What is Cumulative Layout Shift: Understanding Core Web Vitals

Cumulative Layout Shift, or CLS, is a metric that quantifies how often users experience unexpected movement of page content. This instability can occur when elements on a webpage move around as the page loads, leading to a disjointed and frustrating user experience. To provide a superior browsing experience, minimizing these shifts is essential.

One of the Core Web Vitals, CLS, is a key factor in measuring the visual stability of a webpage. The lower the CLS score, the less layout shift a user endures on the site, which contributes to an overall smoother user experience. This aspect is increasingly crucial as web performance becomes not just a matter of speed, but also of predictability and comfort for site visitors.

Key Takeaways

  • CLS is a critical metric for assessing visual stability on websites.
  • It is integral to Core Web Vitals, influencing the user experience.
  • Maintaining a low CLS ensures a consistent and user-friendly page display.

Understanding Cumulative Layout Shift

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As we navigate the digital space, it’s crucial for us to understand the performance metrics that contribute to a seamless user experience. Cumulative Layout Shift, or CLS, is a vital factor that we must consider in evaluating and optimizing web performance.

Definition and Importance

Cumulative Layout Shift (CLS) refers to the unexpected shifting of webpage elements while the page is still downloading. Websites should aim for a low CLS to prevent disorienting the user, as a high CLS score can indicate a poor user experience. Simply put, the more stable the page, the better the experience.

Calculation of CLS Score

To calculate the CLS score, we combine the impact fraction and distance fraction. The impact fraction measures the viewport space that an unstable element impacts, while the distance fraction quantifies the movement of the unstable elements. CLS score is the sum of all individual layout shift scores for every unexpected layout shift that occurs during the lifespan of the page.

Components of CLS

Components of CLS include two key metrics:

  1. Impact Fraction: This entails the space that shifts and impacts the user-visible area of a webpage.
  2. Distance Fraction: This quantifies how much the unstable elements have moved in the viewport. Together, these paint a picture of how dramatically content shifts on the screen.

By understanding and improving CLS, we ensure a smoother, more predictable browsing experience for our website visitors.

Core Web Vitals and User Experience

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We recognize the importance of a website’s performance in relation to user satisfaction. Core Web Vitals are the subset of Web Vitals crucial for providing a good user experience. These metrics measure the visual stability, interactivity, and loading performance of web pages.

Core Web Vitals Explained

Core Web Vitals consist of three specific page performance metrics that Google considers important:

  • Largest Contentful Paint (LCP): This measures the loading performance of the largest image or text block visible within the viewport, relative to when the page first starts loading. A good LCP score ensures that the page is useful within the first 2.5 seconds of the page starting to load.
  • First Input Delay (FID): This metric assesses a page’s interactivity by measuring the time from when a user first interacts with a page to the time when the browser is able to respond to that interaction. A page with an FID of less than 100 milliseconds offers a responsive user experience.
  • Cumulative Layout Shift (CLS): Measures the sum total of all individual layout shift scores for every unexpected layout shift that occurs during the entire lifespan of the page. A low CLS ensures visual stability and that users won’t experience unexpected movement of page content.

CLS and Other Web Vitals

CLS is a key performance metric within Core Web Vitals that quantifies the amount of unexpected movement of page content. A high CLS indicates a poor user experience, as content shifting abruptly can be disorienting, leading users to lose their place or make unintended clicks.

Maintaining a CLS score of less than 0.1 is vital for a good user experience. By optimizing images, ads, embeds, and iframes to avoid sudden rearrangement, we can improve CLS.

Integrating a user-focused approach that includes measuring and enhancing Core Web Vitals ensures that we provide a smooth and stable experience for our website’s visitors.

Optimizing Content for Stability

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Minimizing Cumulative Layout Shift (CLS) ensures a stable and predictable browsing experience. We’ll focus specifically on how to optimize images, ads, and dynamically loaded content to maintain visual stability on a webpage.

Images and Ad Optimization

To prevent layout shifts, we must specify width and height dimensions for our images and ads. Doing so allows the browser to allocate space while the content is loading, thus preventing any unexpected movement. For example, in CSS, we can use:

img, .ad {
    width: auto;
    height: auto;
    aspect-ratio: 1 / 1;

For advertisements, AMP (Accelerated Mobile Pages) formats can significantly reduce CLS, as they’re built to load quickly and display ads in a stable manner without disturbing the content layout.

