Everything You Need to Know About Cumulative Layout Shift

Introduction

Cumulative Layout Shift (CLS) is an important measure of how stable a webpage looks while it’s loading. When things move around unexpectedly as the page loads, it can be annoying for users. A high CLS score means a lot of these unexpected movements, which can frustrate users and make them leave your site.

To provide a smooth experience for your visitors, it’s crucial to understand and improve CLS. By minimizing layout shifts, you ensure that users can engage with your content without any surprises.

Core Web Vitals, including CLS, are key indicators of a site’s quality. Google uses these metrics to assess user experience and determine search rankings. Optimizing for Core Web Vitals not only improves user satisfaction but also boosts your site’s visibility in search results.

Understanding Cumulative Layout Shift (CLS)

How CLS is Calculated During Page Load

Cumulative Layout Shift (CLS) measures visual stability by calculating unexpected layout shifts during the page load. Each time a visible element changes its position between rendered frames, a layout shift occurs. The CLS score is the sum of all individual layout shift scores for every unexpected shift that happens during the entire lifespan of the page.

Individual Layout Shift Score

This score is determined by multiplying two key metrics:

• Impact Fraction: Measures the area affected by the shift relative to the viewport.
• Distance Fraction: Measures how far the elements have moved relative to the viewport size.

The formula can be simplified as:

[ \text{Layout Shift Score} = \text{Impact Fraction} \times \text{Distance Fraction} ]

Importance of Measuring CLS Over a Five-Second Window

Capturing real user experiences accurately is essential. Google recommends measuring CLS over a five-second window, focusing on user-centric interactions. This timeframe excludes shifts within 500 milliseconds of user interaction, ensuring only unexpected shifts impacting user experience are recorded.

For example, if an image loads late and pushes down content already visible to the user, it contributes to a higher CLS score.

Understanding and improving your CLS score can significantly enhance your site’s visual stability, delivering a seamless browsing experience.

Common Causes of Cumulative Layout Shift

Several factors contribute to high CLS scores, impacting your website’s visual stability. Understanding these common causes of CLS helps you mitigate layout shifts effectively.

Images Without Defined Dimensions

Images that load without specified width and height attributes cause unexpected shifts in content. Browsers allocate space for images based on these attributes, preventing layout shifts as the image loads.

Example: An image tag should include width and height attributes: html

Late-Loaded Fonts

Fonts that load late can cause text to shift when the font file is finally available. This issue is often referred to as Flash of Invisible Text (FOIT) or Flash of Unstyled Text (FOUT).

Solution: Use the font-display: swap; property in your CSS. This ensures text remains visible while the font loads. css @font-face { font-family: ‘CustomFont’; src: url(‘customfont.woff2’) format(‘woff2’); font-display: swap; }

Ads, Embeds, and Iframes Without Reserved Space

Dynamic content like ads, embedded media, and iframes can also lead to layout shifts if they are inserted into the document without reserving adequate space.

Tip: Reserve space using CSS aspect-ratio or fixed dimensions. html

JavaScript-Injected Content

Content injected via JavaScript after the initial page load can unexpectedly alter the layout.

Best Practice: Preallocate space for such content or dynamically adjust layout without causing shifts.

Addressing these common causes of CLS by defining dimensions, optimizing font loading, reserving space for dynamic content, and managing JavaScript injections ensures a more stable and user-friendly website experience.

Measuring Cumulative Layout Shift Performance

Tools for Measuring CLS

To gauge your site’s Cumulative Layout Shift (CLS) performance, several tools are available:

• PageSpeed Insights: This tool provides insights into how well a page performs in terms of Core Web Vitals, including CLS. It offers both lab and field data, making it useful for diagnosing issues and verifying real-world performance.
• Lighthouse: Integrated into Chrome DevTools, Lighthouse allows you to run audits on web pages. It simulates different conditions to provide a detailed report on CLS and other performance metrics.

Using these tools, you can pinpoint specific layout shifts and understand their impact on the user experience.

Field Data vs. Lab Data

When measuring CLS, it’s crucial to distinguish between field data and lab data:

• Field Data: This is real user data collected from actual visits to your site. It reflects the true user experience by accounting for various devices, network conditions, and user behaviors. Field data is invaluable for understanding how users experience your site in the wild.
• Lab Data: Generated from controlled tests in a simulated environment, lab data helps identify potential issues by providing consistent results. While it offers a high degree of repeatability, it may not capture all real-world scenarios due to its controlled nature.

