Cumulative Layout Shift: How to Improve Page Stability for a Better User Experience

Cumulative Layout Shift (CLS) is a performance metric that matters both to us as developers and to our users, especially as it plays a significant role in user experience. We know that when a website’s content moves around unexpectedly while it’s loading, it can be frustrating and even result in accidental clicks. Since Google included CLS in its Core Web Vitals—a set of metrics related to speed, responsiveness, and visual stability—it’s become crucial for us to optimize for a stable page experience.

Improving CLS is not always straightforward, given the dynamic nature of many modern web pages. However, by understanding what causes layout shifts and the factors that impact CLS, we can implement strategies to keep our pages visually stable. This involves identifying and measuring the elements that contribute to layout shifts, reserving space for dynamic content, and carefully managing resources that load asynchronously.

Key Takeaways

  • We recognize the importance of a stable and predictable page to provide a superior user experience.
  • Our strategy involves diagnosing and mitigating factors that contribute to layout shifts.
  • We embrace ongoing measurement to ensure adherence to Core Web Vitals, with an emphasis on CLS.

Understanding CLS and Core Web Vitals

YouTube video

In our exploration of website performance, it’s crucial to grasp Cumulative Layout Shift (CLS) and Core Web Vitals as they play pivotal roles in the overall user experience.

What Is Cumulative Layout Shift?

Cumulative Layout Shift, or CLS, is a performance metric that quantifies how often users experience unexpected movement of page content. Essentially, it measures the sum of all individual layout shift scores for every unexpected layout shift that occurs during the lifespan of a page. A layout shift happens whenever a visible element changes its position from one rendered frame to the next. For us to maintain a high-quality user experience, minimizing CLS is paramount.

Importance of Core Web Vitals

Core Web Vitals are a set of specific factors that Google considers important in a webpage’s overall user experience. These vitals include metrics like Largest Contentful Paint (LCP), which measures loading performance; First Input Delay (FID), which quantifies interactivity; and CLS, illuminating visual stability issues. To maximize user satisfaction and engagement, focusing on these Core Web Vitals is essential. They are not just indicators of a website’s usability but are also intertwined with its performance on search engine results pages.

Measuring and Assessing CLS

YouTube video

Cumulative Layout Shift (CLS) is a vital metric for understanding the visual stability of a web page. We’re focusing on the specific methods to calculate the CLS score and the tools available for its measurement to ensure a superior user experience.

CLS Score Calculation

CLS measures the sum of all individual layout shift scores for every unexpected layout shift that occurs during the entire lifespan of the page. A shift occurs whenever a visible element changes its position from one rendered frame to the next. To calculate the CLS score, we need to multiply the impact fraction (the viewport area impacted by the shift) by the distance fraction (the distance the elements have moved, relative to the viewport). This score reflects the severity of shifts on the page—the lower the CLS score, the better:

  • Impact Fraction (IF): The portion of the viewport affected by a shift.
  • Distance Fraction (DF): The distance moved by the shifted element, as a fraction of the viewport.
Layout ShiftImpact FractionDistance FractionCLS Score Calculation
Shift * 0.25 = 0.025
Shift * 0.5 = 0.1
Total CLS0.025 + 0.1 = 0.125

Each shift is calculated and added to the total CLS score of the page.

Tools for CLS Measurement

Several tools can help us measure CLS effectively. Lighthouse, an open-source, automated tool for improving the quality of web pages, provides a comprehensive report that includes the CLS score. PageSpeed Insights is another powerful tool that assesses the content of a web page, then generates suggestions to make that page faster, which can include improving the CLS.

  • Lighthouse: Integrated into many developer tools and can be run on any web page, public or requiring authentication.
  • PageSpeed Insights: Uses data from the Chrome User Experience Report and provides both lab and field data about a page.

By using these tools during and after page load, we can pinpoint layout shifts, identify their causes, and implement the necessary changes to reduce the CLS score, leading to an enhanced user experience.

Factors That Influence CLS

YouTube video

To effectively mitigate Cumulative Layout Shift (CLS), we must understand the elements that commonly contribute to layout instability. We’ll address how images and media, fonts and text, and ads and embeds can significantly influence CLS and strategies for minimizing their impact.

