What is Cumulative Layout Shift (CLS)?#

In the digital landscape, user experience (UX) has become paramount, and Cumulative Layout Shift (CLS) plays a significant role in determining it. CLS refers to the unexpected shifting of webpage elements during the loading process, causing frustration to users.

Understanding Cumulative Layout Shift#

CLS measures the visual stability of a webpage by quantifying the amount of unexpected layout shifts that occur during the loading process. These shifts can be triggered by various factors, such as images and ads loading dynamically, font rendering delays, or asynchronous content loading.

Key Factors Influencing CLS#

Several factors contribute to CLS, impacting the overall user experience and SEO rankings:

  • Images and Videos: Unoptimized media files can cause layout shifts as they load, disrupting the user experience.
  • Ads and Embeds: Third-party content like ads and embeds can trigger layout shifts, affecting page stability.
  • Font Loading: Delayed font rendering can lead to text shifting, causing unexpected layout changes.
  • Dynamic Content: Content that loads asynchronously can shift the layout, impacting user interaction.

Impact of CLS on User Experience#

High CLS scores can frustrate users, leading to increased bounce rates and decreased engagement. When elements on a page suddenly move or resize, users may accidentally click on the wrong elements, resulting in a negative experience. Poor user experience not only affects user satisfaction but also reflects poorly on the website's credibility and trustworthiness.

Optimizing CLS for Enhanced Performance#

Mitigating CLS involves implementing strategic optimizations aimed at enhancing visual stability and user experience:

  • Optimize Images and Videos: Compress and resize media files to reduce layout shifts caused by their loading.
  • Preload Critical Resources: Preload essential content to prevent layout shifts and improve page stability.
  • Set Dimensions for Media: Specify dimensions for images and videos to reserve space and prevent layout recalculations.
  • Avoid Inserting Content Dynamically: Load content statically to prevent layout shifts and maintain visual stability.

Utilizing Preloading Techniques#

Leveraging preloading techniques can significantly reduce CLS and enhance user experience:

  • Preconnect: Establish early connections to essential resources to reduce latency and prevent layout shifts.
  • Prefetch: Load resources in advance to ensure they are readily available when needed, minimizing layout recalculations.
  • Prerender: Render pages in the background to preload content and improve page loading speed and stability.

Strategies to Reduce CLS#

Fortunately, there are several strategies that web developers and designers can employ to reduce Cumulative Layout Shift and improve overall user experience:

Properly Size Images and Videos#

Specify dimensions for all images and videos on your website to ensure that the browser can allocate the necessary space before loading them.

Avoid Dynamically Injected Content#

Minimize the use of dynamically injected content, such as ads or pop-ups, that can cause layout shifts during page load.

Optimize Web Fonts#

Ensure that web fonts are loaded asynchronously and use font-display: swap; to provide fallback options while fonts are loading.

Prioritize Critical CSS and Lazy Loading#

Prioritize the loading of critical CSS to display essential content first, and implement lazy loading for below-the-fold images and videos to reduce CLS.

Test and Monitor CLS Scores#

Regularly test and monitor CLS scores using tools like PulseWeb or Lighthouse to identify and address potential issues proactively.

FAQs (Frequently Asked Questions)

  1. What causes Cumulative Layout Shift (CLS)?

    CLS is often caused by unoptimized images and videos, third-party scripts, font-loading issues, and dynamically injected content.

  2. How does CLS affect user experience?

    CLS leads to unexpected layout shifts during page loading, causing frustration and decreasing user engagement.

  3. Why is CLS important for SEO?

    CLS is a key metric in Google's Core Web Vitals and impacts user experience, SEO rankings, and overall website performance.

  4. How can I reduce Cumulative Layout Shift on my website?

    Optimizing images and videos, preloading critical resources, setting dimensions for media, and avoiding dynamically injected content are effective strategies for reducing CLS.

  5. What tools can I use to measure CLS?

    Tools like PulseWeb, Lighthouse, and Google PageSpeed Insights provide detailed insights into CLS and other Core Web Vitals metrics.

  6. Is there an ideal CLS benchmark to aim for?

    A CLS score below 0.1 is considered good, while scores between 0.1 and 0.25 indicate room for improvement, and scores above 0.25 are considered poor.

  7. How can I test my website's CLS score?

    You can test your website's CLS score using tools like PulseWeb Website Speed Tool or Lighthouse, which provide detailed performance reports and recommendations for improvement.