Cumulative Layout Shift: Why CLS Matters for Your Website?

In today’s fast-paced digital world, user experience is paramount to the success of any website. Among the various metrics that measure user experience, Cumulative Layout Shift (CLS) has emerged as a crucial Core Web Vital that directly impacts how users interact with and perceive your website. In this comprehensive guide, we’ll explore what CLS is, why it matters, and how you can optimize your website to achieve better scores.

What Is Cumulative Layout Shift?

Cumulative Layout Shift is a metric that measures visual stability by quantifying how much unexpected movement of visible page content occurs during the entire lifespan of a page. In simpler terms, it measures how much your page’s elements jump around as the page loads.

Imagine reading an article on your phone, and just as you’re about to click a link, an image loads above it, pushing the link down. You end up clicking an advertisement instead. Frustrating, right? This is exactly what CLS measures and aims to minimize.

Why CLS Matters

User Experience Impact

Poor CLS scores can significantly impact user experience in several ways:

• Reading Interruption: Users lose their place when text suddenly shifts

• Misclicks: Users accidentally click wrong elements

• Frustration: Constant layout shifts create a feeling of instability

• Time Waste: Users need to re-locate content they were trying to read SEO Implications

Since May 2021, Google has incorporated Core Web Vitals, including CLS, into its ranking factors. A poor CLS score can:

• Negatively affect your search rankings

• Reduce your site’s visibility

• Impact your overall SEO performance

How Is CLS Calculated?

CLS is calculated by multiplying two metrics:

• Impact Fraction: The amount of viewport area affected by the shift

• Distance Fraction: The distance elements have moved relative to the viewport The formula is: CLS = Impact Fraction × Distance Fraction

Google considers a CLS score of:

• Good: Less than 0.1

• Needs Improvement: Between 0.1 and 0.25

• Poor: Above 0.25

Common Causes of Poor CLS

1. Images Without Dimensions

When images lack width and height attributes, browsers can’t allocate the correct space before the image loads, leading to layout shifts.

2. Advertisements and Embeds

Dynamic content like ads and embedded videos often cause significant layout shifts as they load and resize.

3. Web Fonts

Custom fonts can cause text to reflow when they finally load, especially if they’re significantly different in size from the fallback font.

4. Dynamic Content

Content that loads after the initial page render, such as AJAX-loaded content or infinite scrolling implementations.

5. Animations and Transitions

Poorly implemented animations can cause layout shifts, especially if they affect the positioning of other elements.

Best Practices to Improve CLS

1. Always Set Dimensions for Media

<img src="image.jpg" width="800" height="600" alt="Description">

Always specify width and height attributes for images and videos. This allows browsers to allocate the correct space before the media loads.

2. Reserve Space for Ads

.ad-slot {
    min-height: 250px;
    width: 300px;
}

Use CSS to reserve space for ad containers before they load.

3. Optimize Web Fonts

@font-face {
    font-family: 'Your Font';
    font-display: swap;
}

Use font-display: swap and preload critical fonts to minimize font-related layout shifts.

4. Handle Dynamic Content Carefully

• Add new content below the viewport

• Use placeholder skeletons for loading content

• Implement fixed-size containers for dynamic content

5. Use Transform for Animations

/* Instead of */
.element {
    margin-top: 100px;
}

/* Use */
.element {
    transform: translateY(100px);
}

Use transform instead of properties that trigger layout changes.

Tools to Measure and Monitor CLS

• Google PageSpeed Insights

• Provides CLS scores and suggestions for improvement

• Shows real-world performance data

• Chrome DevTools

• Performance panel

• Experience section

• Layout Shift regions

• Lighthouse

• Automated auditing tool

• Detailed reports with improvement opportunities

• Web Vitals Extension

• Real-time CLS monitoring

• Easy-to-understand metrics

Advanced CLS Optimization Techniques

1. Implement Content Placeholders

<div class="placeholder">
    <div class="animated-background"></div>
</div>

Use skeleton screens to indicate loading content while maintaining layout stability.

2. Optimize Critical Rendering Path

• Minimize render-blocking resources

• Inline critical CSS

• Defer non-essential JavaScript

3. Implement Progressive Loading

Load content in a way that maintains visual stability:

• Load above-the-fold content first

• Gradually load remaining content

• Use intersection observer for lazy loading

Conclusion

Cumulative Layout Shift is more than just another web metric—it’s a crucial factor in creating a positive user experience and maintaining good search engine rankings. By understanding what causes poor CLS scores and implementing the best practices outlined in this guide, you can significantly improve your website’s visual stability.

Remember that optimizing CLS is an ongoing process. Regular monitoring and testing are essential to maintain good scores as your website evolves. Start implementing these improvements today, and your users will thank you with increased engagement and conversions.