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Improving Cumulative Layout Shift (CLS) scores is essential for providing a smooth user experience in Progressive Web Apps (PWAs). A low CLS score indicates that your app’s content loads predictably, preventing unexpected movements that can frustrate users. Here’s how you can enhance your PWA’s CLS performance.
Understanding CLS in PWAs
CLS measures how much visible content shifts during the loading phase. Factors contributing to high CLS include images without dimensions, dynamically injected content, and slow-loading fonts. Reducing these shifts makes your app more stable and user-friendly.
Strategies to Improve CLS Scores
1. Specify Size Attributes for Images and Videos
Always include width and height attributes on images and videos. This reserves space during page load, preventing layout shifts when media loads.
2. Use CSS Aspect Ratios
Modern CSS allows you to set aspect ratios, ensuring containers maintain their dimensions regardless of content. Example:
img {
aspect-ratio: 16 / 9;
}
3. Optimize Web Fonts
Web fonts can cause shifts when they load. Use font-display: swap in your CSS to display fallback fonts immediately and swap in the custom font when ready.
4. Avoid Injecting Content Dynamically Without Reserve Space
When adding content dynamically, allocate space beforehand. Use CSS to reserve space for ads, banners, or other elements that load asynchronously.
Tools to Measure and Monitor CLS
Use tools like Google Lighthouse, WebPageTest, or Chrome DevTools to analyze your PWA’s CLS scores. Regular testing helps identify issues early and track improvements over time.
Conclusion
Reducing CLS scores enhances user experience and can positively impact your PWA’s performance metrics. Implementing proper image sizing, reserving space for dynamic content, and optimizing fonts are effective strategies. Regular testing ensures your app remains stable and user-friendly.