Cumulative Layout Shift (CLS): What Is CLS & How to Improve It

You’ve felt it. That sudden, jarring lurch on a webpage. The button you were about to tap vanishes, replaced by an intrusive ad. Or maybe the text shifts, making you lose your place entirely. This isn't just annoying; it's a brutal user experience. And for Google, it’s a critical red flag.

Here's the truth: that digital rug pull has a name. It’s called Cumulative Layout Shift (CLS), and it’s one of the most important metrics for your website's success. Ignoring it means you're actively pushing users away and hindering your search rankings. But don't worry. You're about to get the rock-solid strategies you need to fix it.

The Brutal Truth About Cumulative Layout Shift

So, what exactly is Cumulative Layout Shift? Simply put, it measures the visual stability of your webpage. It quantifies how much unexpected movement of visible content occurs during the loading and lifespan of your page. A high CLS score means your page is jumping all over the place, causing massive frustration for anyone trying to interact with it.

This isn't some obscure technical detail. CLS is one of Google's Core Web Vitals, a set of metrics that Google uses to evaluate user experience. And because user experience is paramount, a poor CLS score can directly impact your search engine rankings. Google wants to send users to stable, reliable sites, not those that feel like a digital rollercoaster.

Think about it this way: you wouldn't build a house with a constantly shifting foundation, right? Your website is no different. Users expect a stable, predictable interface. When content unexpectedly moves, it breaks their concentration, leads to misclicks, and shatters trust. It's like trying to read a book while someone keeps shaking the page.

The impact isn't just about user frustration, though that's significant enough. High CLS translates directly into business losses. You'll see higher bounce rates because users give up and leave. Conversion rates will drop because misclicks lead to abandoned carts or unsubmitted forms. And yes, your SEO will suffer, pushing your valuable content further down the search results.

I once worked on an e-commerce site where users consistently complained about accidentally clicking on competitor ads that suddenly appeared above product descriptions. This wasn't just a minor annoyance; it was a direct funnel for lost sales. Our CLS score was abysmal, and the layout shifts were literally costing us money every single day. Fixing it became a top priority, not just for SEO, but for the bottom line.

Unmasking the Culprits: What Causes CLS?

Layout shifts don't happen by magic. They are the direct result of specific coding and loading patterns. Understanding these common culprits is your first step toward achieving a stable, user-friendly page.

Images Without Dimensions

This is the classic, undisputed champion of CLS offenders. You see, when a browser loads an image, it needs to know how much space to reserve for it. If you don't explicitly tell it the image's width and height, the browser initially renders the page without that space. Then, once the image file finally loads, the browser suddenly has to make room, pushing all the content below it down the page. This creates a massive, jarring shift.

Dynamically Injected Content

Imagine you're reading an article, and suddenly, a banner ad or a newsletter signup form pops into existence right in the middle of the text. This is dynamically injected content, and it’s a huge source of layout shifts. These elements often load after the main content, pushing everything else around to make space for themselves. This is especially common with third-party ads or widgets that have unpredictable loading times.

Web Fonts Causing FOIT/FOUT

Web fonts are beautiful, but they can be tricky. When a browser loads your page, it might initially display text using a default system font. Then, once your custom web font finishes downloading, the browser "swaps" it in. If the custom font has different metrics (like character width or line height) than the fallback font, this swap can cause the text to reflow, shifting surrounding elements. This is known as a Flash of Invisible Text (FOIT) or Flash of Unstyled Text (FOUT).

Third-Party Embeds

Social media feeds, video players, map embeds, comment sections – these are all examples of third-party content. They're incredibly useful, but they often come with their own loading behaviors and unpredictable dimensions. If you embed a YouTube video without specifying its size, the browser won't know how much space to reserve, leading to a layout shift once the video player finally renders. It’s a common problem with content you don't fully control.

Animations Without transform

Animations can add a dynamic, engaging feel to your site. But if you're animating properties like top, left, width, or height, you're directly manipulating the page's layout. Each frame of that animation forces the browser to recalculate the entire page's layout, which is expensive and often causes noticeable shifts. These types of animations are layout-triggering, and they are a direct path to a high CLS score.

Measuring Your CLS Score: Tools of the Trade

You can't fix what you don't measure. Understanding your current Cumulative Layout Shift score is the essential first step. Fortunately, Google provides a suite of powerful tools to help you diagnose and track your performance.

Lab Data vs. Field Data

Before diving into specific tools, you need to grasp the difference between lab data and field data. Lab data comes from simulated environments; it's what tools like Lighthouse can tell you about a page before it's even live or under controlled conditions. Field data, on the other hand, comes from real users interacting with your site in the wild. Both are crucial. Lab data helps you debug and test fixes quickly, while field data confirms whether your changes are actually improving the experience for your audience.

Google PageSpeed Insights

This is your go-to diagnostic tool. Just plug in your URL, and PageSpeed Insights will give you a comprehensive report, including your CLS score. It clearly labels whether your score is "Good," "Needs Improvement," or "Poor." More importantly, it provides actionable recommendations on how to fix identified issues. It's like a quick health check for your page.

