This guide will unpack CLS, its significance for your website, and its intricate relationship with SEO and user experience.
What is Cumulative Layout Shift (CLS)?
Cumulative Layout Shift (CLS) quantifies how often users experience unexpected layout shifts during a page’s lifespan. A high CLS score indicates a poor user experience, while a low score suggests a stable and predictable page layout.
CLS is a performance metric that measures the sum total of all individual layout shift scores for every unexpected layout shift that occurs during the entire lifespan of the page. A layout shift occurs any time a visible element changes its position from one rendered frame to the next.
Why CLS Matters for Your Website
- User Experience: A website with a high CLS can frustrate users, leading to a poor user experience. Users may struggle to interact with the page, resulting in decreased engagement and increased bounce rates.
- SEO Rankings: Search engines like Google have started to incorporate page experience signals, including CLS, into their ranking algorithms. A poor CLS can negatively impact your site’s search rankings.
- Conversion Rates: Websites that are visually stable and provide a smooth user experience are more likely to retain users, improving the chances of converting visitors into customers.
The Relationship Between CLS and SEO
The connection between CLS and SEO is significant, with direct and indirect impacts on search rankings and overall website performance.
How CLS Affects Search Rankings
- In May 2021, Google officially made page experience a ranking factor, including metrics such as CLS. Websites that demonstrate better page experience, signified by lower CLS scores, are likely to enjoy higher search rankings.
- CLS is part of the Core Web Vitals, a set of real-world, user-centered metrics that quantify key aspects of the user experience. A poor CLS score can signal to search engines that your site may not provide a good user experience, potentially lowering its rankings.
CLS and User Experience
- Direct Impact on Engagement: Users are less likely to stay on a site that shifts around as it loads. This can lead to increased bounce rates and reduced time on site, both of which are negative signals to search engines.
- Indirect Impact Through Behavioral Signals: Search engines use behavioral signals, such as bounce rate and session duration, as indirect indicators of page quality. A poor CLS can worsen these signals, further impacting SEO.
- Conversion Optimization: Beyond SEO, a stable page layout is crucial for maintaining user trust and facilitating conversions. Users are more likely to complete purchases and engage with content on sites that offer a seamless and stable browsing experience.
Improving CLS is not just about boosting SEO rankings; it’s about committing to providing a superior user experience. By focusing on visual stability, you can enhance user satisfaction, increase engagement, and ultimately achieve better search engine visibility.
Essential Tools for Measuring and Diagnosing CLS Issues
Addressing CLS issues not only enhances user experience but also contributes positively to a site’s SEO performance. This section covers the essential tools for measuring and diagnosing CLS issues, offering a comprehensive guide to monitoring and improving your site’s visual stability.
Key Tools for Monitoring Website CLS
Below are several tools that you should employ to monitor the cumulative layout shift of your site pages.
Using Google’s Core Web Vitals Report
The Core Web Vitals Report in Google Search Console provides a user-centric analysis of real-world data, focusing on metrics that impact user experiences, including CLS.
How to Use:
- Navigate to Google Search Console, select your property, and click on “Core Web Vitals” under “Enhancements”. This report categorizes URLs based on their performance (Good, Needs Improvement, or Poor), allowing you to identify pages that require attention.
Benefits:
- Real-world user data: Understand how your visitors experience your site.
- Actionable insights: Prioritize which pages to fix based on their impact on user experience.
Leveraging Chrome User Experience Report for CLS Insights
The Chrome User Experience Report (CrUX) provides public, real-world usage data from millions of Chrome users worldwide, which can be accessed via BigQuery, PageSpeed Insights, and other tools.
How to Use:
- Utilize the CrUX Dashboard on Data Studio to visualize metrics or query the CrUX dataset on BigQuery for advanced analysis. Look specifically at the `layout_instability.cumulative_layout_shift_score` metric.
Benefits:
- Comprehensive: Offers insights across millions of sites and users.
- Customizable: Allows for deep dives into specific segments or timeframes.
Chrome DevTools
- Offers a visual representation of shifts, allowing developers to pinpoint the problematic elements.
- How to Use: Open DevTools > More Tools > Rendering > Enable “Layout Shift Regions”.
WebPageTest
- Provides detailed information on layout shifts and other performance metrics for any webpage.
- How to Use: Run a test on WebPageTest.org and review the “Cumulative Layout Shift” timeline under the “Performance review” section.
Lighthouse
- An open-source, automated tool for improving the quality of web pages. It has audits for performance, accessibility, progressive web apps, and more.
