Cumulative Layout Shift (CLS) is a vital metric employed by Google to assess the visual stability of a webpage during its loading process. It constitutes one of the three Core Web Vitals metrics and is computed within a maximum shifting period of five seconds.
CLS disregards anticipated layout shifts following user actions within 500 ms, as unexpected shifts can inadvertently lead to clicks and disrupt the user experience. A satisfactory CLS score is defined as ≤ 0.1, necessitating that 75% of page loads achieve a score of 0.1 or lower.
CLS can be measured using field data obtained from Chrome User Experience Report (CrUX) for real-world performance or lab data for repeatable testing. To enhance CLS, recommended strategies include:
- Reserving space for images, videos, iframes, and dynamic content
- Optimizing fonts to minimize delays and swapping
- Utilizing transform animations instead of properties like box-shadow
- Ensuring page eligibility for bfcache to prevent layout shifts during back/forward navigation.
Additional resources and tools such as Pagespeed Insights and Google Search Console aid in the measurement and optimization of CLS.
What is CLS?
Cumulative Layout Shift (CLS) is a metric that measures the visual stability of a page during loading, and it is one of the three Core Web Vitals metrics used by Google. CLS is defined as the sum of all individual layout shift scores within a page. It quantifies the amount of unexpected content movement that occurs during the loading process.
CLS has a significant impact on user experience, as it can lead to accidental clicks or hinder the ability to interact with the page. Common CLS issues include images and videos without specified dimensions, dynamically injected content, and ads.
To improve CLS, best practices include reserving space for images and dynamic content, optimizing fonts to avoid swapping and using transform animations instead of properties like box-shadow. Several case studies have demonstrated the effectiveness of these strategies in reducing CLS and improving user satisfaction.
Measurement of CLS
The measurement of CLS involves assessing the visual stability of a webpage during its loading process, taking into account unexpected layout shifts that can occur within a five-second timeframe. CLS measurement techniques include field data from Chrome User Experience Report (CrUX), which provides real-world performance, and lab data that is useful for repeatable tests but not used for Core Web Vitals.
Factors affecting the CLS score include images and videos without specified dimensions, fonts causing delays or swapping, and animations without proper optimization. Common CLS issues can lead to poor user experience, such as elements shifting during page load.
Solutions for reducing CLS include reserving space for elements, optimizing fonts to prevent delays and swapping, and using transform animations instead of properties that cause shifts. CLS also has an impact on SEO, as Google considers it one of the Core Web Vitals metrics.
Implementing best practices for reducing CLS can improve user experience and search engine rankings.
Improving Layout and Space
Reserving space for elements and optimizing layout can contribute to the improvement of Cumulative Layout Shift (CLS). By considering the responsive design and ensuring elements have predetermined dimensions, unexpected shifts can be minimized.
One way to achieve this is by specifying the height and width of images using the “img” tag and utilizing the “srcset” attribute for responsive images. Additionally, reserving space for videos, iframes, and dynamic content such as ads can help maintain visual stability. Incorporating the “aspect-ratio” CSS property can assist in dynamic width calculations.
Font loading optimization is another important aspect of improving CLS. Loading fonts quickly and avoiding font swapping can prevent delays and shifts. Utilizing system fonts whenever possible and combining “link rel=preload” with “font-display: optional” for custom fonts can enhance performance.
Animation optimization, such as using transform animations instead of properties like box shadow, can also contribute to minimizing layout shifts. Lastly, implementing cache strategies, like ensuring pages are eligible for bfcache and testing for instant cache loading, can further enhance CLS performance.
| Improving Layout and Space | Ensure the eligibility for bfcache Avoid unloading events and minimize the use of Cache-Control: no-store Test for instant cache loading |
|---|---|
| Responsive design | Specify height and width for images Use srcset for responsive images Reserve space for videos, iframes, and dynamic content Consider aspect-ratio CSS property for dynamic width calculations Avoid layout shifts caused by certain CSS property values |
| Dynamic content | Specify height and width for videos, iframes, and dynamic content Use aspect-ratio CSS property for dynamic width calculations Avoid layout shifts caused by certain CSS property values |
| Font loading | Load fonts as fast as possible Avoid font swapping Use system fonts when possible Combine link rel=preload and font-display: optional for custom fonts Avoid layout shifts caused by certain CSS property values |
| Animation optimization | Use transform animations instead of properties like box-shadow Prevent layout shifts during animations |
| Cache strategies | Ensure the eligibility for bfcache Avoid unloading event and minimize the use of Cache-Control: no-store Test for instant cache loading |
Optimizing Fonts
Optimizing fonts is an important aspect of improving the visual stability of a webpage and reducing unexpected layout shifts. To achieve this, several strategies can be employed:
- Preloading fonts: By preloading fonts, the browser can start fetching them early in the loading process, reducing the delay in displaying text content.