Images and Media

Images without explicit width and height can cause layout shifts when they load. To prevent this:

  • We always include width and height attributes in our image tags.
  • We utilize CSS aspect ratio boxes to hold space for media until it fully loads.

For videos and other media elements, the same principles apply; we set fixed dimensions to ensure stability.

Fonts and Text

The way fonts load can affect layout shifts. We adhere to these practices:

  • We use font-face display:swap to control how fonts render as they load.
  • We pre-load key fonts to prevent visible text shifting.

By doing so, we ensure text remains stable, preserving our CLS score.

Ads and Embeds

Ads and embeds are dynamic content that can disrupt a page’s layout unexpectedly. Here’s our approach:

  • We define reserved space for ads.
  • We ensure embeds have explicit dimensions.

By implementing these measures, we minimize the risk of sudden layout changes that could result in a poor CLS score. We commit to these strategies to enhance user experience by improving the visual stability of our web pages.

Optimization Strategies for CLS

YouTube video

To effectively minimize Cumulative Layout Shift (CLS), we focus on enhancing content stability and implementing resize and aspect ratio techniques. These strategies directly address the causes of layout shifts, ensuring a more stable and predictably performing website.

Improving Content Stability

We can significantly reduce CLS by ensuring that elements on our page are stable as they load. To do this, we predefine the size attributes for images and media. This involves setting explicit width and height attributes, which prevent unexpected reflow from occurring. It’s crucial that we also apply this practice to advertisements, embeds, and iframes.

For example, for an image that should display at 300×200 pixels, we would include this in our HTML:

<img src="path-to-image.jpg" width="300" height="200" alt="descriptive text">

This code ensures the space required for the image is reserved during loading, thus preventing layout shifts.

Resize and Aspect Ratio Techniques

By meticulously defining the aspect ratio for media elements, we prevent layout shifts caused by dynamically loaded content. An efficient technique we employ is utilizing the CSS property aspect-ratio to maintain the size consistency of items without explicitly declaring both height and width.

Utilizing modern CSS framework features, we might write:

.item {
  aspect-ratio: 16 / 9;

Here, a video container maintains a 16:9 aspect ratio, reserving the required space while the video loads. This approach ensures that the browser allocates the appropriate space for content, aiding in overall layout stability.

Moreover, optimizing for CLS involves a comprehensive analysis and testing strategy. By regularly reviewing our pages and monitoring their stability, we continue to enhance user experience and performance. With these specific strategies, we confidently work towards a website that offers a seamless and enjoyable experience for all users.

Mitigating Layout Shifts

Effective management of cumulative layout shift (CLS) ensures a stable and user-friendly experience. We can significantly enhance website performance by focusing on how elements load and interact on a page.

Handling Dynamic Content

When we introduce dynamic content to our pages, such as ads or other loading elements, it’s vital to manage them carefully to prevent layout shifts. We can achieve this by:

  • Pre-setting dimensions: Providing width and height attributes for any dynamic content ensures the browser allocates the correct amount of space even when the content hasn’t loaded yet.
  • Stable loading sequences: Prioritize the loading of above-the-fold or critical path content to stabilize the visible portion of the page as quickly as possible.

Reserving Space for Elements

It’s important for us to reserve space for page elements before they load. This practice eliminates unexpected layout shifts. Here’s how we can do that:

  • Placeholder graphics: Use lightweight placeholders or skeleton screens for media, like images or videos, which define their space on a page before fully loading.
  • Font loading strategies: Implement font swap or fallback strategies to ensure text remains stable during font loading.

By adhering to these practices, we can maintain a seamless content flow, enhance user satisfaction, and improve our website’s CLS performance.

Technological Considerations

In addressing cumulative layout shift (CLS), it’s vital we focus on the technical aspects of website development, specifically in the realm of CSS and JavaScript usage. We’ll explore practices and technologies that can substantially minimize CLS, improving user experience.

