CLS: Enhancing User Satisfaction and Conversion Rates on Your Website

In today's digital landscape, web performance and user experience play a crucial role in the success of any website or online business. Users expect fast-loading pages that deliver a seamless browsing experience. One of the key metrics that directly impacts web performance and page speed is Cumulative Layout Shift (CLS).

Cumulative Layout Shift (CLS) measures the visual stability of a web page during the loading process. It quantifies the extent of unexpected layout shifts that occur as content loads, which can be highly frustrating for users. These sudden shifts can cause buttons to move, text to resize, or images to jump around, leading to a disruptive and confusing user experience.

Cumulative Layout Shift (CLS): Improving Web Performance and Page Speed

The goal of this article is to provide a comprehensive guide to Cumulative Layout Shift (CLS), explaining its impact on web performance and page speed. We will delve into detection methods, utilizing tools like Lighthouse, and highlight the importance of addressing CLS issues. Furthermore, we will explore effective strategies to fix Cumulative Layout Shift and optimize websites for a smoother user experience.

By the end of this article, you will have a solid understanding of CLS, its implications for user engagement, conversion rates, and search engine rankings. You will also gain practical insights on how to detect CLS using tools like Lighthouse and, most importantly, learn actionable techniques to fix and prevent Cumulative Layout Shift issues. Let's dive in and improve your web performance!

Understanding Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) is a metric that measures the visual stability of a web page during the loading process. It quantifies the extent of unexpected layout shifts that occur as content loads, resulting in elements moving around on the page.

The significance of CLS cannot be overstated when it comes to user experience. When users visit a website, they expect a smooth and predictable browsing experience. However, when layout shifts occur unexpectedly, it can be highly disruptive and frustrating. Users may accidentally click on the wrong element, lose track of their position on the page, or have difficulty reading content.

Not only does CLS negatively impact user experience, but it also has implications for engagement, conversion rates, and even SEO rankings. Users who encounter frequent layout shifts are more likely to abandon the page, resulting in higher bounce rates and lower session durations. This, in turn, can lead to decreased conversions and revenue.

Search engines, including Google, have recognized the importance of user experience in determining search rankings. With the introduction of Core Web Vitals, CLS has become one of the key metrics considered for evaluating a website's performance. Websites with a high CLS score may experience a drop in search rankings, as search engines prioritize websites that deliver a stable and seamless browsing experience.

Several components contribute to Cumulative Layout Shift. Images without specified dimensions, ads that load after the main content, dynamically injected content, and embeds (such as iframes) are common culprits of layout shifts. As these elements load, they can push down or shift existing content, resulting in an unstable visual experience for users.

Understanding the factors that contribute to CLS is crucial for effectively diagnosing and fixing layout shift issues. In the next section, we will explore how to detect CLS and utilize tools like Lighthouse to assess its impact on web performance.

Common Examples of CLS

Cumulative Layout Shift (CLS) can manifest in various ways, causing frustration and confusion for website visitors. Let's explore some common examples of CLS that many of us have encountered during our browsing experiences:

  • Images without specified dimensions: Have you ever loaded a webpage where images suddenly appear and push down the content you were reading? This is a typical example of CLS caused by images without specified dimensions. As the images load, they adjust the layout, resulting in a jarring shift of the surrounding elements.

<img src="image.jpg" alt="An image without specified dimensions">

To recreate the CLS caused by images without specified dimensions, you can use the above code snippet. Make sure to replace "image.jpg" with the actual image source.

  • Ad loading delays: Advertisements are a common source of CLS issues. Imagine scrolling through an article when suddenly an ad loads, pushing the content down and forcing you to lose your reading position. These unexpected shifts can disrupt your flow and lead to frustration.

This is probably the biggest offender because ad scripts inject ads dynamically and they are often very slow. It is not uncommon for 3rd party ad vendors to inject several hundred or even a thousand JavaScript requests into a web page before injecting the actual ad on the page. Not only is this slow, if you do not have the layout prepped for the final ad it can cause the layout to shift, often 'violently'.

<div class="content">
  <p>Content that is being read...</p>
  <script src="ad.js"></script>

To simulate the CLS caused by ad loading delays, include the above code snippet within your webpage. Ensure that "ad.js" references a script that introduces a delay before loading the ad content.

