Reducing Cumulative Layout Shift (CLS): Best Practices for Web Performance
Understanding Cumulative Layout Shift
Cumulative Layout Shift (CLS) is a crucial metric for assessing website performance and user experience. It measures how much page elements move unexpectedly during loading, affecting visual stability and usability.
Defining CLS and Its Impact on User Experience
CLS quantifies the visual instability of a webpage as it loads. It calculates the sum of all layout shifts not caused by user interactions. A layout shift occurs when visible elements change position from one frame to the next.
High CLS scores can frustrate users. Imagine reading an article when suddenly the text jumps due to an ad loading above it. This disrupts the reading flow and may cause accidental clicks.
Low CLS scores, on the other hand, indicate a stable, pleasant browsing experience. Users can interact with the page confidently, knowing elements won’t shift unexpectedly.
Measuring CLS: Metrics and Tools
CLS is measured on a scale from 0 to 1, where lower scores are better. Google considers a score of 0.1 or less as “good,” 0.1 to 0.25 as “needs improvement,” and above 0.25 as “poor.”
Several tools can help measure CLS:
- Chrome DevTools
- Lighthouse
- PageSpeed Insights
- Web Vitals Extension
These tools provide insights into which elements contribute most to layout shifts. They often offer visualizations of shifts, helping pinpoint problem areas.
To calculate CLS, browsers look at viewport size and movement of unstable elements. The formula considers both the impact fraction (how much visible content shifted) and the distance fraction (how far the elements moved).
Optimization Strategies for CLS
Reducing Cumulative Layout Shift requires a multi-faceted approach. We’ll examine key techniques for optimizing images, fonts, and ad placements to minimize unexpected content shifts and improve user experience.
Image and Video Rendering Techniques
Specifying image and video dimensions in HTML is crucial for preventing layout shifts. We recommend using width and height attributes or CSS aspect ratio boxes. This allows browsers to reserve space before the media loads.
For responsive images, the srcset attribute helps serve appropriately sized images based on screen size. Lazy loading can improve performance, but implement it carefully to avoid shifts as content enters the viewport.
Consider using modern image formats like WebP, which offer smaller file sizes and faster loading. Compress images appropriately to balance quality and performance.
For videos, use poster images to reserve space while the video loads. This prevents sudden layout changes when the video becomes available.
Font Loading Best Practices
Web fonts can cause significant layout shifts if not handled properly. We suggest using font-display: optional or font-display: swap to control font loading behavior.
Preload critical fonts using to ensure they’re available as soon as possible. This reduces the likelihood of text shifting as custom fonts load.
Specify fallback fonts that closely match your custom fonts in size and style. This minimizes layout changes when switching between fallback and custom fonts.
Consider using variable fonts to reduce the number of font files needed, potentially speeding up load times and reducing shifts.
Ad Placement Optimization
Ads are a common source of layout shifts. We recommend reserving space for ad units in your layout, even before they load. Use placeholder elements with specific dimensions to hold the space.
Implement ads below the fold where possible to reduce their impact on initial page load. If ads must be placed above the fold, ensure they load quickly and don’t cause shifts in critical content.
Use sticky ads or fixed position elements for ads that remain in view while scrolling. This prevents content from jumping as users navigate the page.
Consider lazy loading ads as users scroll, but be cautious to avoid sudden shifts. Implement smooth transitions or animations when revealing new ad units to minimize perceived disruption.
Code-Level Improvements
Optimizing code can significantly reduce Cumulative Layout Shift. We’ll explore techniques for managing resource loading and implementing best practices in CSS and JavaScript to enhance layout stability.
Asynchronous and Deferred Loading
Asynchronous and deferred loading techniques help prevent layout shifts caused by late-loading resources. We can use the ‘async’ attribute for scripts that don’t affect the initial render, allowing them to load in the background.
For scripts that need to execute in order, the ‘defer’ attribute is ideal. It ensures scripts load after HTML parsing is complete but before the DOMContentLoaded event.
<script async src="analytics.js"></script>
<script defer src="non-critical.js"></script>
Lazy loading images and iframes also reduces initial page load time and minimizes layout shifts. We implement this using the ‘loading=”lazy”‘ attribute:
<img src="image.jpg" loading="lazy" alt="Description">
CSS and JavaScript Best Practices
Proper CSS practices are crucial for maintaining layout stability. We should set explicit width and height attributes for images and video elements to reserve space before they load:
<img src="image.jpg" width="640" height="360" alt="Description">
In CSS, using the ‘aspect-ratio’ property helps maintain consistent spacing:
.video-container {
aspect-ratio: 16 / 9;
}
For dynamically inserted content, we can pre-allocate space using placeholder elements or skeleton screens. This approach prevents sudden layout shifts when content loads.
