Cumulative Layout Shift (CLS): Why It Matters and How to Improve It

Introduction

Have you ever tried clicking a button on a website, only for it to move suddenly as the page loads, making you click something else by mistake? This frustrating experience is caused by Cumulative Layout Shift (CLS). CLS measures how much elements on a webpage unexpectedly move around during loading, which can lead to a poor user experience. In this article, we’ll explain what CLS is, why it’s important, and how you can reduce it to make your website more stable and user-friendly.

What is Cumulative Layout Shift (CLS)?

Cumulative Layout Shift (CLS) is a metric that measures how much visual content on a page shifts unexpectedly while loading. These shifts can happen when images, ads, or other elements load in a way that pushes content around on the screen.

Understanding CLS Score Ranges

  • Good: A CLS score of 0.1 or less is ideal and indicates that the page layout is stable, offering a smooth experience for users.
  • Needs Improvement: A score in this range suggests moderate layout shifts, which could be annoying but aren’t overly disruptive.
  • Poor: A score above 0.25 is considered poor, meaning that users are likely experiencing noticeable shifts, leading to frustration and mistakes.

A low CLS score makes for a smoother and more reliable experience, where buttons, images, and text stay in place as the page loads.

Why is CLS Important?

  1. Better User Experience:

    When elements don’t shift unexpectedly, users can interact with your page confidently, without worrying about buttons or links moving. This improves overall satisfaction.

  2. Improved Core Web Vitals Score:

    Google uses CLS as one of its Core Web Vitals for ranking websites. Sites with a good CLS score are more likely to rank higher, helping improve visibility and traffic.

  3. Increased Conversions:

    A stable layout keeps users focused and helps avoid accidental clicks or misclicks. This can lead to higher conversions, as visitors are more likely to complete intended actions, like signing up or making a purchase.

Common Causes of High CLS

  • Images Without Dimensions: When images load without specified width and height, they can shift content around as the browser figures out how to display them.
  • Ads and Embedded Content: Ads or third-party embeds can change size dynamically, pushing content unexpectedly.
  • Fonts Loading Late: If custom fonts load after the initial page content, they can cause text to re-render, leading to layout shifts.
  • Injected Content: Banners, pop-ups, or other elements injected after the page loads can create shifts, especially if they appear at the top of the screen.

How to Reduce CLS

  • Set Size Attributes for Images and Videos: Always define the width and height for images and video elements. This tells the browser exactly how much space to reserve, preventing shifts as the media loads.
  • Use CSS Aspect Ratio Boxes: For elements that load dynamically, like responsive images or iframes, use CSS aspect ratio boxes to maintain their shape and size as they load.
  • Reserve Space for Ads and Embedded Content: For ads and embeds, reserve a static area on the page to prevent other content from shifting when these elements load. Using a fixed size or minimum height for ads can also help.
  • Load Fonts Early: Use font-display: swap in your CSS to ensure text displays quickly with a fallback font, reducing the risk of layout shifts when custom fonts load.
  • Avoid Injecting Content Above the Fold: If adding banners, pop-ups, or similar elements, place them below the fold (outside the initial visible area) or ensure they’re loaded in a way that doesn’t push other content around.

CLS and Related Metrics

While CLS focuses on layout stability, other metrics like First Input Delay (FID) and Largest Contentful Paint (LCP) provide additional insights into loading speed and interactivity. Together, these Core Web Vitals give a comprehensive view of user experience and performance.

Conclusion

Cumulative Layout Shift (CLS) is essential for providing a smooth and predictable experience for users. By setting dimensions for images, reserving space for ads, loading fonts quickly, and avoiding unexpected content injections, you can reduce CLS and make your website more user-friendly. A low CLS score improves user experience, enhances SEO, and reduces the chances of user frustration – all key factors in creating a successful website.