What is Cumulative Layout Shift? Guide to a Smoother UX


May 2024
Cumulative Layout Shift

Cumulative Layout Shift (CLS) is a critical metric for web development and digital marketing, forming part of Google’s Core Web Vitals, which are key to enhancing user experience on the web. CLS measures the frequency of unexpected shifts in webpage content, which can frustrate users by causing accidental clicks, degrading user experience and potentially affecting search engine rankings. As websites increasingly adopt dynamic and visually engaging designs, the potential for such shifts grows. This guide delves into the definition, causes, measurement, diagnosis, and optimization of CLS, providing developers and website owners with comprehensive insights to create smoother, more stable user experiences. Improved handling of CLS can lead to better user engagement, higher conversion rates, and enhanced SEO performance, equipping both seasoned developers and business owners with essential knowledge to boost their site’s effectiveness. Understanding the impact of First Input Delay and Largest Contentful Paint is essential for tackling Cumulative Layout Shift effectively, ensuring smoother browsing experiences for your visitors.

What is Cumulative Layout Shift

Cumulative Layout Shift (CLS) measures the stability of a webpage by quantifying how much visible content shifts unexpectedly during the browsing experience. This metric is crucial as it directly affects user interactions, causing issues like accidental clicks when elements move unpredictably. Google includes CLS in its Core Web Vitals, emphasizing its importance for providing a good user experience and influencing SEO rankings.

Measurement of CLS

CLS is calculated by assessing all individual layout shift scores throughout the page’s lifespan. A layout shift occurs whenever a visible element changes its start position from one frame to the next without user initiation. The score for each shift is computed based on the impact fraction and distance fraction:

  • Impact Fraction: The portion of the viewport affected by unstable elements.
  • Distance Fraction: The distance these elements have moved, relative to the viewport size.

These metrics combine to give a score representing the severity of shifts during a page visit. The goal for websites is to maintain a CLS score of 0.1 or lower to ensure a stable and enjoyable user experience.

Causes and Effects of High CLS

Understanding the causes of Cumulative Layout Shift is key to minimizing its occurrence. Several factors can contribute to high CLS scores:

  • Images and Videos without Dimensions: When media elements load without defined width and height attributes, they can cause significant layout shifts as they populate the space unexpectedly.
  • Ads, Embeds, and Iframes: Dynamic content such as ads or embedded videos that load asynchronously can disrupt the layout if space isn’t reserved for them in advance.
  • Dynamically Injected Content: Content added to the page after initial load, like pop-ups or banners, often triggers layout shifts.
  • Fonts: The way web fonts load can cause text to shift. For instance, if the fallback font and the web font have different sizes, it will cause a shift when the web font loads.

Each of these factors can negatively impact user experience by causing unexpected movement of page content, leading to frustration and reduced engagement.

Impact on User Experience

High CLS can severely disrupt the user experience. Key impacts include:

  • Navigation Difficulty: Users may struggle to interact with buttons and links, which move unexpectedly.
  • Reading Disruptions: Text shifting while reading leads to a poor experience, causing users to lose their place on the page.
  • Conversion Rates: Unexpected shifts can lead to accidental clicks on unintended links or buttons, potentially decreasing conversion rates by frustrating users.

By addressing the causes of CLS, developers can enhance site usability and user satisfaction, which in turn supports SEO performance and overall site success. Cumulative layout shift can significantly contribute to increased bounce rates, emphasizing the importance of responsive design and smooth user interactions.

Tools for Measuring CLS

To effectively manage and minimize CLS, it’s crucial to accurately measure it. Several tools are available that help developers diagnose and understand CLS on their web pages:

  • Google Lighthouse: This automated tool, available in the Chrome browser, provides a comprehensive audit of a webpage’s performance, including CLS. Lighthouse measures CLS during both the lab testing phase and under simulated user conditions, giving insights into how the content might shift.
  • Chrome DevTools: Chrome DevTools offers a performance panel that tracks layout shifts and identifies the elements responsible for causing these shifts. This tool is invaluable for real-time debugging and understanding specific incidents of layout shift.
  • Web Vitals Chrome Extension: This extension provides real-time CLS scores as you browse, which can help in understanding how often and severely a page is experiencing shifts during everyday use.
  • Chrome UX Report (CrUX): CrUX is a public dataset of real user experiences on millions of websites, which includes CLS data. It’s useful for understanding how real users experience content shifts across different devices and connections.

Using these tools, developers can pinpoint the specific issues contributing to high CLS scores and prioritize fixes that will improve user experience.

Analyzing CLS Issues

Once the necessary data is collected, the next step is to analyze it to identify patterns or recurring issues that cause layout shifts:

  • Identify Elements Causing Shifts: Tools like Lighthouse and Chrome DevTools highlight which elements on the page contribute most significantly to CLS.
  • Assess Impact of Changes: By making adjustments and monitoring their effects in development tools, developers can see how changes affect the CLS score.
  • Monitor Changes Over Time: Ongoing monitoring is crucial as updates to web content and structure can introduce new CLS issues.