You'll see both field data (from the Chrome User Experience Report, if available) and lab data (from a Lighthouse run). Pay close attention to the "Opportunities" section; it often highlights specific elements causing layout shifts.

Chrome DevTools (Lighthouse & Performance Panel)

For a deeper dive, Chrome DevTools is your best friend. Open DevTools (F12 or right-click > Inspect) and navigate to the "Lighthouse" tab. Run an audit, and you'll get a detailed CLS score along with a breakdown of contributing factors.

But for true investigative work, the "Performance" panel is invaluable. Record a page load, and then look for the "Experience" section. Here, you'll see a visual representation of layout shifts, often highlighted in red. Clicking on these shifts will show you exactly which elements moved and why, allowing you to pinpoint the exact culprit. It’s a powerful way to see the shifts in action.

Google Search Console (Core Web Vitals Report)

This is where you monitor your site's real-world performance over time. The Core Web Vitals report in Search Console shows you which pages on your site are performing well, which need improvement, and which are performing poorly across all Core Web Vitals, including CLS. It categorizes URLs based on their field data, helping you prioritize your optimization efforts across your entire site. This report is essential for understanding the aggregate impact of your changes.

Web Vitals Extension

For quick, real-time feedback, install the Web Vitals Chrome extension. As you browse any page, it displays the Core Web Vitals scores right in your browser toolbar. This is fantastic for instantly checking your own site as you develop, or even for seeing how your competitors are performing. It gives you immediate visual cues about layout shifts as they happen.

Your Battle Plan: Rock-Solid Strategies to Conquer CLS

Now that you understand what CLS is and how to measure it, it's time for action. These strategies are your blueprint for building a stable, high-performing website.

Always Specify Image and Video Dimensions

This is the foundational fix, and it's surprisingly simple. For every <img> or <video> tag, include width and height attributes.

<img src="your-image.jpg" width="600" height="400" alt="Descriptive alt text">

Even if you're using responsive images with srcset and sizes, those base width and height attributes provide the browser with the intrinsic aspect ratio. The browser can then calculate the correct space to reserve, preventing shifts.

For a modern approach, leverage the CSS aspect-ratio property. You can apply this directly to images or containers. For example:

img {
  aspect-ratio: 16 / 9; /* Or whatever ratio your images typically use */
  width: 100%;
  height: auto; /* Ensures the aspect ratio is maintained */
}

This tells the browser exactly what shape the image will eventually take, reserving that crucial space before the image even loads.

Reserve Space for Ads and Embeds

Dynamically loaded content, especially ads, is a notorious CLS offender. The key here is proactive space reservation. You need to tell the browser exactly how much space these elements will occupy, even if they load slowly or fail to load.

For ads, use min-height and min-width on their container elements. If you know an ad slot will usually be 300x250 pixels, set that min-height and min-width. If the ad doesn't load, the space remains, preventing a shift.

.ad-container {
  min-height: 250px;
  min-width: 300px;
  display: block; /* Ensure it takes up space */
}

For embeds like YouTube videos, use the aspect-ratio trick again. Wrap the embed in a container and apply the ratio:

<div class="video-wrapper">
  <iframe src="your-youtube-video" frameborder="0" allowfullscreen></iframe>
</div>

.video-wrapper {
  position: relative;
  width: 100%;
  padding-bottom: 56.25%; /* For 16:9 aspect ratio (9 / 16 * 100) */
  height: 0;
  overflow: hidden;
}
 
.video-wrapper iframe {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}

Alternatively, consider using skeleton loaders or placeholders. These are lightweight visual representations of the content that will eventually load. They reserve space and give users a visual cue that content is coming, making the transition much smoother.

We once ran an A/B test on a news site where we implemented skeleton loaders for ad slots instead of just empty space. The CLS score for those pages dropped significantly, and user feedback indicated a much smoother reading experience. The reserved space, even with a placeholder, made all the difference.

Optimize Web Font Loading

Taming web fonts involves a few key strategies to prevent layout shifts.

First, use font-display: swap in your @font-face declarations. This tells the browser to use a fallback system font immediately, and then "swap" in your custom font once it's loaded. While a swap can still cause a minor shift if font metrics differ, it's generally preferable to a Flash of Invisible Text (FOIT), which leaves users staring at blank space.

@font-face {
  font-family: 'MyCustomFont';
  src: url('mycustomfont.woff2') format('woff2');
  font-display: swap; /* Crucial for CLS */
}

Second, preload critical web fonts. Use <link rel="preload" as="font" crossorigin> in your <head> to tell the browser to fetch these fonts early in the loading process. This makes them available sooner, reducing the time the fallback font is displayed.

Third, consider using the new CSS size-adjust, ascent-override, descent-override, and line-gap-override properties. These allow you to fine-tune the metrics of your fallback font to match your custom font more closely, minimizing or even eliminating layout shifts during the font swap. It's a powerful, albeit more advanced, technique.

