CLS

What does CLS mean?

CLS (Cumulative Layout Shift) is a web performance Metric that measures the amount of unexpected layout changes on a web page. It occurs when a visible element on a page moves its position before the page has fully loaded, leading to an unpredictable and potentially frustrating user Experience.

Layout shifts can result from various factors, including dynamic content loading, image resizing, and font rendering. They can be particularly noticeable on mobile devices due to the smaller Screen size and the frequent need to adjust content to fit within the viewport.

CLS is measured in seconds and ranges from 0 to 1. A CLS score of 0 indicates no layout shifts, while a score of 1 represents a severe layout shift that covers the entire viewport. The ideal CLS score as recommended by Google is less than 0.1.

Applications

CLS plays a crucial role in improving website usability and enhancing user experience. Here are key applications:

• Improved Accessibility: CLS can hinder accessibility for users with disabilities, such as screen readers or mobility impairments. When elements on a page shift unexpectedly, it becomes difficult for users to navigate the page effectively.
• SEO Impact: Google has incorporated CLS into its ranking algorithm, making it a key factor in Search engine optimization. Websites with high CLS scores may rank lower in search results, affecting their visibility and traffic.
• Conversion Rate: Unexpected layout shifts can disrupt the user journey, affecting conversions and lead generation. Users are more likely to abandon a website if they encounter frequent or severe layout shifts.
• Improved User Experience: CLS has a direct impact on user satisfaction. Websites with minimal or no layout shifts provide a predictable and enjoyable experience, increasing engagement and retention.

History

The concept of CLS has been around for several years, but it gained significant attention with the introduction of Google’s Core Web Vitals in 2020. Core Web Vitals are a set of performance metrics that Google uses to measure the user experience of web pages. CLS is one of the three Core Web Vitals, along with First Input Delay (FID) and Largest Contentful Paint (LCP).

Prior to the introduction of Core Web Vitals, CLS was not widely measured or optimized by web developers. However, its incorporation into Google’s ranking algorithm has made it a critical factor for website performance and SEO.

Today, CLS remains a significant metric in web performance optimization. Browser vendors, such as Google and Mozilla, are actively working to reduce CLS by improving rendering techniques and providing tools for developers to identify and fix layout shift issues.