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Understanding Cumulative Layout Shift

Cumulative Layout Shift

A desk with a keyboard and a setup with two monitors understanding cumulative layout shift.

Stop The Movement

Making The Page Solid

Understanding Cumulative Layout Shift

By: Simon | Category: Pagespeed
Published Date: 9th January, 2024

Cumulative Layout Shift (CLS) has emerged as a crucial metric that directly affects how users perceive and interact with websites.

As a key component of Google’s Core Web Vitals, CLS plays a significant role in determining a website’s overall performance. In this article, we will delve into the concept of Cumulative Layout Shift, exploring its impact, the factors contributing to it, and effective strategies to minimise it.

What Is Cumulative Layout Shift?

Cumulative Layout Shift is a metric used to quantify the visual stability of a web page as it loads and becomes interactive. In simpler terms, CLS measures how much the content on a webpage shifts or moves around during the user’s interaction. This movement can be frustrating for users, especially when they are attempting to click on a specific element or read content.

CLS is calculated by multiplying the impact fraction (the amount of visible space affected by the shift) with the distance fraction (the distance the elements have shifted relative to the viewport). A higher CLS score indicates a more disruptive and unpleasant user experience.

Impact of CLS On User Experience

User Frustration

When elements on a webpage unexpectedly move, users can become frustrated, leading to a poor overall experience. Frequent and significant layout shifts can disrupt user engagement and hinder the accomplishment of tasks, such as clicking on buttons or filling out forms.

Reduced Conversion Rates

High CLS scores are often associated with lower conversion rates. Users are less likely to complete transactions or engage with content if they encounter a visually unstable website. This underscores the importance of CLS not only for user satisfaction but also for the success of online businesses.

Search Engine Rankings

Google has incorporated Core Web Vitals, including Cumulative Layout Shift, into its ranking algorithm. Websites with better performance, as measured by these metrics, are more likely to rank higher in search engine results. Therefore, addressing CLS issues is not only beneficial for user experience but also for maintaining and improving search visibility.

Watch Your Page Load

Watch For Jumps

Check For Redraws

Causes of Cumulative Layout Shift

Images & Media Without Dimensions

One common cause of CLS is the inclusion of images and media elements without specified dimensions. When these elements load, they may cause the layout to shift as the space required for them is allocated dynamically. Specifying width and height attributes for images allows browsers to reserve the required space, reducing the likelihood of layout shifts.

Asynchronous Loading Of Content

Content that loads asynchronously, such as ads or dynamically generated elements, can contribute to layout shifts. If the space for these elements is not reserved beforehand, the layout may shift when the content finally loads. Prioritising the loading of critical elements or using placeholders can mitigate this issue.

Web Fonts Loading

Web fonts, especially those with different loading times or delays, can trigger layout shifts. When the font size changes after the initial rendering, it can lead to unexpected shifts in the layout. Utilising font-display property and preloading fonts can help prevent such shifts.

Strategies To Minimise Cumulative Layout Shift

Preload Key Resources

To improve page loading and reduce CLS, consider preloading critical resources such as images, fonts, and stylesheets. Preloading ensures that these resources are fetched early in the page loading process, minimising the chances of layout shifts.

Specify Image Dimensions

Explicitly specifying the dimensions of images and media elements allows browsers to allocate the required space before the content loads. This simple practice significantly reduces the potential for layout shifts caused by dynamically sized images.

Prioritise Loading Of Critical CSS

Loading critical CSS first ensures that the essential styles for the initial view of the page are applied promptly. This helps in avoiding the flash of unstyled content (FOUC) and contributes to a more stable layout.

Lazy Loading

Implement lazy loading for images and other non-critical resources that appear below the fold. Lazy loading defers the loading of these elements until they are about to enter the viewport, preventing unnecessary layout shifts during the initial page load.

Monitor & Analyse

Regularly monitor your website’s performance using tools like Google PageSpeed Insights, Lighthouse, or other web performance tools. These tools provide insights into CLS and other Core Web Vitals metrics, helping you identify and address issues proactively.

My Conclusions

Cumulative Layout Shift is a critical aspect of web performance that directly influences user experience and search engine rankings. Web developers and site owners must prioritise strategies to minimize CLS, ensuring that visitors have a seamless and visually stable experience.

By addressing the causes and implementing best practices, websites can not only enhance user satisfaction but also improve their overall online presence. As the digital landscape continues to evolve, staying vigilant about performance metrics like CLS is crucial for maintaining a competitive edge online.

Technial blog author

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