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Understanding Cumulative Layout Shift

The world of web development is filled with complex terms and concepts, one of which is Cumulative Layout Shift (CLS). CLS is a vital aspect of web performance and user experience. It’s a metric that quantifies how much a webpage layout shifts during the user’s viewport. Let’s delve into the details of CLS, its importance, and how to optimize it for a better user experience.

What is Cumulative Layout Shift?

Cumulative Layout Shift, or CLS, is a part of Google’s Core Web Vitals, a set of metrics that measure the speed, responsiveness, and visual stability of a webpage. CLS specifically measures the unexpected shifting of web elements while a page is still loading.

When a webpage is loading, you might have noticed that some elements tend to move around, causing you to click on something you didn’t intend to. This is what CLS measures. A lower CLS score indicates a better user experience, as it means there are fewer unexpected shifts on the page.

Why is Cumulative Layout Shift Important?

CLS is crucial because it directly impacts the user experience. A high CLS can lead to a frustrating user experience. Imagine reading an article online, and the text keeps shifting, causing you to lose your place. Or worse, you’re about to click a link or button, and it moves, leading you to click something else. Not a great experience, right?

Moreover, Google has started using Core Web Vitals, including CLS, as ranking factors for search results. This means that if your website has a high CLS, it could negatively impact your SEO ranking. Therefore, optimizing your website’s CLS is not just about improving user experience, but also about improving your website’s visibility on search engines.

How to Measure Cumulative Layout Shift

There are several tools available to measure your website’s CLS. Google’s own tools like PageSpeed Insights, Search Console, and Lighthouse can provide you with your CLS score. Other third-party tools like WebPageTest also offer this functionality.

These tools measure CLS by looking at the viewport size and the movement of unstable elements in the viewport between two rendered frames. The CLS score is a product of the impact fraction (how much an unstable element impacts the viewport between two frames) and the distance fraction (the greatest distance an element has moved).

How to Optimize Cumulative Layout Shift

Use Set Size Attribute Dimensions for Media

One of the most common causes of layout shifts is media elements like images and videos without specified dimensions. When the browser doesn’t know the size of an element beforehand, it can cause the layout to shift when the element finally loads. By specifying the width and height attributes for your media elements, you can prevent this from happening.

Reserve Space for Ads and Embeds

Ads and embedded elements are another common cause of layout shifts. Like media elements, if the browser doesn’t know the size of these elements beforehand, it can cause the layout to shift when they load. By reserving space for these elements, you can prevent these shifts.

Ensure Fonts Don’t Cause Layout Shifts

Web fonts can also cause layout shifts if they’re not loaded properly. To prevent this, you can use font: display optional or font: display swap in your CSS to ensure text remains visible during web font load.


Understanding and optimizing Cumulative Layout Shift is crucial for providing a smooth and enjoyable user experience, as well as improving your website’s SEO ranking. By measuring your CLS and taking steps to optimize it, you can ensure your website is visually stable and user-friendly.

Remember, a great user experience leads to higher user engagement, lower bounce rates, and ultimately, better SEO rankings. So, start optimizing your CLS today!

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