Avoid Inserting Content Above Existing Content

This rule is simple: don't unexpectedly push down content that a user is already viewing or interacting with. If you have content that needs to appear, ensure its space is reserved from the start, or design it to appear in a way that doesn't cause a layout shift.

For example, if you have a cookie consent banner, display it as an overlay (modal) or at the very top of the page before any other content loads. Don't let it pop in after the page has rendered and push everything down.

User-initiated layout changes, like expanding an accordion or opening a navigation menu, are generally acceptable. The user expects these changes because they triggered them. The problem arises from unexpected shifts. Server-side rendering (SSR) or static site generation (SSG) can also help here, as the full page structure is available from the initial server response, reducing dynamic injections on the client side.

Use CSS transform for Animations

When you need to animate elements, always prioritize CSS transform properties like translate, scale, rotate, and skew. These properties are handled by the browser's compositor thread, which can animate them without triggering a full layout recalculation. This means smooth, performant animations that won't cause layout shifts.

Avoid animating properties like margin, padding, width, height, top, or left if you want to prevent CLS. These properties directly affect the document flow and will force the browser to re-layout the page with every frame of the animation, leading to jank and layout shifts. It’s a fundamental performance optimization that directly impacts CLS.

Implement contain Property (Advanced)

For more complex layouts, the CSS contain property can be a game-changer. It allows you to isolate a subtree of your page from the rest of the document, preventing layout changes within that subtree from affecting the parent or sibling elements.

You can use values like layout, paint, or size. For example, contain: layout tells the browser that nothing inside this element will cause a layout change outside of it. This is particularly useful for widgets or components that might have internal dynamic content.

A newer, even more powerful property is content-visibility. When set to content-visibility: auto, the browser will skip rendering and layout work for off-screen content, only doing so when it becomes visible. This can dramatically improve initial load performance and reduce CLS by deferring the layout calculations for non-critical sections of your page. It's a powerful tool for large, complex sites.

The Payoff: Why a Low CLS Score Is Your Undisputed Advantage

You’ve put in the work. You’ve tackled the layout shifts, optimized your fonts, and reserved your spaces. So, what's the reward? The payoff for a low Cumulative Layout Shift score is massive, touching every aspect of your online presence. This isn't just about pleasing Google; it's about building a better, more successful website.

First, you deliver a superior user experience. Your users will thank you for it. No more accidental clicks, no more losing their place in an article. They can interact with your content smoothly and predictably. This builds trust and reduces frustration, making them more likely to stay, explore, and return.

Second, you gain a significant SEO advantage. Google explicitly states that Core Web Vitals, including CLS, are ranking signals. A stable, performant site is favored in search results. This means better visibility, more organic traffic, and a stronger position against your competitors. It's a non-negotiable aspect of modern SEO.

Third, you'll see higher conversion rates. When users aren't fighting your website, they're more likely to complete their goals. Whether it's making a purchase, filling out a form, or signing up for a newsletter, a stable layout removes friction from the conversion funnel. Every smooth interaction nudges them closer to becoming a customer.

Fourth, your bounce rates will plummet. Users are quick to abandon sites that feel broken or frustrating. By eliminating layout shifts, you create a more enjoyable browsing experience, encouraging visitors to spend more time on your pages and explore more of your content. They stick around because your site feels reliable.

Finally, a low CLS score gives you a competitive edge. In today's crowded digital landscape, every advantage counts. While many sites still struggle with layout shifts, your commitment to visual stability will make your site stand out as professional, reliable, and user-centric. You're not just meeting expectations; you're exceeding them.

The journey to a perfect CLS score might seem daunting, but it's an exciting opportunity for growth. You're not just fixing a technical issue; you're fundamentally improving how users experience your brand online. Embrace these strategies, measure your progress, and watch your website transform into a stable, high-performing powerhouse. Your users, and Google, will reward you for it.

Frequently Asked Questions (FAQ)

Q1: What is a good Cumulative Layout Shift score?

A CLS score of 0.1 or less is considered "Good" by Google. Scores between 0.1 and 0.25 "Need Improvement," and anything above 0.25 is "Poor."

Q2: Is CLS only about images?

No, while images are a common culprit, CLS also accounts for shifts caused by dynamically injected content, web fonts, third-party embeds, and animations that trigger layout recalculations.

Q3: Can user-initiated layout changes cause CLS?

Generally, no. Layout changes that occur in response to a user's explicit interaction (like clicking to expand an accordion or open a navigation menu) are expected and do not count towards CLS.

Q4: How often should I check my CLS score?

You should regularly monitor your CLS score using Google Search Console's Core Web Vitals report, especially after making significant changes to your website's layout or adding new dynamic content.

Q5: Does CLS affect mobile and desktop performance differently?

CLS impacts both mobile and desktop experiences, but it can be particularly noticeable and frustrating on mobile devices due to smaller screen sizes and often slower network conditions.