How to fix cwv cls on BigCommerce

Steps for BigCommerce

1. Images: In Storefront > My Themes > Edit Theme Files (Stencil), open the relevant template (e.g. templates/components/products/card.html). Find <img> tags and add width and height attributes using the Handlebars image helper: {{getImageManagerUrl image.data '500x500'}} and set the dimensions as attributes.
2. CSS aspect ratio: In assets/scss/components/products/_card.scss (or similar), add an aspect-ratio rule to the image container class so it reserves space before the image downloads.
3. Fonts: In assets/scss/settings/foundation/base/_settings.scss (or your theme's font file), find @font-face declarations and add font-display: swap;. If loading Google Fonts via a <link> in templates/layout/base.html, append &display=swap to the URL.
4. Announcement bars and promotional banners: Use the built-in BigCommerce 'Carousel' or 'Banner Manager' (Marketing > Banners) rather than custom injected scripts, as these are rendered server-side. If using a Script Manager script (Storefront > Script Manager) for a banner, wrap the target div with an explicit min-height in CSS.
5. Third-party scripts: In Storefront > Script Manager, for any widget scripts set the 'Load Method' to 'Defer' and 'Location' to 'Footer' to prevent them from blocking layout.
6. Cookie banners: In your GDPR/consent app's settings, set the banner to 'bottom fixed' position so it does not push page content.

<img
  src="product-photo.jpg"
  alt="Red running shoes"
  width="800"
  height="800"
  style="aspect-ratio: 1/1; width: 100%; height: auto;"
>

/* CSS: reserve space for an ad slot before it loads */
.ad-container {
  min-height: 90px;   /* match your largest ad unit height */
  width: 100%;
}

/* CSS: prevent font-swap layout shift */
@font-face {
  font-family: 'MyFont';
  src: url('/fonts/myfont.woff2') format('woff2');
  font-display: swap;
}

What is cwv cls?

Cumulative Layout Shift (CLS) measures how much the visible content on your page unexpectedly moves while it is loading. Every time an image pops in and pushes a button out of reach, a font swaps and reflowing text, or a banner ad shoves your product description down the page, that counts as a layout shift. Google's "good" threshold is a CLS score below 0.1; a score of 1.05 — as in the example above — means content is shifting dramatically, which is easily noticeable to shoppers. The most common causes are images and videos without explicit width/height attributes, web fonts that change character sizes when they load, third-party widgets (chat bubbles, review carousels, cookie banners, email pop-ups) injected above existing content, and dynamically inserted banners or promotional bars.

CLS is one of Google's three Core Web Vitals, which are a direct ranking signal in Google Search — a poor score can push you below competitors even if your content is better. Beyond rankings, layout shift is a serious conversion killer: a shopper who taps "Add to Cart" while the page is still loading can accidentally hit the wrong element, leading to frustration and cart abandonment. Google's own research links poor page experience scores to higher bounce rates and lower revenue per session. On mobile devices — where Google indexes first — layout shift is even more pronounced because screens are small and a single banner inserting itself at the top can move everything else off screen.

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