Cumulative Layout Shift Examples: Understanding Visual Stability Issues

Cumulative Layout Shift, commonly referred to as CLS, is a metric that quantifies how often users experience unexpected layout shifts—a major factor of poor user experience—during their visit to a website. It’s a component of Google’s Core Web Vitals, which are a set of metrics that Google considers important for a webpage’s overall user experience. High CLS scores are indicative of a website where elements shuffle around as the page loads, causing users to misclick or lose their place, which can be frustrating and may lead to a decrease in user satisfaction.

In the world of web development, understanding and optimizing CLS is crucial. It is measured throughout the lifespan of a page session, reflecting the sum of all individual layout shift scores for every unexpected layout shift that occurs. This is important not only for providing a better user experience but also because it can influence a website’s search engine ranking. As Google’s algorithms increasingly prioritize user experience, mastering CLS will become instrumental in search engine optimization (SEO) strategies.

Key Takeaways

  • CLS is a key metric for evaluating a website’s visual stability and user experience.
  • Improving CLS involves identifying and rectifying common causes of layout shifts.
  • A better CLS score can enhance a site’s usability and contribute to SEO.

Understanding CLS and its Importance

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In our focus on web performance and user experience, we must pay attention to metrics that impact how users interact with our sites, with Cumulative Layout Shift (CLS) being a prime consideration.

Defining Cumulative Layout Shift

Cumulative Layout Shift, core web vital, measures the stability of content as a page loads. If elements move around unexpectedly as the page loads, the CLS score increases. A high CLS score is an indicator of poor user experience.

Significance in Core Web Vitals

CLS is a significant component of the Core Web Vitals, a set of metrics part of Google’s criteria for measuring web usability. High CLS can affect your site’s ranking on Google, as a stable page layout is considered an essential aspect of a high-quality web page.

Calculating CLS Score

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In our exploration of webpage performance, we focus on Cumulative Layout Shift (CLS), a critical factor affecting user experience. CLS measures visual stability, quantifying how often users experience unexpected layout shifts during a page’s lifespan.

Components of CLS

CLS is composed of two main metrics: impact fraction and distance fraction. The impact fraction measures the viewport area that an unstable element impacts during a layout shift. Conversely, the distance fraction assesses how much an unstable element moves from its original to final position, expressed as a percentage of the viewport.

Impact and Distance Fractions

To calculate a layout shift score for an individual shift, we multiply the impact fraction by the distance fraction. For instance, if an element moves down by 15% of the viewport height and impacts 25% of the viewport, the layout shift score for this shift would be 0.0375 (0.15 * 0.25).

Tools for Measuring CLS

For assessing CLS, we have robust tools at our disposal. Lighthouse is an open-source, automated tool integrated into Chrome DevTools, while PageSpeed Insights is a web-based tool utilizing Lighthouse for analysis. These provide actionable feedback on how to improve page stability, reflecting the CLS score clearly in their reports.

Optimizing for Different Devices

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When we talk about optimizing for different devices, we primarily focus on enhancing user experience by addressing performance and layout issues such as Cumulative Layout Shift (CLS) across desktop and mobile environments.

Desktop vs Mobile

On desktop platforms, users generally have a wider viewport and a more stable internet connection, which allows us to design with fewer constraints on image and component sizes. However, the mobile experience is often subject to variable network conditions and smaller screen sizes. We ensure that our layout is flexible and adjusts fluidly to different screen dimensions without causing unexpected layout shifts. Insights from this study on optimization guide us in refining our CLS strategy for both desktop and mobile.

By examining these variances, it becomes clear that we must handle resources differently. On mobile devices, we prioritize speed and interactivity by reducing file sizes and simplifying designs. This ensures a low CLS, minimizing the amount of movement a page element undergoes after it has been displayed, making for a more enjoyable user experience.