In JavaScript, we should avoid directly manipulating styles that affect layout. Instead, we use CSS classes to toggle visibility or positioning:
element.classList.add('visible');
Layout Stability Techniques
Implementing effective layout stability techniques is crucial for reducing Cumulative Layout Shift. These methods help maintain visual consistency as pages load, enhancing user experience and improving Core Web Vitals scores.
Using Size Attributes for Media Elements
We recommend specifying width and height attributes for images, videos, and other media elements. This practice allows browsers to allocate the correct amount of space before the content loads. For images, we use the following HTML:
<img src="image.jpg" width="640" height="360" alt="Description">
CSS can be used to make images responsive while maintaining aspect ratio:
img {
max-width: 100%;
height: auto;
}
For videos, we apply similar principles:
<video width="640" height="360" controls>
<source src="video.mp4" type="video/mp4">
</video>
These techniques prevent layout shifts as media loads, creating a smoother viewing experience.
Reserving Space for Dynamic Content
To accommodate dynamic content like ads or asynchronously loaded elements, we reserve space in advance. This approach prevents unexpected shifts when content appears. We use placeholder divs with minimum heights:
<div class="ad-slot" style="min-height: 250px;"></div>
For variable content, we estimate the largest possible size and adjust as needed:
.dynamic-content {
min-height: 100px;
max-height: 300px;
overflow-y: auto;
}
We also use skeleton screens to indicate loading states, maintaining layout consistency until content arrives. This combination of techniques significantly reduces layout shifts and improves overall page stability.
Performance Monitoring and Maintenance
Continuous monitoring and maintenance are crucial for optimizing Cumulative Layout Shift. We’ll explore effective strategies to track performance and implement sustainable improvements.
Regular Audits with Performance Tools
Performance audits are essential for identifying CLS issues. We recommend using tools like Google Lighthouse, which provides detailed reports on layout shifts. Chrome DevTools offers a Performance tab to visualize layout shifts during page load. Web Vitals extension can be installed for real-time CLS monitoring.
PageSpeed Insights combines lab and field data, giving a comprehensive view of CLS performance. For larger sites, automated tools like Screaming Frog can crawl multiple pages and flag CLS problems. Regular audits help spot trends and regressions quickly.
Incorporating Performance Budgets
Performance budgets set clear targets for CLS and other metrics. We suggest establishing a CLS budget of 0.1 or lower for optimal user experience. This budget should be integrated into the development process.
Automated testing tools can be configured to alert when CLS exceeds the budget. Continuous integration pipelines should include CLS checks before deployment. Performance dashboards help teams track CLS over time and across different pages.
Budget enforcement encourages developers to consider layout stability in their designs. It’s important to review and adjust budgets periodically based on real-world data and changing site requirements.
Frequently Asked Questions
Cumulative Layout Shift (CLS) can be complex to understand and optimize. We’ll address key questions about measuring, fixing, and improving CLS across different platforms to enhance user experience and SEO performance.
What are some strategies for fixing Cumulative Layout Shift on WordPress sites?
Setting placeholders for dynamically-injected content is an effective strategy for WordPress sites. This prevents unexpected shifts as content loads. Using custom fonts with fallback options can also minimize layout shifts.
Another approach is optimizing image dimensions. By specifying width and height attributes for images, browsers can allocate the correct space before loading.
How can Cumulative Layout Shift be tested and measured effectively?
Chrome DevTools offers a straightforward method to measure CLS. Open DevTools, navigate to the Performance tab, and enable Web Vitals. Record the page load to view the Cumulative Layout Shift score.
Lighthouse is another valuable tool. Generate a performance report in Chrome DevTools to get a detailed CLS analysis. This provides insights into specific elements causing shifts.
In what ways can improving Cumulative Layout Shift impact SEO performance?
Reducing CLS can significantly boost SEO performance. Search engines like Google consider page experience signals, including CLS, in their ranking algorithms.
A low CLS score indicates a stable, user-friendly page. This can lead to improved search rankings and increased organic traffic. It also enhances user engagement, potentially reducing bounce rates.
What are common causes of large layout shifts and how can they be avoided?
Images without specified dimensions are a frequent cause of layout shifts. Always include width and height attributes for images to reserve space during loading.
Ads and embeds can also trigger shifts. Implement size-specific ad slots and use placeholders for embeds to maintain layout stability.
How does Cumulative Layout Shift affect user experience, and what are the best practices to improve it?
CLS directly impacts user experience by causing frustration when page elements move unexpectedly. This can lead to accidental clicks and difficulty reading content.
To improve user experience, prioritize above-the-fold content stability. Use CSS to reserve space for slower-loading elements. Avoid inserting content above existing content, except in response to user interactions.
What are the techniques to address Cumulative Layout Shift issues on Shopify platforms?
On Shopify, optimizing product images is crucial. Ensure all product images have consistent aspect ratios and specify dimensions in the theme code.
Lazy-loading off-screen images can improve initial load times. However, it’s important to reserve space for these images to prevent layout shifts as users scroll.
