CLS
Definition
Cumulative Layout Shift (CLS) is a key performance metric in web design that evaluates the visual stability of a webpage. It specifically measures how often and how much elements on a page shift unexpectedly during the loading process. A high CLS score indicates that users might experience frustrating layout shifts, which can negatively impact their experience and lead to higher bounce rates.
CLS is part of the Core Web Vitals, a set of metrics established by Google to assess real-world user experience. It is calculated by adding up the scores of all unexpected layout shifts that occur during the lifespan of a page. The score is represented as a decimal, with lower scores indicating better performance. A score of 0.1 or less is considered good, while a score above 0.25 is deemed poor.
Understanding and optimizing CLS is crucial for UX professionals as it directly affects user satisfaction and engagement. Users tend to abandon sites that are difficult to navigate due to layout instability, making it imperative for designers to focus on minimizing CLS in their projects.
Expanded Definition
CLS was introduced as part of Google's initiative to improve web performance and user experience. It specifically addresses a common issue where users experience sudden shifts in content, which can lead to misclicks or frustration. This issue often arises from images or ads that load after other content, causing the layout to change unexpectedly.
To effectively measure and optimize CLS, UX designers can implement strategies such as setting explicit width and height for images and videos, avoiding inserting content above existing content, and utilizing CSS to manage the layout. By addressing these factors, designers can create a more stable and enjoyable user experience.
Key Activities
Analyzing CLS scores using web performance tools.
Implementing best practices to minimize layout shifts.
Testing web pages under different loading conditions.
Collaborating with developers to optimize asset loading.
Monitoring user feedback related to page stability.
Benefits
Improves overall user experience by reducing frustration.
Enhances engagement and retention rates on websites.
Boosts SEO performance as search engines prioritize stable pages.
Reduces the likelihood of misclicks and navigation errors.
Encourages faster load times and better performance metrics.
Example
For instance, an e-commerce website that experiences significant layout shifts while loading product images may frustrate users, leading them to abandon their shopping carts. By optimizing image loading and ensuring all elements are properly sized, the site can achieve a lower CLS score, resulting in a smoother shopping experience and potentially higher conversion rates.
Use Cases
Improving the loading stability of e-commerce sites.
Enhancing user experience on news and media websites.
Optimizing performance for mobile applications.
Minimizing layout shifts in web applications.
Ensuring consistent user experience across different devices.
Challenges & Limitations
Difficulty in predicting all layout shifts during development.
Challenges in balancing visual design and performance optimization.
Need for continuous monitoring as content changes.
Potential trade-offs between aesthetics and performance.
Tools & Methods
Google PageSpeed Insights
Lighthouse
Web Vitals Chrome Extension
GTmetrix
Chrome DevTools
How to Cite "CLS" - APA, MLA, and Chicago Citation Formats
UX Glossary. (2025, February 11, 2026). CLS. UX Glossary. https://www.uxglossary.com/glossary/cls
Note: Access date is automatically set to today. Update if needed when using the citation.