Font Display Techniques

Appropriate font display strategies are crucial for stability. To avoid the flash of unstyled text (FOUT) or flash of invisible text (FOIT), we can use font-display: swap; in our CSS. This ensures users see text in a fallback font while the chosen one loads, thus avoiding shifts.

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

Embeds and Iframes

When incorporating embeds and iframes, loading them asynchronously prevents layout shifts. We must also reserve space with an aspect ratio box technique to maintain the layout’s integrity before they load. Additionally, setting explicit width and height in the embed or iframe tag is essential:

<iframe src="video.html" width="560" height="315"></iframe>

To ensure stability with animations and dynamic content, restraint and proper triggering are key. Avoid triggering large, layout-affecting animations based on initial page load. Instead, conditionally apply animations when elements become visible with IntersectionObserver, effectively managing potential layout shifts.

Tools for Measuring and Diagnosing CLS

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Accurate measurement is crucial for diagnosing Cumulative Layout Shift (CLS) issues. We use various tools to gain insights into both lab and real-world user experiences.

Lighthouse and PageSpeed Insights

Lighthouse is an open-source, automated tool integrated into the Chrome Developer Tools. It’s designed to audit web pages by simulating a page load in a controlled environment, also referred to as lab data. This process provides a CLS score that reflects the shifts happening on the page as it loads. Similarly, PageSpeed Insights uses Lighthouse but also incorporates field data from the Chrome User Experience Report to give a broader picture of CLS over time from real users.

  • Use cases: Diagnostic tool for devs; quality check before go-live.

Chrome DevTools

For a hands-on approach, Chrome DevTools offers an array of options. It not only helps in identifying layout shifts but also points to potential causes. With the Performance tab, we can record a session and analyze the rendering timeline, where shifts are visually annotated. This detailed feedback supports us in pinpointing and correcting specific issues causing layout instability.

  • Use cases: Interactive debugging; immediate feedback during development.

Search Console Reports

Google Search Console provides reports that help us understand how our site is performing in search results, including visibility into CLS issues. It distinguishes between lab and field data, helping us to identify whether issues are widespread or isolated incidents. By monitoring these reports, we can detect and address layout shifts before they negatively impact user experience or search ranking.

  • Use cases: Long-term monitoring; benchmarking against industry standards.

In our pursuit of excellent user experience, we carefully consider these tools as part of our toolkit to diagnose and measure CLS, thereby ensuring stability and smoothness in the browsing experience for our audience.

Improving Layout Shift During Development

Cumulative Layout Shift (CLS) is a core web vital metric that measures the sum of all individual layout shift scores for every unexpected shift that occurs during the entire lifespan of a page. Focusing on this metric during development can significantly enhance the user experience by ensuring the page remains stable as it loads, preventing elements from moving unexpectedly on the screen.

Responsive Design Best Practices

Responsive design is essential in preventing layout shifts across various devices. We ensure that our design is fluid and adaptable to different viewports, utilizing relative units like percentages or viewport-width (vw) and viewport-height (vh). It’s important to define aspect-ratio to maintain consistency in media dimensions, reducing shifts when resizing. We also embrace CSS transform properties for animations, as they are less likely to cause layout changes compared to properties that affect the document flow, such as width or height.

  • Utilize relative CSS units (%, vw, vh) for responsive sizing.
  • Maintain consistent media dimensions through the aspect-ratio property.
  • Prefer the CSS transform property for animations and transitions.

Using Size Attributes

It’s crucial to prevent content from jumping around as it loads, which we achieve by using size attributes on images and videos. We define the expected size or aspect ratio of these elements directly in the HTML. This way, even if an image or video takes time to load, the space is already allocated for it, and other content will not shift into its place.

<!-- Example of using `width` and `height` attributes -->
<img src="image.jpg" width="600" height="400" alt="description">
  • Predefine the width and height of images and videos.
  • Reserve space for media elements using placeholders.