- How to Use: Run Lighthouse in Chrome DevTools under the “Lighthouse” tab or as a CLI tool, focusing on the “Avoid large layout shifts” audit.
How To Diagnose CLS Problems
Once you’ve leveraged the tools above to collect data about potential CLS problems on your site, the next step is to understand what elements on page are contributing to a high CLS score.
Identifying Common Layout Shifts
- Common causes include images without dimensions, dynamically injected content, web fonts causing FOIT/FOUT, and actions waiting for a network response before updating DOM.
- Use the Performance tab in Chrome DevTools to record and review layout shifts.
- Pay attention to elements tagged with a red ‘shift’ line in the experience section.
Other Common Causes of Poor CLS Performance and How to Address Them
Understanding the common causes of poor CLS performance and how to address them is essential for web developers and site owners. This section delves into the typical culprits behind CLS issues and offers strategies to mitigate their impact.
Common Causes of CLS Issues
Below are some of the primary causes that contribute to cumulative layout shifts:
Images and Ads Without Dimensions:
- Cause: When images or ads lack specified dimensions (width and height), the browser cannot allocate the appropriate space while the page is loading. This results in content jumping around as images and ads load, contributing to a higher CLS.
- Solution: Always include width and height attributes for images and ads. This practice allows the browser to reserve the necessary space, reducing unexpected layout shifts.
Dynamically Injected Content:
- Cause: Content added to the page dynamically (e.g., through JavaScript) without reserving space can push down existing content, increasing CLS. This is common with ads, pop-ups, or content loading based on user interaction.
- Solution: Reserve space for any dynamically injected content by setting minimum height or width, depending on the content’s orientation. For ads, consider using a placeholder or skeleton screens to maintain layout stability.
Strategies for Mitigating CLS Impact
Below are tactics to address any cumulative layout shift impact that your pages may be experiencing:
Implementing Aspect Ratio Boxes:
- To ensure images and videos don’t cause layout shifts as they load, employ aspect ratio boxes. This technique involves:
- Calculating the aspect ratio (width divided by height) of the media.
- Using CSS to set a container’s `padding-top` to match this ratio, creating a box that reserves the exact space needed for the media.
- This prevents shifts by maintaining the layout as the media loads.
Optimizing Font Loading and Visibility
- Font loading can significantly affect CLS, especially if fallback fonts have different sizes or styles. To minimize this:
- Preload key fonts: Use `<link rel=”preload”>` in the header for critical fonts to prioritize their loading.
- Use font-display: CSS property `font-display: swap;` ensures text remains visible during font loading, using fallback fonts and then swapping to the intended font once loaded. This reduces layout shifts related to font loading.
- Match fallback font sizes: Ensure fallback fonts closely match the intended font’s size and line-height to minimize layout adjustments when the final fonts load.
By addressing these common causes and implementing the suggested strategies, web developers and site owners can significantly improve their site’s CLS performance. This not only enhances the user experience but also contributes positively to SEO, as search engines increasingly prioritize page experience metrics.
Advanced Techniques for CLS Optimization
Below are additional, more advanced techniques to address cumulative layout shift issues:
Utilizing CSS for Layout Stability
CSS can be a powerful tool for enhancing layout stability. By leveraging CSS properties and techniques, you can significantly reduce CLS and improve the overall user experience.
- Content-visibility Property: Use the `content-visibility` property to skip rendering offscreen content until needed. This can reduce layout shifts and improve loading times.
- Transforms over Top/Left Positioning: For animations or dynamic elements, use CSS transforms instead of changing the top or left properties. Transforms are less likely to cause layout shifts.
- FOIT/FOUC Strategies: Avoid flash of invisible text (FOIT) and flash of unstyled content (FOUC) by using font display strategies and ensuring stylesheets load promptly.
- Debounce Scrolling Events: Implement debouncing for scroll events to prevent layout shifts caused by rapid firing events.
- Load on User Request: For non-essential elements, consider loading them only upon user request or interaction. This approach can significantly reduce initial page load time and minimize potential layout shifts.
- Animation and Transition Use: Apply subtle animations or transitions for elements that may shift or load dynamically. This approach can make shifts appear intentional and less jarring, improving the perceived stability.
By implementing these foundational strategies and advanced techniques, you can effectively reduce CLS, enhancing both the user experience and your website’s SEO performance. Remember, the goal is to create a stable, responsive, and engaging environment for your users, which in turn supports your site’s visibility and success.