Introduction: In the realm of web performance, Cumulative Layout Shift (CLS) is a crucial metric that directly impacts user experience. In this blog post, we’ll explore what CLS is, why it matters for website performance, and how to measure it effectively.
What is CLS? Cumulative Layout Shift (CLS) is a metric that quantifies the visual stability of a web page during the loading process. It measures the sum total of all unexpected layout shifts that occur as a user interacts with the page, such as images or ads loading dynamically and pushing content down, causing the layout to shift.
Why Does CLS Matter? CLS is essential for user experience for the following reasons:
- User Frustration: Unexpected layout shifts can be frustrating for users, especially when they are trying to interact with content or complete tasks on a webpage.
- Accessibility: Layout shifts can negatively impact users with disabilities or those using assistive technologies, making it difficult to navigate and interact with content.
- SEO Impact: Google considers CLS as a ranking factor for search engine optimization (SEO), with pages that provide a better user experience, including minimal layout shifts, potentially ranking higher in search results.
- Ad Revenue: For websites that rely on advertising revenue, minimizing layout shifts is crucial to maintain engagement and prevent accidental clicks due to shifting elements.
How to Measure CLS: Measuring CLS involves using tools and techniques to quantify layout shifts on a webpage. Here’s how to measure CLS effectively:
- Chrome DevTools: Chrome DevTools offers a CLS report in the Performance panel, allowing developers to visualize layout shifts and identify contributing elements.
- Web Vitals Extension: The Web Vitals Chrome extension provides real-time insights into key web performance metrics, including CLS, allowing developers to monitor and analyze CLS on different websites.
- Lab Testing: Use lab testing tools like Lighthouse or PageSpeed Insights to assess CLS scores and identify opportunities for improvement.
- Field Data Analysis: Analyze real-user data from tools like Google Analytics or Google Search Console to understand CLS performance across different devices and user segments.
How to Improve CLS: Once CLS is measured, take proactive steps to improve it:
- Optimize Images and Videos: Use proper dimensions and aspect ratios for images and videos to prevent unexpected layout shifts when they load.
- Set Dimensions for Ads and Embeds: Specify dimensions for ad units, iframes, and embedded content to allocate space in the layout and prevent them from causing layout shifts.
- Preload Critical Resources: Use resource hints like preload to prioritize loading critical resources, reducing the likelihood of layout shifts caused by late-loading elements.
- Avoid Dynamic Content Insertions: Minimize the use of dynamically injected content, especially above-the-fold, to prevent unexpected layout shifts during page load.
- Implement CSS Transitions: Use CSS transitions to animate layout changes smoothly, reducing the visual impact of shifts on the page.
Conclusion: Cumulative Layout Shift (CLS) is a critical metric that directly impacts user experience and search engine rankings. By understanding what CLS is, why it matters, and how to measure and improve it effectively, web developers and site owners can enhance the visual stability of their web pages, providing a better experience for users and improving their website’s performance in search results. Incorporate CLS measurement and optimization into your web performance strategy to ensure a smoother and more engaging browsing experience for your visitors.