Handling Dynamic Content

Dynamic content, such as ads or widgets, might load at different times, causing shifts. We manage this by reserving space with a placeholder or skeleton screen before the content loads, and we use the CSS transform property rather than top or left properties, which cause reflows. When handling images, we opt for the srcset attribute to appropriately choose image sizes based on the user’s device, further preventing layout shifts.

  • Reserve sufficient space with placeholders before dynamic content loads.
  • Implement srcset for responsive and optimized image loading.

By adhering to these development practices, we provide a smoother, more stable browsing experience, paving the way for better user satisfaction and improved performance metrics.

CLS as a Ranking Factor in SEO

Cumulative Layout Shift (CLS) has become a crucial metric for understanding user experience and, by extension, search engine rankings. We’ll explore why it’s a vital component of SEO strategy.

Search Engine Optimization and CLS

CLS is a performance metric that measures the stability of content on a webpage. A high CLS score indicates a poor user experience, as content shifting unexpectedly can be jarring and frustrating for users. In terms of SEO, this translates into a negative ranking signal for search engines like Google. We must ensure that webpage layouts remain stable as the page loads to maintain a low CLS score and uphold a positive user experience.

The Impact of CLS on Traffic and Ranking

It’s clear that CLS directly affects traffic; pages with high layout shifts may be penalized in search engine rankings, resulting in decreased visibility. Google, which is constantly updating its algorithms to focus on user experience, now considers CLS as part of its Core Web Vitals—a set of metrics forming a significant ranking factor. For us, improving CLS is part of an effective SEO strategy to boost organic traffic and improve our site’s position on the search engine ranking ladder.

Real User Monitoring and CLS

When assessing the stability of a webpage as it loads, it’s crucial to understand how real user interactions influence Cumulative Layout Shift (CLS) metrics. We analyze the impact through field data, which represents the actual experiences of real users visiting a page.

Field Data vs. Lab Data

  • Field Data: Captures real-world user experience. It relies on metrics collected from actual users under a variety of conditions. This data is particularly valuable for measuring CLS because it reflects real interaction patterns and device performance, accounting for expected layout shifts.
  • Lab Data: Generated in a controlled testing environment. It allows us to simulate and debug issues in a consistent setting. However, it may not capture the full range of experiences real users encounter.

CRUX and Real User Data

  • Chrome User Experience Report (CRUX): Provides user experience measurements from millions of websites. Through CRUX, we gather field data about CLS to understand how real users perceive the stability of a page.
  • Real User Data: This data is sourced directly from interactions on the webpage. It provides us with the most accurate representation of how users experience layout shifts during their browsing session.

By integrating CRUX data with our internal metrics, we can create a detailed picture of our site’s performance from the lens of real-world usage.

Future Directions and Best Practices

In the evolving landscape of web performance, maintaining visual stability is paramount. With cumulative layout shift (CLS) as a core metric, we’re looking toward a future where stable elements support a seamless user experience, and unstable elements are mitigated.

Upcoming Changes to CLS Metric

Google’s Layout Instability API has made strides in quantifying visual stability on the web. Recent enhancements to CLS tracking focus on transitions that occur during user interactions. This shift acknowledges that some layout changes are intentional and should not be penalized if they’re a response to user actions. This nuanced approach is expected to encourage developers to script transition animations in a way that contributes positively to the CLS metric. By anticipating these updates, we can begin to integrate smoother transitions that don’t affect the stability score negatively.

Staying Ahead with Stable Layouts

To sustain high performance, we need to prioritize the design of stable elements from the outset:

  • Identify and monitor unstable elements: Regularly use tools to detect elements causing layout shifts.
  • Reserve space for dynamic content: Wherever content is dynamically loaded, we should allocate space in the initial layout to prevent unexpected shifts.
  • Optimize images and ads: Ensure proper aspect ratios are defined for images and ads to avoid reflows.
  • Preload important assets: This strategy can help prevent sudden layout changes when the asset becomes visible.
  • Use CSS containment: This confines the impact of an element’s layout to itself, preserving the surrounding layout stability.

Our shared goal is to solidify web stability where both the developers and users benefit from an internet with minimal unexpected layout shifts. By aligning with the evolving CLS metric and designing for stability, we can achieve a more consistent and user-friendly web.

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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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