How to fix cwv cls on BigCommerce

Steps for BigCommerce

1. Images: In Storefront → My Themes → Edit Theme Files, open templates/components/products/product-view.html (and card.html for listing pages). Add width and height attributes to every <img> tag. BigCommerce's Stencil framework provides {{getImageSrcset}} helpers; use {{image width=800 height=800}} to output sized images with explicit dimensions.
2. Theme CSS: In assets/scss/settings/foundation/images/_settings.scss, ensure img { max-width: 100%; height: auto; } is present so explicit width/height attributes work correctly with responsive layouts.
3. Banner / promo: In Marketing → Banners, avoid rotating hero banners injected via JavaScript without a reserved container. In your theme's homepage template (templates/pages/home.html), wrap the banner region in a container with a fixed aspect-ratio or min-height.
4. Fonts: In assets/scss/settings/global/color-and-type/_settings.scss, locate @font-face declarations and add font-display: optional;.
5. Apps: For review or chat apps installed from the BigCommerce App Marketplace, wrap their embed scripts in a sized container div in the relevant template file.
6. Verify: Check PageSpeed Insights on your storefront URLs; CLS should be < 0.1 on both mobile and desktop.

What is cwv cls?

Cumulative Layout Shift (CLS) measures how much page content unexpectedly moves around while a visitor is looking at it. Every time an image pops in and shoves text down, a banner slides in from a cookie notice, or a font swap causes a paragraph to reflow, that counts as a layout shift. Google adds all those shifts together into a single score; anything above 0.1 is considered "needs improvement," and above 0.25 is "poor." A score of 1.35 (like the example above) means the page is shifting dramatically — visitors are misclicking, losing their place, and bouncing before they buy.

CLS is one of Google's three Core Web Vitals and is a direct ranking signal in Google Search — a poor score actively suppresses your position in search results, reducing organic traffic and revenue. On mobile, where most ecommerce shopping happens, layout shifts are especially jarring: a customer tapping "Add to Cart" can accidentally tap a shifted link instead, killing the conversion. High CLS also signals a low-quality page experience, which increases bounce rate and reduces the trust visitors place in your store. Google's own data shows that sites meeting Core Web Vitals thresholds see measurably better business outcomes than those that don't.

See the complete Cwv cls guide for every platform and the full background.