Best Practices to Reduce CLS: Image Dimensions, Font Loading, and Third-Party Scripts

Best practices to reduce Cumulative Layout Shift (CLS) focus on stabilizing the page layout during loading by managing image dimensions, font loading, and third-party scripts effectively.

1. Define Image and Video Dimensions Explicitly
   Always specify the width and height attributes for images, videos, and iframes in the HTML markup. This reserves the exact space needed before these elements load, preventing sudden layout shifts when they appear. Alternatively, use the CSS aspect-ratio property to maintain the correct space dynamically. This ensures the browser allocates the right amount of space upfront, improving visual stability.

2. Optimize Font Loading
   Web fonts can cause layout shifts when they load and replace fallback fonts, changing text size or spacing. To reduce this:

• Use the preload attribute to load fonts early.
• Consider font-display strategies like swap to minimize invisible text or reflows.
• Optimize font subsets to reduce load time.
  Proper font loading prevents text from jumping or resizing after initial render.

3. Manage Third-Party Scripts and Dynamic Content
   Third-party ads, embeds, and scripts often inject content dynamically, causing layout shifts. To mitigate this:

• Reserve space for ads and embeds by setting fixed dimensions or minimum heights in CSS before the content loads.
• Use placeholders or skeleton screens to maintain layout structure.
• Avoid injecting content that changes layout size after initial load without reserved space.
• Lazy load offscreen content but ensure dimensions are defined to prevent shifts when they appear.

Additional Tips:

• Prefer CSS transform properties for animations instead of properties that trigger layout recalculations.
• Monitor CLS scores regularly using tools like Google PageSpeed Insights or Web Vitals to identify and fix new issues.

Implementing these practices helps maintain a stable visual experience, improving user engagement and Core Web Vitals scores.