CLS optimization: How to stabilize layouts, improve SEO & boost conversions

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What is CLS? (Cumulative Layout Shift)

Simply put: CLS measures how much your page’s elements shift unexpectedly as it loads. Have you ever tried clicking a button, only for it to move at the last second, making you click something else? That’s CLS in action—and it’s a major frustration for users.

Why CLS is important?

CLS isn’t just another fancy metric—it directly affects user experience, engagement, and your site’s credibility. If your CLS score is poor, visitors are more likely to become frustrated, and abandon your site, and Google could demote your rankings. With a strong CLS, you get a smooth, stable site that users trust and engage with. Other benefits of a well-performing CLS include:

🔍 SEO Boost: CLS is a core part of Google’s Core Web Vitals. High CLS can negatively impact rankings, reducing your organic traffic.

💡 User Experience: Sudden shifts make a site feel broken. A stable page keeps users engaged and prevents frustrating missclicks.

💰 Revenue Impact: Shifty layouts hurt conversions. Whether it’s an “Add to Cart” button or a checkout form, unexpected movements can cost you real money.

How to test CLS?

Before optimizing, you need to measure your CLS score. Here’s how:

1️⃣ Google PageSpeed Insights (PSI):

To start, visit Google’s PSI assessment page here. Then, simply enter your URL, and it will analyze your page’s performance on both mobile and desktop devices. It provides a CLS score based on real-world user data (from the Chrome User Experience Report or CrUX) and lab data for controlled testing.

🔍What makes Google’s Page Speed Insights unique?

• It combines field data (real-world user experiences) and lab data (simulated performance tests) for a well-rounded analysis.
• It offers a tailored to-do list to help you tackle the most urgent CLS issues your site is facing.

📌 Pro Tip: Scroll down to the Diagnostics section for detailed insights. You can even filter results to focus on the specific factors affecting CLS.

2️⃣Chrome DevTools (Performance tab):

Open Chrome’s Performance tab and reload your page. You can do so by right-clicking on a website and selecting Inspect. Then navigate to the Performance tab. It will highlight the exact element contributing to CLS and its position.

🔍 What makes this tool unique? Unlike other tools, DevTools lets you see a detailed timeline of your page load, helping you:

• Pinpoint the exact moment and cause of layout shifts.
• Identify whether images, CSS, or third-party scripts are responsible.
• Easily track and debug the elements that are affecting your site’s stability.

3️⃣ Google Search Console (Core Web Vitals report):

This tool gives you a site-wide overview of CLS issues, based on real-world performance data collected from users visiting your site on Chrome.

🔍 What makes this tool unique?

• It provides insights at scale by analyzing CLS across all your pages, not just a single URL.
• It highlights recurring CLS issues on specific devices (desktop vs. mobile) or regions, helping you identify patterns.
• It tracks performance changes over time, so you can measure the impact of any optimizations you make.

📌 Pro Tip: Use the Core Web Vitals Report to prioritize fixing issues on pages with the highest traffic first—this will give you the biggest SEO gains with the least effort.

What’s a good CLS score?

✅ Good: ≤ 0.1 (Stable and user-friendly)
⚠️ Needs Improvement: 0.1–0.25 (There’s potential here)
❌ Poor: > 0.25 (Users are bouncing, and Google notices)

How to optimize CLS?

Fixing CLS is about ensuring your page’s elements don’t shift unexpectedly. This means setting proper dimensions, avoiding dynamically injected content, and optimizing fonts and ads. Here’s how to stabilize your site:

Set explicit width & height for images and videos

💡 Why? If an image or video loads without predefined dimensions, it causes a shift when the browser figures out its size.

📌 How to do it? Use width and height attributes in <img> and <video> elements. For responsive images, use the CSS aspect ratio.

Reserve space for ads & embeds

💡 Why? Third-party content like ads, iframes, or dynamic widgets often loads after the main content, causing layout shifts. To prevent this, you should reserve space for these elements during the page load.