  • Dynamically injected content: Websites that dynamically inject content, such as user-generated comments or social media feeds, can experience CLS problems. As these elements load, they often push down existing content, causing layout shifts and making it difficult for users to maintain their focus. Again, this is often what ad scripts do.

<div class="content">
  <p>Content that is being read...</p>
  <div id="comments"></div>
  setTimeout(function() {
    document.getElementById("comments").innerHTML = "User-generated comments";
  }, 3000);

This code simulates the CLS caused by dynamically injected content. It delays the injection of user-generated comments within the "comments" div element after 3 seconds. Adjust the delay time and the injected content to suit your needs.

  • Delayed loading of web fonts: Web fonts are used to enhance the visual appearance of a website. However, if the web fonts are not properly optimized or there are delays in loading them, the text on the page can shift once the fonts are applied. This sudden shift can be disorienting and make it challenging to read the content.

<link rel="stylesheet" href="">
  body {
    font-family: 'Open Sans', sans-serif;

To replicate the CLS caused by delayed loading of web fonts, use the above code snippet. It loads the 'Open Sans' font from Google Fonts, and the body text is styled to use this font. You can observe a shift in text once the font is applied due to the font loading delay.

  • Embeds and iframes: Embedding content from external sources, such as videos or social media posts, can introduce CLS issues. When these embeds load, they can cause significant shifts in the layout of the surrounding content, leading to a less stable and visually pleasing experience.

<div class="content">
  <p>Content that is being read...</p>
  <iframe src=""></iframe>

Replace "VIDEO_ID" with the actual YouTube video ID. Embedding a YouTube video within an iframe can introduce CLS when the iframe loads, causing shifts in the surrounding content.

Remember to insert these code snippets appropriately within your webpage to recreate the CLS scenarios. This will allow you to observe the layout shifts and analyze the impact on web performance using tools like Lighthouse.

By highlighting these common examples of CLS, we can relate to the frustrations and annoyances caused by unexpected layout shifts. Understanding these pain points helps us recognize the importance of addressing CLS issues and striving for a more stable and user-friendly browsing experience. In the next sections, we will explore how to detect CLS using tools like Lighthouse and provide effective strategies to fix and prevent Cumulative Layout Shift.

The Effects of CLS on Web Performance and Page Speed

Cumulative Layout Shift (CLS) has profound effects on web performance and page speed, directly impacting user experience and various performance metrics. Let's explore the significant effects of CLS:

  1. User Frustration, Particularly for Mobile Users

CLS can be highly frustrating for users, especially those browsing on mobile devices with smaller screens. When layout shifts occur unexpectedly, users may accidentally tap on incorrect elements, lose their reading position, or experience a general sense of disorientation. These frustrating experiences can lead to increased user dissatisfaction and negatively impact overall engagement.

  2. Impact on Bounce Rates, Session Duration, and Conversion Rates

Layout shifts caused by CLS have a tangible impact on website performance metrics. High CLS scores are associated with higher bounce rates, as users may quickly abandon pages that exhibit constant layout disruptions. Additionally, CLS can reduce session durations, as users become more likely to leave a site prematurely due to poor visual stability. Furthermore, CLS issues can hinder conversion rates, as users may feel hesitant to interact or make purchases on websites that exhibit unstable layouts.

  3. Connection to Core Web Vitals and Search Engine Rankings

CLS is one of the Core Web Vitals, a set of performance metrics identified by Google as crucial for delivering a great user experience. Google has indicated that Core Web Vitals, including CLS, play a role in search engine rankings. Websites that prioritize optimizing their CLS scores may gain a competitive advantage in search rankings, as Google aims to promote websites that provide stable and user-friendly experiences.

Improving CLS is not only vital for enhancing user experience but also for maintaining a strong online presence and visibility. By minimizing layout shifts and prioritizing a visually stable website, you can increase user satisfaction, reduce bounce rates, enhance session duration, boost conversion rates, and potentially improve your search engine rankings.

In the next section, we will explore how to detect CLS using tools like Lighthouse and delve into the practical steps to address and fix Cumulative Layout Shift issues effectively.

