Cumulative Layout Shift: Tips for Reducing CLS in Websites
Cumulative Layout Shift (CLS) is a key metric that gauges how stable your website’s content is visually during user interaction. Introduced by Google as part of Core Web Vitals, CLS tracks unexpected element movements within the viewport as a page loads.
Think about trying to click the “buy now” button on a product page, but an ad loads at the top, pushing the button down right when you’re about to click it. This annoying shift is exactly what CLS measures and helps you avoid.
Maintaining a good CLS score is important for a smooth user experience. It not only affects how visitors interact with your site but also plays a role in your search engine rankings.
CLS is one of Google’s Core Web Vitals, alongside Largest Contentful Paint (LCP) and Interaction to Next Paint (INP). Optimizing CLS can greatly enhance your website’s performance and boost user satisfaction.
In this article, I’ll explore Cumulative Layout Shift, explaining what it is, why it matters, and how you can reduce CLS on your website to improve user experience and SEO.
Understanding Cumulative Layout Shift
Defining CLS
Cumulative Layout Shift (CLS) is a metric within Google’s Core Web Vitals that measures the visual stability of a webpage during loading. It quantifies how much elements shift unexpectedly within the viewport as the page loads.
CLS is calculated using two components: the impact fraction and the distance fraction. The impact fraction measures the area affected by the shift relative to the viewport size. The distance fraction assesses how far elements move compared to the viewport height.
The CLS score is the product of these two fractions. A lower CLS score means better stability and user experience. Scores below 0.1 are good, while scores above 0.25 are considered poor.
Why It Matters
CLS is vital because it directly affects user experience and your website’s performance in search rankings. Unexpected layout shifts can frustrate users, leading to accidental clicks on the wrong buttons or links.
This frustration can increase your bounce rate and decrease overall engagement.
From an SEO standpoint, CLS is a ranking factor that contributes to your PageSpeed score. Google’s algorithm uses CLS, along with LCP and INP, to assess the quality of a webpage’s user experience.
Therefore, maintaining a good CLS score is essential for staying competitive in search rankings.
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Strategies to Minimize CLS
Reserve Space for Media and Ad Elements
To reduce CLS, reserve space for media and ad elements on your webpage. Without defined dimensions for images and videos, the browser may cause sudden shifts once these elements load.
Add width and height attributes to your image and video tags in HTML. This ensures the browser allocates the correct space, preventing unexpected shifts.
Similarly, reserve space for ads using CSS. Set a minimum height and width for ad containers with the `min-height` and `min-width` properties.
This practice maintains visual stability, even with dynamic ad sizes. Use media queries to adjust reserved space for different screen sizes, ensuring a stable layout across devices.
Load Assets Strategically
Strategic asset loading is another method to reduce CLS. Implement lazy loading for non-critical resources like images or scripts. This defers their loading until needed, preventing shifts from late-loading elements.
Use resource hints such as `preload` or `prefetch` to help the browser fetch essential assets early, minimizing unexpected layout changes.
Optimize animations by placing them in CSS. Control the timing and transitions of layout changes to ensure they occur smoothly after critical content loads.
This approach helps minimize abrupt shifts that can disrupt the user experience.
Optimize Font Loading
Web fonts can cause CLS due to loading delays. Use the `preload` attribute in the HTML `` tag or the `font-display: swap` CSS property. These methods ensure fonts load promptly, reducing layout shifts.
Use a fallback or system font initially, then load the custom font asynchronously. This prevents major layout shifts as custom fonts load. Additionally, specify font size and line height in CSS to help the browser allocate space for text.
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Advanced Techniques for Reducing CLS
Utilize New CSS Features
Leverage advanced CSS features to further reduce CLS. Use the `transform` property for animations instead of properties that trigger layout changes. Transformations do not cause layout shifts, making them ideal for animations and transitions.
Use CSS grid or flexbox for layout management. These systems are flexible and can adapt to content size changes without causing significant shifts.
Defining grid or flexbox layouts ensures elements adjust smoothly when new content loads or existing content changes size.
Monitor and Measure CLS Regularly
Regularly monitor and measure CLS to maintain a stable website. Tools like Google Pagespeed Insights and the Layout Instability API provide insights into your CLS score.
These tools help identify specific elements causing layout shifts, allowing you to address these issues effectively.
Analyze field data from real user sessions to understand how users experience your site in real conditions. This data-driven approach helps improve CLS.
Additionally, use synthetic testing in lab settings to identify CLS issues before they affect real users.
Adopt a Performance-first Design Approach
A performance-first design approach is essential for minimizing CLS. Design your website with performance in mind from the start, rather than as an afterthought.
Focus on lightweight and efficient code, minimize dynamic content, and ensure all elements load in a way that maintains visual stability.
Plan ads, iframes, and other embeds carefully. Reserve space for these elements to avoid disrupting the layout. A well-thought-out design strategy that considers each element’s impact on layout stability is key to achieving a good CLS score.
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Conclusion
In summary, Cumulative Layout Shift (CLS) is a vital metric affecting user experience and SEO rankings. To achieve a good CLS score, reserve space for media and ad elements, load assets strategically, and optimize font loading.
Use advanced CSS features, regularly monitor CLS, and adopt a performance-first design approach. Specifying image dimensions, reserving space for ads, avoiding dynamic content above existing content, and using optimal font loading techniques are essential for minimizing CLS.
Implementing these strategies will enhance user experience, improve SEO rankings, and drive more traffic and conversions to your website.
Take action today to optimize your CLS and provide a seamless user experience.