In simple terms: Think of it like reserving a parking spot for an incoming guest—by making sure the space is already set aside, nothing else gets moved around when they arrive.

1. Set fixed sizes for ads & iframes Ads and embeds are often the culprits behind unexpected shifts. By assigning fixed dimensions to these elements, you ensure that the rest of your page layout doesn’t get impacted when they load.

📌 How to do it?

• Define a fixed width and height for ad containers or embedded content (like videos or widgets).
• Use CSS to reserve space for third-party content.

<div style="width: 300px; height: 250px;">
  <!-- Ad script here -->
</div>

2. Load Content on a Different Layer (Without Affecting Layout)

By ensuring that a pop-up or banner loads on a separate layer, you avoid shifting any content that is already rendered. This technique allows for elements like pop-ups to appear over the existing page without disturbing its layout.

In simple terms: It’s like placing a transparent sheet over a well-organized desk rather than shifting the items around.

📌 How to do it?

• Set the element’s position to fixed or absolute to ensure it doesn’t disrupt the layout of other elements.
• Use z-index to ensure it appears on top of the content.
• Add a transparent background behind the pop-up content to ensure it doesn’t interfere with other content.

Preload Fonts to avoid Flash of Invisible Text (FOIT)

💡 Why? Web fonts can load late, causing a flash of invisible text or layout shifts as the fonts appear. To solve this, preload fonts to ensure they are ready before the text is displayed.

In simple terms: It’s like ensuring your guests have their name tags ready before they enter the room, so they don’t appear as blank spaces.

Preload Critical Fonts Fonts should be preloaded so that text appears immediately without a flash of invisible text.

📌 How to do it?

• Add a <link rel="preload"> tag to the <head> to load the fonts in advance.

<link rel="preload" href="/fonts/brand.woff2" as="font" type="font/woff2" crossorigin>

How Uxify’s agents help with CLS

Most of the fixes above live in your code, and they’re worth doing. Here’s where agents fit, and where they don’t.

Watching CLS: Vita. Layout shift regresses quietly. A new app, a late banner or a theme update can push CLS past 0.1 on one template or one device while the site-wide number still looks fine. Vita is the Uxify agent that watches Core Web Vitals around the clock, CLS included, on every page and device. When a metric really moves, it tells you the page, when it changed and the likely cause, so you find out before your CrUX data does.

Fixing CLS: Unshift (coming soon). Unshift is the agent we’re building for CLS itself. It finds the element that moves, on which page and which device, from real visitor sessions. Then it applies a fix and tests it before keeping it.

Where Navigation AI fits. Navigation AI isn’t a CLS fixer. It owns navigation speed: it predicts the next click from real sessions and prerenders that page with the Speculation Rules API. A prerendered page does its loading in the background, so when the visitor clicks, it lands already laid out. Shifts that happen once the page is on screen, like a chat widget that pops in or an ad that resizes, still need the fixes above.

🌎 Works on any website

Vita and Navigation AI switch on from the one Uxify snippet, with no code changes, no config and no rules to write. Unshift will run on the same snippet.

💡 Why This Matters for Your Business

• Better SEO: Google rewards stable, fast-loading sites with higher rankings.
• More Conversions: Stable experiences lead to more sales and sign-ups.
• Happier Users: No unexpected layout shifts = better engagement and enhanced user perception.

Final Thoughts

CLS is more than an inconvenience. It’s a dealbreaker for engagement, rankings, and conversions. Fixing it means better SEO, happier users, and higher revenue. Start with the fixes above, let Vita tell you the moment CLS slips on a page, and keep an eye out for Unshift, the agent that will fix layout shifts for you.

Lower CLS = Better UX, Higher Conversions, and More Revenue.

Looking for more performance tips?

If you’re interested in diving deeper into performance optimization strategies, be sure to explore our Performance Tips section.