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When designing websites, one of the key factors for a positive user experience is page stability. Cumulative Layout Shift (CLS) measures how much content shifts unexpectedly during page load. High CLS can frustrate users and harm SEO rankings. Fortunately, optimizing CSS and JavaScript can significantly reduce CLS and improve overall page stability.
Understanding CLS and Its Impact
CLS is a Core Web Vital that quantifies visual stability. A high CLS score indicates that elements on the page move around, causing a poor user experience. Common causes include late-loading images, dynamically injected content, or fonts that cause layout shifts.
CSS Tips to Minimize CLS
- Specify size attributes for images and videos: Always include width and height attributes or CSS aspect ratio boxes to reserve space before loading.
- Use font-display: swap; in @font-face rules to prevent invisible text during font loading, reducing layout shifts caused by font swaps.
- Optimize CSS delivery: Minimize and inline critical CSS to ensure essential styles load quickly and prevent layout shifts.
- Avoid inserting new styles dynamically: Predefine styles to prevent unexpected shifts when styles are applied after load.
JavaScript Tips to Reduce CLS
- Defer non-essential JavaScript: Use the defer attribute to delay script execution until after the main content loads.
- Lazy load third-party scripts: Load scripts like ads or social widgets asynchronously to prevent blocking rendering.
- Manage dynamic content carefully: When injecting content dynamically, reserve space beforehand to avoid shifts.
- Optimize event handling: Minimize layout thrashing by batching DOM reads and writes efficiently.
Additional Best Practices
Consistently testing your website with tools like Google Lighthouse or WebPageTest can help identify CLS issues. Regularly monitor and optimize your CSS and JavaScript to ensure a smooth, stable user experience with minimal layout shifts.