Adaptive Images and Responsiveness

For responsive images, we use techniques that serve different image sizes based on the user’s device. This is particularly important to prevent layout shifting when an image is loaded. By specifying dimensions and using modern image formats, we adaptively deliver images that are optimized for both desktop and mobile viewports, as suggested by techniques for optimizing React web-applications.

  • List of methods for responsive images:
    • Srcset attribute: Specifies multiple image sizes.
    • Sizes attribute: Defines a set of media conditions.
    • Picture element: Gives us more control over which image is shown.

The goal is to ensure that our images resize gracefully within a flexible grid structure, maintaining the aspect ratio and providing a seamless shift between different breakpoints. This helps to prevent any unexpected layout changes as the viewport size alters, directly contributing to a stable CLS score.

Common Causes of Layout Shifts

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Cumulative Layout Shift (CLS) primarily stems from elements on a page loading asynchronously or unexpectedly changing in size after the initial render. This can frustrate users as it often leads to a poor web experience. We’ll explore some common causes directly associated with this issue.

Ads and Dynamic Content

Dynamic content, such as ads, can lead to significant layout shifts if not properly managed. For instance, when an ad loads asynchronously or is injected dynamically onto a page, it can push down or rearrange the existing content. This is especially problematic if size attributes are not defined, as the space for the ad isn’t reserved in the document flow. To mitigate this, we ensure that the space for ads is specified upfront, preventing any unexpected layout changes.

Unstyled and Late-Loaded Fonts

Web fonts present another challenge, as they can cause a shift when they are swapped after the initial render. This is often due to a fallback font being displayed initially and then replaced by the web font once it has loaded. To avoid this, we can utilize font-display strategies to control how fonts are loaded and displayed, ensuring a smoother visual transition and reducing layout shifts.

Embeds and Iframes

Iframes and embeds, such as videos or social media widgets, are common culprits of layout shift. These elements often have content that is not immediately loaded with the rest of the page. If we don’t define an explicit size for these elements, they can expand and cause content around them to be pushed out of place. We combat this by setting appropriate dimensions or using aspect ratio boxes to reserve space, preventing any disruptive changes in layout.

Improving CLS Through Techniques and Best Practices

Cumulative Layout Shift (CLS) is crucial in providing a stable and pleasant web experience. We can address CLS issues head-on by implementing key techniques and best practices.

Usage of Aspect Ratios for Media

When defining media elements like images or videos, it’s essential to specify their size attributes or aspect-ratio in the CSS. By declaring an aspect ratio, we reserve the necessary space on the page, preventing unexpected layout shifts when the media load. An example would be:

img {
  aspect-ratio: 16 / 9;
  width: 100%;

This ensures the browser knows exactly how much space the image will occupy, thus improving the CLS.

Preventing FOIT and FOUT

To avoid Flash of Invisible Text (FOIT) and Flash of Unstyled Text (FOUT), which contribute to layout shifts, we ensure that text remains visible during web font load using the font-display property. For instance:

@font-face {
  font-family: 'WebFont';
  src: url('webfont.woff2') format('woff2');
  font-display: swap;

The swap value tells the browser to use a fallback font until the custom font is fully loaded, maintaining the layout stability.

Animations and Transforms

Animations can enhance user experience, but they can also cause layout shifts if not handled correctly. By using the CSS Transform property for animations, rather than properties that trigger layout changes (like width or top), we can prevent CLS. For example:

.element-to-animate {
  transform: translateX(50px);

This approach allows us to move elements around visually without affecting the actual layout of the page. It’s powerful for animations and keeps our CLS low.

Analyzing Real-World CLS Performance

In today’s web ecosystem, Cumulative Layout Shift (CLS) signifies the stability of visual page content. We focus on understanding how real-world performance impacts user experience.

Chrome User Experience Report

The Chrome User Experience Report provides insights into real-world CLS performance across millions of websites. This report aids us in gathering field data that reflect actual user experiences. By assessing this rich dataset, we can pinpoint common issues that lead to poor CLS scores and identify patterns in shifts that affect user satisfaction.

