CLS

⚡ What is Cumulative Layout Shift (CLS)?

Cumulative Layout Shift (CLS) is a metric used in web performance to measure the visual stability of a page. It looks at how much a webpage's elements unexpectedly shift during the loading phase, disturbing the user experience.

⭐ Why is CLS Important in Site Performance?

CLS is a key factor in evaluating user experience as part of Google's Core Web Vitals. Unexpected layout shifts can lead to a frustrating experience for users, causing them to click on the wrong elements and detracting from the goal of a seamless visit.

⚙️ How Does CLS Work?

1. As a page loads and elements show up, CLS calculates the total of all unexpected layout shifts.
2. A layout shift score is calculated by multiplying the impact fraction (how much space it takes) and the distance fraction (how far it moves).
3. The final CLS score for a page is the sum of all individual layout shift scores occurring from load to when the page becomes visible.

📌 Examples of CLS Issues

• Images without size attributes that load later and push down content.
• Adverts or embeds loading asynchronously and causing a shift.
• Dynamically injected content like banners or forms pushing other content.

✅ Best Practices for Reducing CLS

• Always include size attributes on your images and video or reserve the required space with CSS.
• Avoid inserting content above existing content, except for when responding to a user interaction.
• Utilize transitions for animations to help minimize layout shifts.

⚠️ Common Mistakes Leading to High CLS

• Omitting width and height attributes for images.
• Adding new elements dynamically without considering space hanging.
• Using unstable third-party content like ads that have shifting dimensions.

🛠️ Tools for Measuring CLS

• Web.dev Test - Insights into Core Web Vitals including CLS.
• Google Search Console - Check your website's performance and see the Web Vitals reports.
• Lighthouse in Chrome DevTools - Analyze CLS score along with other performance metrics.

📊 Quick Facts About CLS

• CLS is part of Google's Core Web Vitals, a collection of metrics needed for ranking.
• A good CLS score is 0.1 or less, whereas scores above 0.25 are considered poor.
• Non-visual elements or those below the fold can also contribute to CLS.

❓ Frequently Asked Questions About CLS

🔍 Related Topics in Site Performance

• CDN Tiered Caching
• Client Hints
• Code Splitting
• Connection Pooling

📚 Learn More About CLS

• Google: Core Web Vitals

📝 Key Takeaways

• CLS measures visual stability and is a crucial metric in Core Web Vitals.
• A lower CLS score indicates a more stable page layout, enhancing user experience.
• Adhering to best practices for CLS can prevent disruptive shifts and improve SEO.