Cumulative Layout Shift (CLS) measures unexpected page movements that frustrate users, such as when images or buttons shift during loading. Common causes on Shopify include not reserving space for images, dynamically injected content, delayed CSS, improper animations, and font swaps. To minimize CLS, ensure that your site reserves space for images and dynamic content, avoids lazy-loading critical CSS, uses CSS transforms for animations, and employs optimized fallback fonts. Use tools like PageSpeed Insights and WebPageTest to detect and troubleshoot layout shifts.
Optimizing Cumulative Layout Shift (CLS) on Shopify Sites
Cumulative Layout Shift (CLS) represents one of the key Web Vitals that impact user experience, specifically tracking visual stability. A poor CLS score can cause user frustration when elements unexpectedly shift while interacting with a page, leading to missed clicks or a disjointed experience.
What Causes CLS on Shopify Stores?
Several common factors can lead to layout shifts, including:
- Images Without Dimensions: When image dimensions aren’t specified, the browser doesn’t reserve space before the image loads, leading to shifts.
- Dynamically Injected Content: Elements added to the page after initial load can cause existing elements to move.
- Late-Loading Web Fonts: If custom fonts load slowly, they can cause text to shift as the fallback font is replaced.
- Non-Reserved Space for Ads or Embedded Elements: Not reserving space for ads or third-party content can also cause layout shifts.
- Animations: Animations that adjust dimensions or layout, rather than using CSS transforms, can cause instability.
Best Practices for Reducing CLS on Shopify Sites
Here’s how to mitigate layout shifts and optimize the CLS score for a Shopify store:
- Define Fixed Dimensions for Images: Always specify image width and height attributes or use a placeholder to avoid unexpected shifts as images load. This ensures the browser reserves the correct amount of space in advance.
- Preload Key Fonts: Use the font-display property to control font loading behavior, reducing layout shifts caused by delayed font rendering. Preloading critical fonts ensures faster load times for visible text.
- Avoid Dynamically Injecting Content Above the Fold: To prevent content shifts, try to load dynamic content below the fold or reserve the necessary space before loading.
- Ensure Ads and Third-Party Embeds Have Reserved Space: By allocating enough space for ads and other third-party content, you avoid sudden shifts when these elements load.
- Use CSS Transform for Animations: When animations are needed, favor CSS transforms over animations that adjust layout properties, which can cause reflow and shift the page content.
Tools to Measure and Improve CLS
Several tools can help you diagnose and improve CLS on your site:
- PageSpeed Insights: This tool offers a detailed report of your website’s performance, including CLS scores.
- WebPageTest: Provides detailed analysis and can help identify specific issues contributing to high CLS.
- Lighthouse: Integrated with the Chrome DevTools, Lighthouse gives actionable insights to optimize your site’s Core Web Vitals, including CLS.
Conclusion
Reducing CLS is essential for providing a smooth and stable experience to users, which can lead to better engagement and conversions. By following best practices like setting dimensions for images, preloading fonts, and reserving space for dynamic content, you can significantly reduce layout shifts and improve your Shopify store’s performance.