Using RUM for CLS Data

Real User Monitoring (RUM) tools offer granular, user-centric performance metrics including CLS. By leveraging RUM, we collect real user data in a way that goes beyond laboratory testing. RUM helps us understand the real-world impact of layout shifts as experienced by actual users under various conditions.

Case Studies and Comparison

Evaluating case studies gives us clear examples of CLS improvements and their effect on user experience. By comparing before-and-after scenarios, we can measure the benefits of optimizations on CLS scores. This comparison not only highlights the direct correlation between stability and user satisfaction but also showcases the best practices in real-world web design.

CLS and Search Engine Optimization

Cumulative Layout Shift (CLS) has become a crucial factor for our website’s performance in Search Engine Optimization (SEO). As we navigate the intersection of technical SEO and user experience, understanding and optimizing for CLS is key to achieving favorable search engine rankings.

Relationship Between CLS and SEO

CLS measures the visual stability of a webpage—how much unexpected shifting of content occurs after a page starts loading. This metric is part of Core Web Vitals, which are integrated into Google’s search algorithms. Strong performance in these metrics, including CLS, enhances user experience by providing a stable and seamless browsing environment. We acknowledge that optimizing CLS may result in higher engagement, lower bounce rates, and longer session durations, which search engines, like Google, interpret as indicators of a quality website. Our examination of website analytics through tools such as Google Search Console can reveal issues with CLS that we need to address to improve our site’s search visibility.

CLS as a Ranking Factor

Google officially recognizes CLS as a ranking factor, emphasizing its importance alongside other key metrics such as Largest Contentful Paint (LCP) and First Input Delay (FID). This means that minimizing layout shifts is essential for retaining and improving our position on the search engine ranking spectrum. To maintain competitive rankings, we must prioritize monitoring and reducing CLS scores. Through the Google Search Console, we can identify specific CLS issues and rectify them to not only enhance user experience but also to potentially gain an edge in the search engine results pages (SERPs). It’s imperative that we apply continuous improvements in response to these insights to remain aligned with SEO best practices and Google’s evolving algorithms.

Advanced Topics in CLS

Before we dive into the depths of Cumulative Layout Shift (CLS), it’s essential to understand that tackling this web performance metric involves leveraging sophisticated APIs and continuous performance evaluation tools. Now, let’s examine two pivotal facets of mastering CLS.

Layout Instability API and Developer Tools

The Layout Instability API offers us a critical toolset for detecting and troubleshooting unexpected layout shifts. By embedding JavaScript to listen for layout shift events, we are able to gather insightful data on contributing elements that cause visual instability. Developer resources such as Lighthouse and WebPageTest have integrated this API, enabling us to analyze and optimize CLS effectively. For instance, Lighthouse produces a comprehensive report highlighting elements that contributed most to the layout shift, which we can utilize to pinpoint and address the root cause.

  • API Usage Example:
    • Listener Setup: new PerformanceObserver((list) => { ... }).observe({ type: 'layout-shift', buffered: true });
    • Data Processing: Iterating through list.getEntries() to analyze layout shift scores and affected elements.

Continuous Monitoring and A/B Testing

To ensure that our websites maintain excellent performance, we espouse the practice of continuous monitoring, employing synthetic monitoring tools such as WebPageTest. By using these tools, we set up a regime that consistently checks for CLS issues, identifying any regressions or improvements over time. Furthermore, it’s beneficial to conduct A/B testing on our sites, especially if we’re running a platform like WordPress. This strategy allows us to compare different versions of our pages and determine which changes specifically reduce CLS, thus enhancing user experience.

  • A/B Testing Flow:
    1. Implement changes on a test page.
    2. Monitor key metrics, including CLS, across variations.
    3. Analyze the performance impact and solidify the most user-friendly experience.
    • Tools for Continuous Monitoring: WebPageTest, Lighthouse, and custom monitoring scripts.
    • Key A/B Testing Consideration: Minimal CLS ensures a stable and enjoyable experience for end-users.

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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.

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