What is Cumulative Layout Shift?

Cumulative Layout Shift (CLS) is a crucial metric for web developers and website owners to track, as it directly impacts user experience. CLS measures the extent to which visible content on a webpage shifts or moves around during its loading process. This shift can occur due to various reasons such as content reflow, content resizing, or unexpected changes in page layout.

  • Content Reflow: When web pages contain multiple elements, such as text and images, they may need to be rearranged as the page loads. This reflow of content can cause visible elements to shift or move, resulting in a poor user experience.
  • Content Resizing: As users interact with a webpage, content may dynamically resize or change its dimensions. This resizing can lead to layout shifts, making it essential for developers to account for these changes.

CLS is significant because it affects how users perceive and engage with a website. A high CLS score indicates that the webpage has a poor layout stability, leading to frustration and decreased user satisfaction. On the other hand, a low CLS score suggests that the webpage provides a smooth and stable layout experience, contributing to increased user engagement and conversion rates.

By understanding how CLS works and its impact on user experience, web developers can take proactive measures to optimize their webpage’s layout stability, ensuring a seamless and enjoyable browsing experience for users.

How Does CLS Measure Layout Shift?

Measuring Layout Shift

CLS measures layout shift by tracking the movement of content on a page over time. When a page loads, its content may not be immediately visible due to various reasons such as ad rendering, script execution, or image loading. During this period, the content is considered “hidden” from the user’s perspective.

  • Content Reflow: One type of layout shift occurs when the browser adjusts the layout of a page in response to changes in screen size or orientation. This can cause content to reflow and move around on the page.
  • Content Resizing: Another type of shift happens when an element is resized, either by changing its font size or image resolution. This can also cause content to move around on the page.
  • Unexpected Changes in Page Layout: The third type of layout shift occurs when the page’s layout changes unexpectedly due to factors such as ad rendering, script execution, or content updates. These changes can cause content to jump around on the page.

CLS calculates the total amount of time that elements have been shifted by more than 1 pixel during a user interaction (such as scrolling or clicking). This value is then normalized and displayed in seconds. A lower CLS score indicates better layout stability, while a higher score indicates worse layout stability.

Best Practices for Reducing CLS

Optimize Images for Better CLS

One of the most common causes of layout shifts is image lazy loading and responsive design. To reduce CLS, it’s essential to optimize your images by using techniques like image compression, resizing, and lazy loading.

Image Compression

Compression reduces the file size of your images, which in turn reduces the amount of data that needs to be loaded, thus minimizing the risk of layout shifts. You can use tools like ImageOptim or TinyPNG to compress your images without sacrificing quality.

Responsive Design

Responsive design ensures that your website adapts to different screen sizes and devices, reducing the likelihood of unexpected layout shifts. Use flexible grid systems and media queries to create a responsive design that works across various devices.

Lazy Loading

Lazy loading defers the loading of non-visible content until it comes into view, which can help reduce CLS. You can use libraries like Lazy Load or Lozad.js to lazy load your images.

CSS Techniques

CSS techniques like object-fit and background-size can also help minimize layout shifts. Use these properties to control the size and positioning of your images, ensuring a smooth user experience.

Here are some additional tips to optimize your images for CLS:

  • Avoid using large images that may cause content reflow or resizing
  • Use SVGs instead of raster images when possible
  • Optimize image formats like WebP and JPEG XR for better compression
  • Implement image placeholders or fallback images to avoid layout shifts during loading

CLS Tools and Resources

To measure and improve Cumulative Layout Shift (CLS), you can leverage various tools and resources provided by Google and third-party developers. One of the most popular tools for measuring CLS is Google’s Web Vitals tool, which provides a comprehensive report on your website’s performance, including CLS scores.

Using Web Vitals Tool

To use the Web Vitals tool:

  1. Sign in to your Google account.
  2. Go to the Web Vitals dashboard and select the URL you want to test.
  3. Run the test to generate a report that includes CLS metrics such as Shift Distance and Shift Ratio.

PageSpeed Insights

Another useful resource for measuring CLS is PageSpeed Insights, which provides detailed insights into your website’s performance and offers actionable recommendations for improvement.

  1. Enter your URL in the PageSpeed Insights tool.
  2. Run the test to generate a report that includes CLS metrics such as CLS Score and Optimization Opportunities.

Third-Party Tools

There are also several third-party tools available for measuring CLS, including:

  • Lighthouse: A popular auditing tool developed by Google that provides insights into your website’s performance, including CLS.
  • WPT (WebPageTest): A free online tool that allows you to test your website’s performance and measure CLS metrics.
  • CLS Meter: A simple tool that provides a CLS score based on your website’s layout shifts.

Tips for Using Tools

When using these tools, keep the following tips in mind:

  • Run tests multiple times to ensure accurate results.
  • Test different browsers and devices to account for varying user experiences.
  • Review optimization opportunities and implement changes to improve CLS scores.
  • Continuously monitor your website’s performance and adjust changes as needed.

By leveraging these tools and resources, you can effectively measure and improve CLS on your website, providing a better user experience for your visitors.

Conclusion: The Importance of CLS

In conclusion, understanding Cumulative Layout Shift (CLS) is crucial for website owners and developers to ensure a seamless user experience on their websites. By measuring and improving CLS, you can improve your website’s performance, accessibility, and overall user satisfaction.

As we’ve seen throughout this article, CLS is a complex issue that requires a comprehensive approach to solve. From understanding the causes of CLS to identifying and fixing issues with various tools and resources, it’s clear that optimizing for CLS is not a simple task. However, by prioritizing CLS optimization and staying up-to-date with the latest best practices and tools, you can significantly improve your website’s performance and user experience.

Here are some key takeaways to remember:

  • Measure and track CLS using Google’s Web Vitals tool and other third-party tools
  • Identify and fix CLS issues by analyzing your website’s layout and content
  • Optimize for mobile-first, as most users access websites on their mobile devices
  • Prioritize CLS optimization, alongside other web performance metrics like LCP, FID, and TTI

By following these best practices and staying committed to improving your website’s user experience, you can ensure a positive impact on your online presence.

In conclusion, CLS is a crucial metric for website owners and developers to measure and improve the responsiveness of their web pages. By understanding how CLS works and applying best practices to reduce layout shift, websites can provide better experiences for users, leading to increased engagement, conversion rates, and overall success.