- Font display options: The font-display property allows web developers to control how fonts are displayed while they are being loaded. Options like ‘optional’ can be used to ensure that the default system font is shown initially, reducing layout shifts caused by font loading.
- System fonts vs Custom fonts: Using system fonts can eliminate delays and shifts caused by font loading altogether. System fonts are already installed on users’ devices, leading to faster rendering.
- Combining fonts and layout shifts: Care should be taken when combining multiple fonts on a webpage, as different font files may have different loading times, leading to layout shifts. Ensuring that font files are similar in size and load time can help minimize these shifts.
Animation and Caching
Animation and caching are two essential factors that contribute to improving the visual stability and user experience of webpages.
Transform animations, when used instead of properties like box-shadow, play a significant role in preventing layout shifts and maintaining visual stability during animations.
Caching techniques, particularly bfcache strategies, are crucial in avoiding layout shifts when users navigate back or forward in their browsing history. To ensure eligibility for bfcache, web developers can follow certain practices such as avoiding the use of the unload event and minimizing the use of Cache-Control: no-store.
Font optimization is another important aspect that can contribute to reducing layout shifts. By loading fonts quickly and avoiding font swapping, delays and shifts can be minimized. Web developers can also enhance visual stability by utilizing system fonts and combining link rel=preload with font-display: optional for custom fonts.
Field Data vs Lab Data
Field data and lab data are two distinct methods used in measuring the Cumulative Layout Shift (CLS) metric.
Field data provide real-world performance information and is useful for assessing Core Web Vitals. It allows for the measurement of CLS in real-world scenarios, providing insights into how a website performs in actual user interactions. Field data reflects the impact of external factors on CLS, such as varying network conditions and user devices. It also captures a wide range of user interactions, providing a comprehensive understanding of CLS in different usage scenarios.
On the other hand, lab data is collected in a controlled environment, allowing for repeatable tests and the identification of specific issues. Lab data helps isolate and address individual CLS-related problems. It provides a controlled environment for precise measurements, which can be useful for troubleshooting and optimizing CLS performance.
However, there are limitations to both field data and lab data. Lab data may not accurately reflect real-world performance due to the absence of external factors. It may not capture all possible scenarios, leading to limitations in its accuracy. Field data, while providing valuable insights into real-world performance, may be influenced by factors that are beyond the control of the website owner.
Pagespeed Insights and Search Console
Transition: Moving on to the comparison of CLS performance analysis between Pagespeed Insights and Search Console, these tools provide valuable insights for understanding and optimizing CLS.
| CLS Performance Comparison | Pagespeed Insights | Search Console |
|---|---|---|
| Metrics Analyzed | Lab data for a single URL | Field data for many URLs or entire site |
| CLS Score | Provides a score for the analyzed URL | Provides CrUX data for multiple URLs |
| Data Granularity | Limited to the analyzed URL | Provides a broader view of CLS trends |
| Recommendations | Offers optimization suggestions for the analyzed URL | Identifies common CLS issues and provides recommendations |
| Improvement Strategies | Use the insights to optimize CLS for the analyzed URL | Leverage the data to implement site-wide CLS improvement strategies |
Analyzing CLS trends in Pagespeed Insights and Search Console data can help identify areas for improvement. Pagespeed Insights provides recommendations specific to the analyzed URL, while Search Console identifies common CLS issues across multiple URLs. By leveraging the insights from both tools, website owners can implement targeted optimization strategies to enhance CLS performance.
Additional Resources
The topic of additional resources for optimizing CLS involves exploring various tools and strategies that can contribute to improving the visual stability of web pages. CLS optimization techniques encompass a range of best practices for reducing layout shifts. These practices include:
- Reserving space and layout for images, videos, iframes, and dynamic content
- Optimizing fonts to minimize delays and swapping
- Utilizing transform animations instead of properties that can cause layout shifts
Case studies on improving CLS scores provide valuable insights into successful optimization strategies implemented by different websites. It is also important to be aware of common mistakes that contribute to poor CLS, such as not reserving space for content and improperly loading fonts.
Understanding the impact of CLS on SEO and user engagement reinforces the significance of optimizing CLS for a positive user experience and improved search engine rankings.
Conclusion
In conclusion, understanding Cumulative Layout Shift (CLS) is crucial for optimizing webpage performance and user experience. By measuring CLS using field data or lab data, website owners can identify areas for improvement.
Reserving space for content, optimizing fonts, using transform animations, and ensuring eligibility for bfcache are effective solutions to reduce CLS.
Tools like Pagespeed Insights and Google Search Console provide valuable insights for measuring and optimizing CLS.
By implementing these recommendations and utilizing additional resources, web developers ca
n achieve a better CLS score and enhance the overall user experience.
