Cumulative Layout Shift
Definition
Cumulative Layout Shift (CLS) is a key metric in web performance that quantifies the amount of unexpected layout shifts that occur during the loading phase of a webpage. It reflects how stable the visual elements of a page are as it fully loads, providing insights into user experience. A high CLS score indicates that users may experience frustration due to shifting content, which can lead to accidental clicks and a negative perception of the site.
CLS is part of Google's Core Web Vitals, a set of metrics aimed at quantifying user experience on the web. It specifically measures the cumulative score of all unexpected layout shifts that occur during the lifespan of a page, focusing on the stability of visual elements such as images, buttons, and text blocks. By analyzing these shifts, web developers and designers can better optimize their pages for a seamless user experience.
Understanding and minimizing CLS is crucial for UX designers and developers. A lower CLS score means that users can interact with the page without unexpected changes, which enhances usability and satisfaction. As websites become more dynamic and content-heavy, managing layout shifts is essential to ensure that users can navigate smoothly and efficiently.
Expanded Definition
The concept of Cumulative Layout Shift emerged from the need to address user experience issues associated with unpredictable layout changes on websites. Historically, as websites evolved from static pages to more dynamic and interactive designs, the potential for layout shifts increased. These shifts can occur due to late-loading images, fonts, or asynchronous scripts that cause content to move unexpectedly. Google's introduction of CLS as a Core Web Vital reflects a growing recognition of the importance of visual stability in user-centric web design.
CLS is measured on a scale from 0 to 1, where a score of 0 indicates no layout shifts, and a score closer to 1 signifies significant shifts that hinder user experience. To achieve a good user experience, a CLS score of less than 0.1 is recommended. This metric encourages developers to implement best practices such as specifying image dimensions, using CSS for layout stability, and avoiding large layout shifts caused by font loading.
Key Activities
Identify and analyze layout shifts using web performance tools.
Implement best practices to minimize unexpected layout changes.
Test web pages across different devices and browsers for consistency in layout.
Monitor CLS scores regularly as part of website performance audits.
Educate stakeholders about the impact of layout shifts on user experience.
Benefits
Enhances overall user satisfaction by providing a stable browsing experience.
Reduces accidental clicks caused by unexpected content movement.
Improves search engine ranking due to better Core Web Vitals scores.
Encourages better design practices and a more predictable layout.
Helps in identifying performance bottlenecks related to visual stability.
Example
Consider a news website that features a variety of articles, images, and advertisements. If an image loads slowly and pushes down the headline, users may inadvertently click on a different article or ad due to the unexpected shift. This scenario highlights the importance of managing CLS; by setting explicit width and height attributes for images, the layout can remain stable while the content loads, significantly improving the user experience.
Use Cases
Assessing the performance of e-commerce websites where users expect stable product displays.
Evaluating news sites that frequently update content and require layout stability.
Optimizing portfolio sites for designers to ensure a smooth viewing experience.
Testing mobile applications that utilize web views to ensure visual consistency.
Improving landing pages for marketing campaigns to enhance conversion rates.
Challenges & Limitations
Difficulty in measuring CLS accurately across different devices and screen sizes.
Potential trade-offs between dynamic content delivery and visual stability.
Legacy systems may require significant rework to minimize layout shifts.
Relying solely on CLS may overlook other critical user experience factors.
Tools & Methods
Google Lighthouse for auditing web performance metrics.
WebPageTest for analyzing layout shifts in real-time.
Chrome DevTools to debug layout shifts during development.
PageSpeed Insights to receive insights related to CLS and other performance metrics.
Real User Monitoring (RUM) tools to gather data from actual users regarding layout stability.
How to Cite "Cumulative Layout Shift" - APA, MLA, and Chicago Citation Formats
UX Glossary. (2025, February 11, 2026). Cumulative Layout Shift. UX Glossary. https://www.uxglossary.com/glossary/cumulative-layout-shift
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