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Understanding CLS: Cumulative Layout Shift

When it comes to creating a seamless user experience on your website, understanding and optimizing for Cumulative Layout Shift (CLS) is crucial. CLS, a key metric in Google’s Core Web Vitals, measures visual stability and can significantly impact your site’s performance and user experience.

In this comprehensive guide, we’ll delve deep into what CLS is, why it matters, and how you can optimize for it. Let’s get started.

What is Cumulative Layout Shift?

Cumulative Layout Shift, or CLS, is a performance metric that quantifies how much a webpage’s content shifts during the user’s interaction. It’s a measure of how stable a webpage is as it loads and while the user interacts with it.

The lower the CLS score, the better. A high CLS score indicates that elements on your webpage are moving around a lot during load or interaction, which can be frustrating for users.

How is CLS Calculated?

CLS is calculated by looking at the impact fraction and the distance fraction. The impact fraction measures the viewport area that is affected by the shift, while the distance fraction measures the distance that the unstable elements have moved.

The CLS score is then calculated by multiplying the impact fraction by the distance fraction. This score can range from 0 to 1, with 0 being the best possible score.

Why Does CLS Matter?

CLS matters because it directly impacts the user experience. If elements on a webpage are constantly shifting, it can be disorienting and frustrating for users. This can lead to a poor user experience and potentially drive visitors away from your site.

Moreover, CLS is a part of Google’s Core Web Vitals, a set of metrics that Google uses to evaluate the quality of a webpage. A high CLS score can negatively impact your site’s SEO ranking.

The Impact of CLS on SEO

Google has made it clear that user experience is a key factor in its ranking algorithm. As part of this, CLS, along with other Core Web Vitals, plays a significant role in determining your site’s SEO ranking.

A high CLS score can lead to a lower ranking in Google’s search results, while a low CLS score can help improve your site’s ranking. Therefore, optimizing for CLS is crucial for SEO.

How to Optimize for CLS

Optimizing for CLS involves identifying and fixing the causes of layout shifts on your webpage. Here are some strategies to help you reduce your CLS score.

Use Set Dimensions for Media

One common cause of layout shifts is media (like images and videos) without set dimensions. When the dimensions aren’t specified, the browser doesn’t know how much space to allocate for the media until it’s fully loaded, which can cause other elements to shift.

By specifying the width and height of your media, you can prevent these layout shifts and reduce your CLS score.

Avoid Inserting Content Above Existing Content

Another common cause of layout shifts is inserting new content above existing content, pushing the existing content down. This is especially common with ads, pop-ups, and banners.

To avoid this, try to insert new content below existing content or reserve space for it ahead of time.

Use Web Fonts Optimally

Web fonts can also cause layout shifts if they’re not loaded properly. To prevent this, you can use font display options to control how fonts are rendered during the load.

For example, you can use the ‘font-display: optional’ property to tell the browser to use a system font until the web font is available, preventing a layout shift.

Tools to Measure and Improve CLS

There are several tools available that can help you measure and improve your CLS score. These tools can identify the causes of layout shifts on your webpage and provide recommendations for improvement.

Google’s PageSpeed Insights

PageSpeed Insights is a free tool from Google that provides a wealth of information about your webpage’s performance, including your CLS score. It also provides suggestions for how to improve your score.


WebPageTest is another free tool that provides a detailed analysis of your webpage’s performance. It includes a visual comparison of how your webpage loads, which can help you identify the causes of layout shifts.

Chrome DevTools

Chrome DevTools includes a Performance panel that can help you visualize layout shifts. It also includes a Layout Shift Regions option that highlights the areas of the page that have shifted.

By understanding and optimizing for CLS, you can provide a better user experience, improve your SEO ranking, and ultimately, drive more traffic to your site. Start optimizing today!

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