Improving website performance has become more important than ever in 2025. Among all Core Web Vitals metrics, Cumulative Layout Shift (CLS) plays a crucial role. It measures how visually stable your page is while loading. A high CLS score can frustrate users, hurt engagement, and even lower your Google rankings.

This complete CLS optimization guide will show you what CLS is, why it happens, and how you can fix it. The strategies we discuss will help you achieve a stable, user-friendly design and better search visibility.

What Is Cumulative Layout Shift (CLS)?

Cumulative Layout Shift (CLS) is a Core Web Vital metric that measures unexpected shifts in web page elements while the page is loading. In simple words, when content moves around before the page finishes loading, the CLS score increases.

For example:

  • A button moves down because a banner loads late.
  • Text jumps when a custom font replaces the default font.
  • Images shift because dimensions were not defined.

Why CLS Matters for SEO and UX

CLS impacts both user experience and search rankings.

  1. User Experience (UX): When elements jump on the screen, users click the wrong buttons or lose their place while reading.
  2. Conversions: A poor CLS score lowers trust and may cause users to abandon shopping carts or forms.
  3. Google Rankings: CLS is part of Core Web Vitals, which directly influence Google’s ranking system.

💡 Best Practice: To pass Google’s CLS test, your CLS score should be less than 0.1.

Main Causes of High CLS

Understanding what causes layout shifts is the first step toward fixing them. Here are the most common issues:

  • No width/height for images or videos
  • Late-loading ads, banners, or embeds
  • Web fonts replacing fallback fonts
  • Animations and transitions without proper stability
  • Dynamically injected content (like pop-ups)

CLS Scoring and Thresholds

Google uses these thresholds:

  • Good CLS Score: 0 – 0.1
  • Needs Improvement: 0.1 – 0.25
  • Poor Score: Above 0.25

Tools like Google PageSpeed Insights, Lighthouse, and Chrome DevTools show you your CLS performance in real time.

Strategy 1: Font Loading Optimization

Fonts are one of the biggest culprits behind CLS. When a custom font loads late, it replaces the fallback font and causes text shifts.

 Fixes for Font Loading:

  • Use font-display: optional or font-display: swap in CSS.
  • Preload key fonts using <link rel="preload">.
  • Use system fonts where possible.
  • Reduce font weights and variations to improve load speed.

💡 Example CSS:

@font-face { font-family: 'CustomFont'; src: url('customfont.woff2') format('woff2'); font-display: swap; }

Strategy 2: Define Image & Video Dimensions

One of the simplest yet most effective CLS fixes is to set explicit width and height attributes for media files.

 Fixes for Images and Videos:

  • Always define width and height in HTML.
  • Use aspect-ratio CSS property for responsive layouts.
  • Consider lazy-loading images but reserve space for them.

💡 Example:

<img src="example.jpg" width="600" height="400" alt="CLS optimization example">

Strategy 3: Reserve Space for Ads and Embeds

Ads, banners, and embeds often load late and push content around.

 Fixes:

  • Reserve fixed space for ads in the layout.
  • Use CSS containers with predefined dimensions.
  • Avoid inserting ads above main content when possible.

Strategy 4: Avoid Unexpected Pop-Ups and Dynamic Elements

Dynamic content injection like cookie banners, pop-ups, and notifications can create layout instability.

 Fixes:

  • Place such elements outside the main content flow.
  • Trigger them after interaction, not on page load.
  • Animate them with transform and opacity, not height or width.

Strategy 5: Use CSS Transformations Instead of Position Shifts

When animating elements, avoid properties that trigger reflows. Instead, use GPU-friendly animations.

Advanced CLS Fixes and Optimization Strategies

Now that we have covered the basics, it is time to go deeper. Advanced CLS optimization not only reduces layout shifts but also enhances the overall website experience.

Strategy 6: Preload Key Resources

Resource preloading ensures that critical assets such as fonts, images, and CSS files are fetched earlier. This prevents last-second replacements that lead to layout jumps.

Implementation:

<link rel="preload" href="style.css" as="style"> <link rel="preload" href="main-font.woff2" as="font" type="font/woff2" crossorigin>

  • Preload hero images.
  • Preload primary fonts.
  • Prioritize above-the-fold CSS.

Strategy 7: Optimize CSS and JavaScript

Excessive CSS and JavaScript can delay rendering and cause elements to load in chunks.

Fixes:

  • Remove unused CSS and JS.
  • Split large JavaScript bundles.
  • Use async or defer for non-critical scripts.
  • Minify and compress all files.

💡 Tools: Webpack, Rollup, PurgeCSS

Strategy 8: Test and Fix CLS with Lighthouse & DevTools

Google Chrome DevTools provides an excellent environment to monitor layout shifts.

✅ Steps:

  1. Open Chrome DevTools > Performance tab.
  2. Record a session.
  3. Identify layout shift regions.
  4. Fix the elements causing instability.

Strategy 9: Avoid Lazy Loading Above-the-Fold Elements

Lazy loading is useful, but if applied incorrectly, it may delay above-the-fold images, which causes CLS.

 Best Practice:

  • Apply lazy loading only to below-the-fold content.
  • Keep hero images preloaded.

Strategy 10: Optimize Third-Party Scripts

Third-party tools such as ads, social media embeds, and analytics can introduce CLS issues.

 Fixes:

  • Load them asynchronously.
  • Use a tag manager.
  • Provide placeholders for third-party embeds.

Tools to Measure CLS

There are several reliable tools that help in tracking and improving CLS:

  • Google PageSpeed Insights – Offers CLS metrics and optimization tips.
  • Google Lighthouse – Helps identify layout shifts.
  • Chrome User Experience Report (CrUX) – Collects real user data.
  • WebPageTest.org – Provides detailed performance insights.

Real-Life Case Studies

 Case Study 1: E-Commerce Website

An e-commerce store suffered from CLS because of dynamically loaded product images. After adding proper width/height attributes and preloading key assets, their CLS dropped from 0.35 to 0.08, improving conversions by 12%.

 Case Study 2: News Portal

A news site used multiple ad placements that caused layout jumps. By reserving space for ads and using placeholders, the CLS score improved from 0.27 to 0.09.

CLS Optimization Checklist (2025)

✅ Define image and video dimensions
✅ Preload fonts and hero images
✅ Use font-display: swap
✅ Reserve space for ads and banners
✅ Avoid injecting dynamic content late
✅ Optimize CSS and JavaScript
✅ Test with Google tools regularly

Future of CLS and Core Web Vitals

Google continues to refine Core Web Vitals, and CLS remains one of the top ranking factors. In 2025, websites with perfect visual stability will enjoy better rankings, higher engagement, and stronger conversions.

Conclusion

Cumulative Layout Shift is one of the most important metrics for building a smooth and stable user experience. By applying the strategies discussed in this complete guide, you can achieve a CLS score below 0.1, improve SEO, and build user trust.

Remember, performance optimization is not a one-time job. Continue testing,