Both types of data have their own strengths:

• Field Data: Offers a comprehensive view of real user experiences.
• Lab Data: Provides a controlled environment to identify and troubleshoot specific issues.

Balancing insights from both field and lab data ensures a more rounded approach to optimizing CLS.

What Makes a Good CLS Score?

A good CLS score is one that is less than or equal to 0.1. This benchmark is based on current industry standards and is crucial for maintaining a visually stable user experience during page load.

Key Statistics:

• According to the Chrome User Experience Report (CrUX) data, as of April 2023, 72.8% of websites achieve a good CLS score.
• This statistic highlights the improvements many sites have made in reducing layout shifts, ensuring a smoother user experience across the board.
• Maintaining a good CLS score helps improve user satisfaction and boosts your site’s overall performance metrics.

Strategies for Improving Cumulative Layout Shift

Understanding how to improve CLS is essential for delivering a smooth user experience. Here are some effective strategies:

Reserving Space for Media Elements

To prevent unexpected layout shifts during page load:

• Define Dimensions: Always set explicit width and height attributes for media elements like images, videos, and iframes. This practice reserves the necessary space in the layout, ensuring that content doesn’t shift as these elements load.
• Aspect-Ratio Property: Utilize the CSS aspect-ratio property to maintain a consistent aspect ratio for responsive images. This helps in reserving dynamic space without causing layout shifts.
• Placeholders: Use low-quality image placeholders (LQIP) or skeleton screens until the full image loads. This keeps the layout stable while improving perceived performance.

Optimizing Fonts

Flash of Invisible Text (FOIT) or Flash of Unstyled Text (FOUT) can cause significant layout shifts. Techniques to avoid these issues include:

• font-display: swap: Implement the font-display: swap property in your CSS. This ensures that text remains visible while web fonts are loading, reducing the chances of layout shifts.
• System Fonts: Consider using system fonts as a fallback. System fonts load faster and can be styled to closely match custom web fonts, minimizing visual disruption.
• Preload Fonts: Use <link rel=”preload”> in your HTML to preload critical font files. This approach speeds up font loading times, reducing the likelihood of noticeable shifts.

By implementing these best practices, you can significantly improve your website’s CLS score and create a more stable and enjoyable experience for users.

Advanced Techniques for Optimization

Using CSS Transform Animations

Using CSS transform animations can significantly reduce layout shifts during page load. Properties like transform and opacity are preferred because they do not trigger reflows or repaints. This results in smoother animations and a more stable layout.

Example:

css .element { transition: transform 0.3s ease-in-out; }

.element:hover { transform: scale(1.1); }

By using transforms, you ensure that the elements remain in their original positions, preventing unexpected shifts.

Making Sure Pages Can Use bfcache

Making sure your pages can use bfcache (back-forward cache) is crucial for improving loading stability and reducing layout shifts when users go back to previously visited pages. This involves several practices:

• Avoiding the use of the unload event since it can disqualify pages from being cached.
• Minimizing Cache-Control: no-store headers, which prevent caching.
• Updating stale or sensitive data post-cache restore to ensure data integrity.
• Avoiding references to window.opener, which can create security vulnerabilities.
• Closing open connections before the user navigates away.

These steps help maintain a consistent user experience by leveraging the benefits of bfcache, ensuring that previous layouts are retained without causing additional shifts.

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Implementing these advanced techniques helps in creating a more visually stable website, improving both performance and user experience.

Conclusion

Continual optimization is essential for delivering a high-quality user experience. By effectively managing Cumulative Layout Shift (CLS), you can ensure that your website remains visually stable and user-friendly. To optimize your website for better Core Web Vitals, it’s important to reserve space for images and media, optimize fonts to prevent issues like Flash of Invisible or Unstyled Text, and employ advanced techniques such as CSS transform animations.

Additionally, ensuring that your pages are bfcache eligible can significantly enhance performance. By prioritizing these strategies, you’ll not only boost your site’s performance but also improve overall user satisfaction. Remember, optimizing your website is an ongoing process—staying updated with best practices and continually refining your approach will help you maintain a competitive edge.