In this in-depth guide, we explore everything from how CLS is calculated, to practical fixes using tools like PageSpeed Insights, Chrome DevTools, and Ahrefs Site Audit. Whether you’re a developer, marketer, or website owner, this is your one-stop guide to improving visual stability and achieving SEO success.
What Is Cumulative Layout Shift (CLS) and Why It Matters for Your Website
CLS measures the visual stability of a page as it loads. Specifically, it captures unexpected shifts in layout caused by elements loading asynchronously or without reserved space.
Understanding the Core Web Vitals and CLS Role
CLS is one of the three Core Web Vitals alongside Largest Contentful Paint (LCP) and First Input Delay (FID). Each one focuses on a different aspect of user experience. CLS measures stability, tracking how often elements jump as a user scrolls or interacts.
Real-World Impact of Layout Shifts on UX and SEO
Unplanned layout shifts cause users to click the wrong links, lose their reading spot, or get frustrated with your page. These experiences lead to higher bounce rates and lower conversions. According to the Chrome User Experience Report (CrUX), 72.8% of websites currently meet the recommended CLS threshold.
Why Google Prioritises CLS in Rankings
Google prioritises CLS because it reflects real-world usability. A clean, stable page that maintains its structure across devices and connections enhances trust and experience. As a result, websites that avoid layout shifts and improve CLS score higher in SERPs.
How CLS Is Calculated and Interpreted
CLS Formula Explained: Impact × Distance
Google calculates CLS with the formula:
Layout shift score = impact fraction × distance fraction
It measures the percentage of the viewport affected and how far elements move. The more disruptive the shift, the higher the score.
Key Metrics and Thresholds: Good vs Poor CLS
- Good: CLS ≤ 0.1
- Needs Improvement: CLS > 0.1 and ≤ 0.25
- Poor: CLS > 0.25
You need 75% of your page loads to meet the “Good” threshold for a passing score.
What Causes High CLS Scores: Visual Instability Explained
- Images without dimensions
- Late-loading fonts (FOIT/FOUT)
- JavaScript-injected content
- Ads or embeds without space reservation
- Uncomposited animations
These disrupt the visual flow, harming your CLS score and overall web performance.
What Are the Most Common CLS Issues in 2025?
Images, Videos, and Iframes Without Dimensions
If you fail to reserve space for media, the browser reflows content when those assets load. This leads to layout shifts on slow connections and higher CLS scores.
Ads and Embeds That Load Without Reserved Space
Display ads, embedded tweets, or YouTube videos without structural placeholders often shift other elements. CLS spikes especially on ad-heavy or editorial websites.
Font Swaps, FOIT, FOUT and Late Styling
When custom fonts load slowly, text switches mid-render. This causes a Flash of Invisible Text (FOIT) or Flash of Unstyled Text (FOUT). Both disrupt user reading experience and affect CLS.
Dynamic Content Injection Using JavaScript
Content injected post-load, such as reviews, banners, or forms, often shifts content downward unless space is pre-reserved.
Third-Party Scripts and Widgets Causing Delays
Over-reliance on third-party dependencies leads to CLS issues. Social widgets, chat tools, and analytics scripts delay content rendering or push other elements.
How to Measure CLS on Your Site
Field Data vs Lab Data: Which One Matters More
- Field data: Real user metrics (from CrUX, GSC)
- Lab data: Simulated, repeatable tests (from Lighthouse, PageSpeed Insights) Google uses field data for rankings, but lab data is useful for debugging.
Tools for Measuring CLS
- PageSpeed Insights: Shows both lab and field data
- Chrome DevTools: Use Performance tab to record layout shifts
- Google Search Console: View field data in the Core Web Vitals section
- Ahrefs Site Audit: Provides CLS and CWV data across domains
Using API and Bulk Checkers for Site-Wide Analysis
Tools like Ahrefs’ bulk CWV checker help analyse up to 10 pages or domains using your PageSpeed Insights API key. Results include:
- Page-level and origin-level CLS
- Desktop and mobile Lighthouse metrics
How to Improve Your CLS Score and Avoid Large Layout Shifts
Reserve Space for Visual Elements
Use HTML or CSS to reserve dimensions:
<img src=”cat.jpg” width=”640″ height=”360″ alt=”Cat with string”>
Use aspect-ratio for responsive designs. Define the largest space needed for ads, embeds, iframes to prevent mid-load shifts.
Optimize Font Delivery
- Use system fonts when possible
- Preload fonts with:
<link rel=”preload” href=”/font.woff2″ as=”font” crossorigin>
- Add font-display: swap in CSS
Manage Dynamic and Injected Content
- Avoid inserting above-the-fold content unless user-triggered
- Add placeholders using min-height or empty divs
Use Composited Animations Only
Avoid animating properties like top, left, height, width. Use transform and opacity instead.
