Understanding Cumulative Layout Shift (CLS): Definition, Measurement, and Solutions

Cumulative Layout Shift (CLS) is a metric that measures the visual stability of a web page. It is an important metric for website owners and developers to understand because it affects user experience and can impact search engine rankings. In this article, we will define CLS, discuss how it is measured, and provide solutions for improving it.

Definition of Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) is a metric that measures the amount of unexpected layout shifts that occur during the loading of a web page. A layout shift occurs when an element on a web page changes position unexpectedly, causing the content to move around. This can be frustrating for users, especially if they are trying to interact with the page.

The cumulative part of CLS refers to the fact that layout shifts can occur multiple times during the loading of a page. The metric takes into account the number of layout shifts and the distance that content moves as a result of those shifts.

Measurement of Cumulative Layout Shift (CLS)

CLS is measured using a formula that takes into account the impact of layout shifts on the visible area of a web page. The formula is:

CLS = Σ impact score / total visible area

The impact score is calculated by multiplying the distance that an element moves by its size. The total visible area is the area of the viewport that is visible to the user.

The result of this formula is a score between 0 and 1, with 0 indicating no layout shifts and 1 indicating significant layout shifts. A score of less than 0.1 is considered good, while a score of 0.25 or higher is considered poor.

Solutions for Improving Cumulative Layout Shift (CLS)

There are several solutions for improving CLS on a web page. Here are some of the most effective:

1. Avoid loading content dynamically: One of the main causes of layout shifts is content that is loaded dynamically. This can include images, videos, and ads. To avoid layout shifts, it is best to load all content statically.

2. Set dimensions for images and videos: Another cause of layout shifts is images and videos that do not have set dimensions. When these elements load, they can push other content around. By setting dimensions for images and videos, you can ensure that they do not cause layout shifts.

3. Use CSS animations: If you need to animate elements on a web page, it is best to use CSS animations. These animations are smoother and more predictable than JavaScript animations, which can cause layout shifts.

4. Preload critical resources: Preloading critical resources, such as fonts and scripts, can help to reduce layout shifts. By preloading these resources, you can ensure that they are loaded before the rest of the page, reducing the likelihood of layout shifts.

5. Optimize server response times: Slow server response times can cause layout shifts by delaying the loading of content. By optimizing server response times, you can ensure that content loads quickly and smoothly.

In conclusion, Cumulative Layout Shift (CLS) is an important metric for website owners and developers to understand. By measuring and improving CLS, you can improve user experience and potentially improve search engine rankings. By following the solutions outlined in this article, you can reduce layout shifts and improve the visual stability of your web pages.

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