Diagnosing CLS not only helps in optimizing current pages but also aids in setting best practices for future design and development to preemptively minimize layout shifts.

Strategies to Optimize and Reduce Cumulative Layout Shift

Optimizing Cumulative Layout Shift (CLS) involves implementing strategies that prevent unexpected movement of page elements. Here we discuss several effective approaches to reduce CLS, ensuring a smoother experience for users. Cumulative layout shift can impact click-through rates by disrupting user engagement, highlighting the need for stable page layouts

Optimizing Images and Media

Proper handling of images and videos is crucial for minimizing CLS:

  • Specify Dimensions: Always include width and height attributes for images and video elements. This helps the browser allocate space before the media loads, preventing layout shifts.
  • Use Aspect Ratio Boxes: For responsive designs, using CSS aspect ratio boxes maintains the space for media elements as the viewport size changes, avoiding shifts.
  • Preload Important Assets: Preloading key media files can ensure they are loaded before the DOM is fully rendered, reducing the likelihood of shifts.

Handling Dynamic Content

Dynamic content such as ads or embeds needs careful management to prevent CLS:

  • Reserve Space for Dynamic Elements: Define explicit dimensions for ad slots and other dynamic content areas. This prevents content from pushing other elements around as it loads.
  • Avoid Inserting Above Existing Content: When dynamically adding content, do so in ways that do not disrupt the user’s current view, such as below the fold or in designated spaces.

Font Loading and Visibility

Fonts can cause layout shifts if not handled properly:

  • Use Font Display Options: CSS properties like font-display: swap can control how fonts are displayed as they load, minimizing visible shifts.
  • Match Font Metrics: Ensure fallback fonts have similar metrics to your primary web fonts to reduce shifts when fonts are swapped.

Implement Stability with CSS and JavaScript

Certain CSS and JavaScript practices can enhance stability:

  • CSS Containment: The contain property in CSS can limit the impact of an element’s layout on the rest of the page, reducing unexpected shifts.
  • Avoid Large DOM Changes: Minimize or batch DOM manipulations during user interactions to prevent major layout shifts during these times.

Continuous Improvement and Monitoring

Reducing CLS is an ongoing process:

  • Use Performance Monitoring Tools: Tools like Google Lighthouse and the Web Vitals Chrome Extension should be used regularly to monitor CLS.
  • Respond to Feedback: User feedback can often highlight issues not caught in testing. Regularly review and address user-reported issues to continuously improve stability.

By implementing these strategies, developers can significantly reduce CLS, leading to improved user satisfaction and potentially higher SEO rankings. The key is to integrate these practices into regular development workflows to maintain low CLS scores as a standard.

Ongoing Monitoring Tools

Ensuring consistent performance regarding Cumulative Layout Shift (CLS) requires ongoing monitoring and maintenance. This continuous vigilance helps in identifying new issues quickly and maintaining a high-quality user experience.

Effective monitoring of CLS is critical for understanding how real users experience the website over time:

  • Real User Monitoring (RUM): Tools that support RUM collect data directly from actual user sessions, providing insights into how content shifts affect users across different devices and network conditions.
  • Automated Performance Testing: Integrating CLS monitoring into continuous integration/continuous deployment (CI/CD) pipelines ensures that new code does not degrade performance.
  • Analytics and Reporting: Regularly reviewing performance metrics through analytics platforms can help track CLS trends and pinpoint periods or updates that caused degradation.

Maintaining Low CLS

Keeping CLS low is an ongoing effort that involves several best practices:

  • Regular Updates and Refinement: As browsers and technologies evolve, so do the strategies for handling layout shifts. Regularly update your practices to include the latest recommendations and solutions.
  • Feedback Loop: Encourage user feedback on usability issues, and use this data to refine user interfaces. What works well on one device or browser might not translate perfectly to another.
  • Education and Training: Keep development teams up-to-date with the best practices for designing and coding to prevent high CLS. This includes training on the importance of layout stability and how to achieve it.

By committing to these monitoring and maintenance practices, organizations can ensure that their websites remain fast, efficient, and user-friendly. This proactive approach not only improves user satisfaction but also supports better SEO rankings by adhering to Google’s Core Web Vitals standards.


Understanding and optimizing Cumulative Layout Shift (CLS) is crucial for enhancing user experience and boosting SEO performance. This guide has explored the nature of CLS, its impacts, and detailed strategies for diagnosis and optimization, highlighting the importance of integrating stable design practices to prevent unexpected content shifts. Effective monitoring and continual improvement are key to maintaining low CLS scores, ensuring websites remain user-friendly and perform well in search rankings. By staying vigilant and adapting to new best practices, developers and site owners can significantly improve their site’s stability and user satisfaction, fostering a positive web environment that benefits both users and businesses alike.