.my-animation {
will-change: transform, opacity;
}
Ensure Changes Occur Within 500ms of User Actions
Delays over 500ms lead to layout shift penalties. Use prefetching, loading indicators, and responsive transitions.
Enable Back/Forward Cache (bfcache) Support
Make your pages eligible:
- Avoid unload event
- Minimize Cache-Control: no-store
- Close connections before navigation
Use Synchronous JavaScript to Control Visibility
Temporarily hide content using CSS. Reveal with render-blocking JS:
<style>.cls-target{display:none}</style>
<script>/* Populate content here */</script>
<style>.cls-target{display:block}</style>
Minimize Third-Party Dependencies
Audit external scripts. Replace with self-hosted alternatives. Lazy-load widgets.
CLS Optimisation for WordPress Sites
Using Breeze Cache Plugin to Improve Font Loading
Cloudways’ Breeze Plugin enables web font preloading and CSS minification, reducing FOUT/FOIT issues.
Fixing Image and Video Dimensions in WordPress Media Library
Upload and edit dimensions directly. WordPress themes like WoodMart or Astra often auto-resize, so override where needed.
Editing Theme Files to Prevent Shifts
Edit CSS via Theme Editor or FTP. Target headings, headers, and media containers with min-height or aspect-ratio rules.
Implementing CSS Fixes via Theme Customizer
Paste optimised CSS in Appearance > Customize > Additional CSS. Use will-change for smoother rendering.
Visual Stability Tips for E-commerce and Mobile UX
Reserving Dimensions for Product Images and Thumbnails
E-commerce stores often face shifting product grids. Set consistent image ratios and scale with media queries.
Avoiding CTA and Checkout Button Shifts
Define consistent container height and avoid injecting elements post-load.
Mobile CLS Challenges and Solutions for Slow Connections
Preload media assets and minimise third-party calls. Use lightweight fonts and native UI elements.
Tools to Monitor and Fix CLS in Real-Time
Top CLS Analysis Tools in 2025
- Ahrefs Site Audit (CrUX + Lighthouse)
- PageSpeed Insights (page/origin-level)
- Chrome DevTools (shift recording)
Using PageSpeed Insights API for Bulk Monitoring
Integrate into custom dashboards or use tools like Ahrefs’ CWV checker for up to 10 domains.
Automating Layout Shift Monitoring With Custom Scripts
Use PerformanceObserver API in JavaScript to capture layout shifts programmatically.
Case Study: Before and After Fixing CLS
Score Improvements (59 to 93)
Cloudways reported a jump in site performance after reducing CLS from 0.024 to 0.0.
Visual Comparison of CLS Reductions
Before: text and images shifted downward. After: locked layout during loading.
WordPress Optimization in Action
Preloading fonts, sizing images, and replacing third-party scripts led to the CLS improvement.
Future of CLS: What to Expect in Google’s Next Web Performance Updates
CSS aspect-ratio and New Layout Standards
Aspect-ratio now enables developers to set dynamic height with less code.
How Browsers Are Adapting to Reduce Layout Shifts
Chrome, Firefox, and Safari all enhance font handling, media allocation, and shift suppression.
Anticipated Changes to Core Web Vitals Scoring
Google may replace FID with Interaction to Next Paint (INP) and update CLS with new thresholds or timing windows.
Final Checklist to Maintain a Healthy CLS Score
- Use fixed width and height for media
- Preload and optimise fonts
- Insert content only after reserving space
- Use transform-based animations
- Reduce third-party reliance
- Monitor with both field and lab tools
Frequently Asked Questions (FAQ)
What is considered a large layout shift?
A layout shift scoring over 0.25 is considered poor by Google. It typically involves major visible changes during load.
Can I fix CLS without a developer?
Yes. WordPress users can use plugins like Breeze, optimise images via the Media Library, and edit CSS via Theme Customizer.
How do fonts cause layout shifts?
Slow-loading custom fonts switch visible text styles, creating a disruptive effect. FOIT/FOUT are common causes.
What’s the best CLS score for SEO?
A CLS score of ≤ 0.1 across 75% of page views is required to be marked “Good” by Google’s Core Web Vitals.
Do CLS improvements affect mobile rankings?
Yes. Mobile CLS data from CrUX is often what Google uses for mobile-first indexing and ranking.
Why Choose Tech Trends as Your SEO Agency?
Tech Trends is a performance-focused SEO agency that specialises in technical optimization, Core Web Vitals audits, and CLS remediation. Unlike generic providers, we bring:
- Real-world field data monitoring
- Expert CLS fixes for WordPress, Shopify, and custom CMS
- CrUX and Lighthouse data integration
- Page-by-page UX audits
Our team draws from real data — not assumptions. Whether you need site speed audits, layout debugging, or strategic SEO consulting, Tech Trends delivers results.
Learn more about Advanced Technical SEO and how Tech Trends can help you dominate SERPs.