Detecting CLS and Using Tools like Lighthouse

To effectively detect Cumulative Layout Shift (CLS) and measure its impact on web performance, we can utilize various tools. One of the most popular and powerful tools for performance auditing is Lighthouse. Let's explore how to use Lighthouse to detect and analyze CLS:

  1. Install Lighthouse: If you are using Google Chrome or Microsoft Edge, Lighthouse is built into the browser's DevTools. Simply open the DevTools by right-clicking on the webpage and selecting "Inspect" or by pressing Ctrl + Shift + I (Windows), Cmd + Option + I (Mac) or my preference F12. Then, navigate to the "Lighthouse" tab.

  2. Select Audits: In the Lighthouse tab, ensure that the "Performance" checkbox is selected. You can also choose additional audits based on your specific needs.

  3. Run the Audit: Click on the "Generate report" button to start the performance audit. Lighthouse will simulate a page load and gather various performance metrics, including CLS.

  4. Interpreting the Results: Once the audit is complete, Lighthouse will present a detailed report. Look for the CLS score, which represents the severity of layout shifts on your webpage. The score ranges from 0 to 1, with lower scores indicating better visual stability. Additionally, Lighthouse provides a breakdown of individual CLS events and their impact on user experience.

devtools lighthouse performance test

My advice is to run this test in an Icognito browser. This will remove any extensions, like Ad Blockers that will not give you a pure report. Extensions themselves can cause all sorts of performance issues. Since you cannot control what extensions a user has, you need to test in a clean environment to isolate just your web page.

lighthouse performance header

Alternative tools and browser extensions can also help in identifying and measuring CLS:

  • WebPageTest: WebPageTest is a free online tool that provides detailed performance testing. It offers CLS analysis, along with various other metrics, and allows you to test your webpage from different locations and devices.

  • PageSpeed Insights: PageSpeed Insights, provided by Google, analyzes the performance of your webpage and provides recommendations for improvement. It includes CLS as one of the key metrics and offers insights into areas that require attention.

  • Browser Extensions: Some browser extensions, such as the "CLS Detector" for Google Chrome, can visually highlight layout shifts on a webpage. This can be useful for manually identifying and understanding the sources of CLS.

By utilizing tools like Lighthouse and other performance analysis tools, you can gain valuable insights into the CLS performance of your website. These tools provide data-driven feedback and enable you to understand the impact of CLS on your users' experience. In the next section, we will delve into the importance of addressing CLS and the benefits it brings to web performance and user satisfaction.

The Importance of Fixing CLS

Addressing Cumulative Layout Shift (CLS) is crucial for website owners and developers for several reasons:

  1. Enhanced User Experience: Fixing CLS contributes to a more stable and visually pleasing user experience. Users appreciate websites that provide consistency and predictability during page loading. By minimizing layout shifts, you can improve user satisfaction, engagement, and overall browsing experience.

  2. Improved Engagement and Conversion Rates: A visually stable website reduces user frustration and encourages prolonged engagement. When users can smoothly interact with your content without distractions caused by layout shifts, they are more likely to stay on your website, explore further, and convert. This can lead to increased conversion rates, higher revenue, and improved business outcomes.

  3. Positive Impact on SEO and Rankings: Search engines, including Google, consider user experience as a critical factor in determining search rankings. With the introduction of Core Web Vitals, which includes CLS, Google has explicitly emphasized the importance of optimizing for a stable browsing experience. Websites that prioritize addressing CLS and other Core Web Vitals metrics may benefit from improved search engine visibility and rankings.

By actively addressing CLS issues, you not only enhance user experience and engagement but also align with search engine optimization best practices. Prioritizing a visually stable website contributes to positive user perceptions, higher conversion rates, and potential improvements in search engine rankings. In the next section, we will explore effective strategies to fix Cumulative Layout Shift and optimize your website's performance.

Effective Strategies to Fix Cumulative Layout Shift

To effectively fix Cumulative Layout Shift (CLS) issues and improve the visual stability of your website, consider implementing the following strategies:

  1. Proper Image and Video Dimensions: Specify the dimensions (width and height) of images and videos in your HTML or CSS. This allows the browser to reserve the necessary space for the content before it loads, preventing unexpected layout shifts. Additionally, consider using placeholder elements with the same aspect ratio as the final image or video to ensure a smoother loading experience.

  2. Preloading and Lazy Loading: Preload critical resources, such as CSS, JavaScript, and fonts, to ensure they are available when needed. This minimizes delays in rendering and reduces the likelihood of CLS caused by delayed loading. For non-critical or below-the-fold content, implement lazy loading techniques to defer the loading until the content is about to become visible, thereby reducing unnecessary layout shifts.

