The Core Web Vitals metric for visual stability — measures how much page content unexpectedly jumps around during load. Target: under 0.1.
CLS (Cumulative Layout Shift) in plain English
CLS measures the sum of all unexpected layout shifts that happen while a page is loading. A layout shift is anything that moves visible content from one position to another after it first rendered — an image loading and pushing text down, an ad slot expanding mid-scroll, a font swap that changes line heights. The score is calculated as shift impact times distance fraction, summed across the page load.
Google's thresholds are: under 0.1 is good, 0.1 to 0.25 is needs-improvement, over 0.25 is poor. Roughly 75% of your pageviews need to hit the good threshold for the page to be classified as good in Search Console.
The most common culprits on ecommerce stores are: product images without width/height attributes (browser doesn't reserve space, so when the image loads it pushes everything down), web fonts that swap from a fallback to the custom font and change line metrics, banner notifications that inject above the fold after page render, and third-party widgets (review badges, chat boxes, ad slots) that load into unreserved space.
The fix pattern is the same across all causes: reserve the space before the content loads. Set explicit width and height attributes on every <img> and <iframe>. Use the CSS aspect-ratio property for responsive media. Preload critical web fonts and use font-display: optional to skip the swap entirely. Reserve space for late-loading widgets with min-height. For dynamic content like notifications, place them in fixed-position elements so they don't push the document flow.
Why cls (cumulative layout shift) matters for ecommerce
For ecommerce, CLS directly affects conversion. Shoppers who go to tap an Add to Cart button just as the page shifts and accidentally tap something else lose trust immediately — they bounce or refund. Google has data showing a strong correlation between CLS scores and bounce rate. For mobile especially, where thumb-tap precision is small, even a 0.15 CLS can cost meaningful conversion. Fix the layout shifts and you improve both ranking signal and revenue at the same time.