ads or widgets that push content down. Each can be fixed by reserving space in advance.
Is CLS a problem if the page looks fine to me?
It may be. Shifts happen during loading and are easy to miss on a fast connection. Field data from real visitors on slower connections is what Google measures.