  3. CSS and JavaScript Optimization: Optimize your CSS and JavaScript to reduce render-blocking and improve loading times. Minify and compress your CSS and JavaScript files, eliminate unused code, and leverage techniques like code splitting and caching to enhance performance. By optimizing these resources, you can minimize the potential for layout shifts during page loading.

  4. Best Practices for Ads, Embeds, and Iframes: Ensure that ads, embeds (such as social media widgets), and iframes are properly implemented to minimize their impact on layout stability. Use appropriate sizes and aspect ratios for ads and ensure they load efficiently. For embeds and iframes, specify their dimensions and leverage techniques like sandboxing and asynchronous loading to prevent CLS.

  5. Monitoring and Testing: Continuously monitor and test your website's CLS performance using tools like Lighthouse or WebPageTest. Regularly analyze the results and address any issues that arise. Additionally, consider user testing and gather feedback to identify and resolve CLS problems that may go unnoticed through automated testing alone.

By implementing these strategies, you can effectively mitigate Cumulative Layout Shift issues and provide a more stable browsing experience for your users. Remember to regularly assess and optimize your website to ensure long-term improvements in CLS and overall web performance. In the next section, we will explore case studies and real-world examples of websites that successfully addressed CLS issues and witnessed significant improvements.

Case Studies and Real-World Examples

Let's explore some real-world examples of websites that successfully resolved Cumulative Layout Shift (CLS) issues and achieved improved performance:

  1. Example 1: E-commerce Website

    • Before: The e-commerce website experienced frequent layout shifts, particularly when images loaded. This resulted in a higher CLS score, increased bounce rates, and lower conversion rates.
    • Solution: The development team implemented proper image dimensions, ensuring that the layout remained stable even as the images loaded. They also utilized placeholder elements to reserve space for images. As a result, the CLS score decreased significantly, leading to improved engagement and a boost in conversion rates.

  .image-container {
    position: relative;
    width: 100%;
    height: 0;
    padding-bottom: 75%; /* Default aspect ratio */

    /* Adjust height and width at different breakpoints */
    @media (min-width: 576px) {
      width: 200px;
      height: 200px; /* Adjust dimensions for phones */

    @media (min-width: 768px) {
      width: 310px;
      height: 310px; /* Adjust dimensions for small tablets */

    @media (min-width: 992px) {
      width: 500px;
      height: 400px; /* Adjust dimensions for tablets */

    @media (min-width: 1200px) {
      width: 610px;
      height: 500px; /* Adjust dimensions for desktop */

  .image-container img {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    object-fit: cover;

<div class="image-container">
    alt="Description of the image"
      image-phone.jpg 200w,
      image-tablet-small.jpg 310w,
      image-tablet.jpg 500w,
      image-desktop.jpg 610w
      (max-width: 575px) 200px,
      (min-width: 576px) and (max-width: 767px) 310px,
      (min-width: 768px) and (max-width: 991px) 500px,

The CSS media queries are used to adjust the dimensions of the image container at different breakpoints. The width and height properties are specified based on the desired dimensions for each device category.

The srcset attribute is used to define different image sources along with their corresponding widths. Adjust the image sources (image-phone.jpg, image-tablet-small.jpg, image-tablet.jpg, image-desktop.jpg) and their widths (200w, 310w, 500w, 610w) according to your specific image files.

The sizes attribute specifies the image sizes to be used based on the available viewport width. The sizes are defined using media queries, ensuring that the appropriate image size is selected for each breakpoint. Adjust the dimensions (200px, 310px, 500px, 610px) and the corresponding media queries to match your desired dimensions for each device category.

By utilizing CSS media queries along with the srcset and sizes attributes, you can ensure that the image container is appropriately sized for different devices, providing a pre-sized placeholder for the image based on the specific breakpoints.

By using the combination of loading="lazy" and srcset, you can achieve lazy loading of images while also providing responsive image support for different device pixel densities.

Key Takeaway: Properly specifying image dimensions and using placeholders can effectively reduce CLS and enhance user experience in e-commerce websites.

  1. Example 2: News Website

    • Before: A news website experienced significant layout shifts due to dynamically injected content, such as user comments. This resulted in a high CLS score and user frustration.
    • Solution: The website implemented a loading indicator to notify users that comments were being loaded. They also allocated space for the comment section before the content appeared, preventing sudden shifts. These changes reduced the CLS score, resulting in longer session durations and increased user engagement.

