CLS, short for Cumulative Layout Shift, is a term that is becoming increasingly important in the world of web design and web development. It refers to unexpected shifts in website elements during the loading process. These shifts can significantly impact the user experience as they can cause users to accidentally click on the wrong buttons or have difficulty reading content. The CLS is part of the Core Web Vitals, an initiative by Google that aims to define a set of specific factors that are crucial for a good user experience on websites.

The importance of CLS lies in its direct impact on the user experience. Websites that flicker or move elements around during the loading process can lead to frustration for users. This is particularly problematic for e-commerce sites, where a poor user experience can lead directly to a loss of sales. At a time when user experience is a crucial factor in the success of a website, minimising CLS is becoming an important task for web developers and designers.

CLS is calculated by determining the sum of all individual layout shifts that occur during the entire lifetime of a page. Each shift is scored based on the extent of the shift and the screen area affected. A high CLS value indicates that a web page has many unexpected layout shifts, resulting in a poor user experience. Conversely, a low CLS value indicates that the page is stable and the content is loading as expected.

The causes of a high CLS can be manifold. Common factors include images or videos without defined dimensions, dynamically loaded content such as adverts or embeds that are suddenly inserted into the DOM, and web fonts that change the layout when they are loaded. Identifying and fixing these causes is a crucial step in improving the CLS. This can include setting fixed sizes for media elements, avoiding sudden content insertions or pre-loading fonts.

Optimising the CLS is not only important for the user experience, but is also becoming increasingly relevant for search engine optimisation (SEO). Google has announced that Core Web Vitals, including CLS, will become part of its ranking factors. This means that websites with a poor CLS may receive lower positions in search results. It is therefore crucial for website owners to monitor and optimise CLS to improve both user experience and search engine visibility.

To summarise, the Cumulative Layout Shift is an important factor that significantly influences the stability and usability of a website. By understanding and minimising CLS, developers and designers can ensure that their websites provide a positive and seamless experience for users. In an increasingly competitive digital landscape, optimising CLS is becoming an essential part of web development and design.