Understanding Cumulative Layout Shift

In the dynamic world of web development, user experience is paramount, and one critical aspect that can greatly influence it is Cumulative Layout Shift (CLS). Cumulative Layout Shift measures the visual stability of a webpage, assessing how much elements shift around as the page loads. In this article, we’ll delve into what CLS is, why it matters, and explore effective strategies to reduce layout shift for an enhanced user experience.

What is Cumulative Layout Shift (CLS)?

CLS is a metric that quantifies the amount of unexpected layout shift that occurs during the loading of a webpage. This shift is particularly problematic as it can lead to a frustrating user experience, causing users to click on unintended elements and potentially leading to errors or unintended actions.

CLS is calculated by multiplying the impact fraction (the fraction of the viewport affected by the shift) by the distance fraction (how far the elements shifted). This cumulative score provides a comprehensive measure of the visual instability experienced by users.

Why Does CLS Matter?

Cumulative Layout Shift is crucial because it directly impacts user experience. A webpage with a high CLS score can be disorienting and frustrating for visitors, leading to a poor overall impression of the site. Moreover, search engines like Google consider CLS as a part of their Core Web Vitals, affecting a site’s search engine ranking. Therefore, addressing CLS is not just about improving user experience but also about enhancing the site’s visibility and accessibility.

How to Reduce Cumulative Layout Shift

  1. Optimize Images and Videos: Large media files can significantly contribute to layout shift. Optimize images and videos by compressing them without compromising quality. Implement lazy loading to ensure that media content loads only when it comes into the user’s viewport.
  2. Set Dimensions for Elements: Assign fixed dimensions (width and height) to images, ads, and other embeds. This prevents the browser from reserving space for an element with unknown dimensions, reducing the chances of layout shift.
  3. Load Fonts Asynchronously: Loading fonts asynchronously ensures that text doesn’t reflow when fonts are being fetched. This can be achieved by using the font-display: swap property in CSS.
  4. Prioritize Critical CSS: Serve critical CSS inline to ensure that the essential styles for above-the-fold content are applied quickly. This helps in rendering the initial view faster and reduces the likelihood of layout shift.
  5. Delay JavaScript Execution: Defer non-essential JavaScript or load it asynchronously to prevent it from blocking the rendering of the page. This helps in minimizing unexpected layout shifts caused by late-loading scripts.

What is a Good CLS Score?

Google recommends a CLS score of 0.1 or less for a webpage to be considered as providing a good user experience. This benchmark ensures that users encounter minimal visual instability during the loading process, resulting in a more satisfying and efficient browsing experience.


Cumulative Layout Shift is a critical metric that significantly influences the user experience and search engine rankings. By understanding the factors contributing to CLS and implementing effective strategies to reduce it, web developers can create visually stable and user-friendly websites. Aim for a CLS score of 0.1 or lower to ensure that your site not only meets user expectations but also stands out in search engine results.

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