CSS and JavaScript Best Practices

When we write and implement CSS and JavaScript, the approach can greatly affect CLS. We recommend asynchronous loading of JavaScript to prevent blocking of page rendering, allowing the page to be interactive faster. Avoiding large shifts requires ensuring that elements on the page have a defined size and space before JavaScript or CSS loads. This can be achieved by explicitly setting width and height attributes for images and other media. Effective CSS practices include:

  • Using transform for animations instead of properties that cause re-layouts.
  • Implementing font-display: swap in our CSS to reduce layout shift due to font loading.

Utilizing the Latest Web Technologies

Leveraging modern web technologies is key to optimizing layout stability. We utilize the content-visibility property, which enables the browser to skip rendering of off-screen content until needed, reducing load time and preventing layout shift. Additionally, by using WebP or AVIF image formats, we reduce file sizes significantly, ensuring images load quickly and thus reducing CLS. Key elements of modern technologies for CLS improvement include:

  • Adopting Responsive Images with srcset and sizes attributes to ensure appropriate image sizes are loaded.
  • Applying CSS containment with contain property to limit the area of the page that requires layout work.

By meticulously applying these practices and technologies, we improve our website’s core web vitals, creating a smoother, more stable browsing experience for our users.

Common Pitfalls and How to Avoid Them

Cumulative Layout Shift (CLS) can significantly affect user experience by causing unexpected movement of page content. We’ll focus on recognizing the difference between unexpected and expected shifts, illustrated through real-world examples, and provide strategies to minimize the former for a more stable and enjoyable user experience.

Unexpected vs. Expected Layout Shifts

Unexpected layout shifts occur when elements on the page move spontaneously, often leading to user frustration and mistakes. These are typically caused by resources loading asynchronously or DOM elements dynamically changing. To mitigate these shifts, we precisely define sizes for media elements and reserve space for ad slots. Additionally, we avoid injecting new content above existing content unless in response to a user action, which would be considered an expected shift.

Real-World Examples and Case Studies

Real users often encounter unexpected layout shifts when browsing websites that load additional content without warning, such as ads or images, which push down the rest of the page content. A study examining eye-tracking research shows how these shifts can disrupt the reading flow and reduce comprehension. By analyzing cases where developers have successfully implemented strategies to reduce CLS, we can adopt best practices such as lazy loading non-critical images and stabilizing fonts to prevent FOIT (Flash of Invisible Text) or FOUT (Flash of Unstyled Text). Through these techniques, the stability of a page is maintained, ensuring that real users have a seamless browsing experience.

Tracking CLS Performance and Reporting

To effectively minimize Cumulative Layout Shift (CLS), it is essential that we accurately track performance and report findings. Proper CLS monitoring involves collecting data through reliable APIs and analyzing measurements from actual users.

APIs for CLS Data Collection

Web Vitals API is fundamental in tracking CLS data. It allows us to capture real-time CLS scores reflective of user experience. Using this API, we can collect granular details per session, allowing for a precise breakdown of page performance issues. For instance, we can measure the impact of dynamic content loading and ad insertions.

To initiate data collection, our code snippet might look like this:

new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    if (entry.hadRecentInput) continue; // Skip shifts following user input
    console.log('CLS:', entry.value);
}).observe({type: 'layout-shift', buffered: true});

This snippet creates a performance observer that logs the CLS value each time a layout shift occurs, except for those caused by user actions.

Analyzing Real User Measurements

We further dissect collected data by tapping into the Chrome User Experience Report — a public dataset of real user experiences on millions of websites. By using this report, we can extract user-centric CLS performance metrics across different devices and connection types.

Here’s how to retrieve CLS data from the report:

  1. Access the dataset on BigQuery.
  2. Run a query for our domain.
  3. Isolate CLS-related data points.
  4. Analyze variances across demographics and tech specs.

With the collected data, we chart trends and correlations, pinpointing what influences CLS on our site. This meticulous process of analysis is critical in implementing high-impact optimizations, ultimately elevating the stability and responsiveness of the user interface.

Google Activity

What can we do for you?

Hello & Welcome! 

My name is Shaheen, and I’m the Founder and President of WebUpon. We’re a digital marketing agency focused on our customers and even more focused on our customer’s customers. I’ve been programming and executing digital marketing strategies for more than 10 years.

The only thing that has stayed the same in that time is the need to innovate and test. We’re excited to share the latest information, perspective, and research from our work with you!