    Key Takeaway: Providing loading indicators and reserving space for dynamically injected content can minimize layout shifts and improve user satisfaction in news websites.

  2. Example 3: Blogging Platform

    • Before: A blogging platform struggled with CLS issues caused by ads loading after the main content. The resulting layout shifts led to a poor user experience and increased bounce rates.
    • Solution: The platform optimized the ad loading process, ensuring that the ad space was reserved beforehand, and implemented lazy loading for ads. They also optimized their CSS and JavaScript to reduce render-blocking. These optimizations significantly reduced CLS, resulting in improved engagement and decreased bounce rates.

    Key Takeaway: Properly managing ad loading, leveraging lazy loading, and optimizing CSS and JavaScript can effectively address CLS issues in blogging platforms.

These case studies illustrate the positive impact of resolving CLS issues on web performance and user experience. By implementing targeted solutions tailored to specific CLS causes, websites were able to reduce layout shifts, improve engagement metrics, and create a more satisfying browsing experience. Consider these examples as valuable insights and adapt the strategies to your own website. In the final section, we will conclude with a summary and encourage you to prioritize CLS optimization for long-term benefits.


Throughout this comprehensive guide, we have explored Cumulative Layout Shift (CLS) and its impact on web performance and user experience. Let's recap the key points discussed:

  • CLS is a critical metric that measures the visual stability of a webpage during the loading process.
  • It directly affects user experience, leading to frustration, increased bounce rates, and lower conversion rates.
  • CLS is an important component of Core Web Vitals, which influences search engine rankings.
  • Detecting CLS can be done using tools like Lighthouse, which provides insights into CLS scores and individual layout shift events.
  • Fixing CLS is crucial for website owners and developers to enhance user experience, engagement, and conversion rates.
  • Strategies to address CLS include proper image and video dimensions, preloading and lazy loading techniques, CSS and JavaScript optimization, and careful handling of ads, embeds, and iframes.
  • Real-world case studies have shown the positive impact of CLS fixes on web performance and user satisfaction.

It is essential to prioritize CLS optimization for the long-term benefits it brings. By implementing the strategies discussed in this guide, you can significantly improve your website's stability, user experience, and search engine visibility. Monitor your CLS improvements and regularly assess your website's performance to ensure ongoing optimization.

Remember, by proactively addressing CLS and delivering a visually stable browsing experience, you can gain a competitive advantage, retain users, and increase conversions. Don't underestimate the power of a seamless user experience. Start optimizing your website for CLS today and enjoy the rewards it brings.

Happy optimizing!

Q & A

Q: How can I detect and measure Cumulative Layout Shift (CLS) on my website?

A: One of the most popular tools for detecting and measuring CLS is Lighthouse, which is built into Google Chrome's DevTools. Simply run a performance audit using Lighthouse to gather CLS scores and detailed information about layout shifts. Additionally, tools like WebPageTest and PageSpeed Insights can also provide insights into CLS performance.

Q: What are some effective strategies to fix CLS issues on my website?

A: Effective strategies to fix CLS include ensuring proper image and video dimensions, implementing preloading and lazy loading techniques, optimizing CSS and JavaScript, and properly managing ads, embeds, and iframes. By implementing these strategies, you can minimize layout shifts and improve visual stability.

Q: How do third-party ads impact CLS and what can I do to mitigate their effects?

A: Third-party ads can impact CLS by causing loading delays, injecting dynamic content, having incorrect dimensions, or containing heavy content. To mitigate their effects, consider implementing lazy loading for ads, reserving space for ad content, ensuring accurate dimensions, and optimizing ad content for performance in collaboration with ad providers.

Q: How does addressing CLS impact SEO and search engine rankings?

A: Addressing CLS is important for SEO as search engines, like Google, consider user experience metrics, including CLS, when determining search rankings. Websites that prioritize optimizing CLS and other Core Web Vitals metrics may benefit from improved search engine visibility and rankings.

Q: What are the long-term benefits of optimizing CLS on my website?

A: Optimizing CLS brings several long-term benefits, including enhanced user experience, increased engagement, improved conversion rates, and potential competitive advantage. By providing a visually stable browsing experience, you can foster positive user perceptions, retain users, and potentially achieve better search engine rankings.

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