Cumulative Layout Shift (CLS) : How to Fix CLS Issues: Improve Core Web Vitals & Boost SEO
Learn how to fix Cumulative Layout Shift (CLS) fast. Get 9 proven fixes for images, fonts, ads and WordPress to hit a CLS score under 0.1.
Have you ever tried to click a button on a website, only for the page to jump at the last second? This frustrating experience is known as Cumulative Layout Shift (CLS). It is a critical metric within Google’s Core Web Vitals, designed to measure the visual stability of your pages.
Google evaluates this metric at the 75th percentile of real-user visits. Because search engines prioritize a smooth user experience, high stability scores are essential for your SEO strategy. If your content moves unexpectedly while loading, visitors may leave, which hurts your rankings.
In this guide, we will show you how to fix these stability issues. We will cover practical steps like reserving space for media, stabilizing web fonts, and managing dynamic advertisements. By testing your templates across different devices, you can ensure a seamless journey for every visitor.
Key Takeaways
- Understand how visual stability impacts your site rankings.
- Learn why Google tracks these metrics at the 75th percentile.
- Discover how to reserve space for images and videos to prevent jumps.
- Find out how to optimize font loading for a better user experience.
- Get tips on controlling ad placements to maintain page integrity.
Why Cumulative Layout Shift (CLS) Matters for SEO and User Experience
Have you ever tried to click a button, only for it to jump away at the last second? This common annoyance is caused by Cumulative Layout Shift (CLS), a metric that tracks how much content moves around while a page loads. When elements shift unexpectedly, it creates a disjointed and frustrating journey for your visitors.
How Unexpected Page Movement Frustrates Visitors
Unexpected movement often forces users to lose their place while reading an article. Even worse, it can lead to accidental clicks on the wrong links or buttons, which is incredibly aggravating. When a site feels unstable, visitors are much more likely to abandon the page before they ever complete their intended action.
A smooth user experience relies on predictability. When a visitor interacts with your site, they expect the interface to remain steady. If your content constantly jumps, it signals a lack of polish and can damage the trust you have built with your audience.
How CLS Affects Google Core Web Vitals and Search Performance
Google recognizes that visual stability is a critical component of a high-quality site. As one of the three Core Web Vitals, CLS serves as a direct measure of how stable your pages are during the loading process. While high-quality content and search intent remain the primary drivers of ranking, poor performance in these metrics can weaken your overall SEO standing.
Search engines prioritize sites that provide a seamless experience. If your pages suffer from frequent layout shifts, you might find it harder to maintain top positions in search results. Improving these metrics is a smart way to ensure your site remains competitive in a crowded digital landscape.
What a Stable, User-Friendly Page Looks Like
A stable page is one where every element—including text, images, buttons, and navigation menus—stays exactly where it belongs. As resources finish loading, the layout remains rock-solid, allowing users to interact with the page immediately. This level of visual consistency is the hallmark of a professional, well-optimized website.
When you prioritize stability, you create an environment where users feel in control. By ensuring that your forms and media containers are properly sized, you prevent the jarring jumps that ruin the browsing experience. Ultimately, a stable design is a user-friendly design that keeps your audience engaged and coming back for more.
How CLS Is Measured and What Counts as a Poor Score
Understanding how Cumulative Layout Shift (CLS) is calculated helps you pinpoint exactly where your site struggles. Rather than simply counting how many elements move, the metric looks at the total impact of these shifts on your visitors.
Understanding the CLS Scoring Formula
The CLS score is determined by multiplying two specific values: the impact fraction and the distance fraction. The impact fraction measures how much an unstable element affects the viewport area between two frames.
The distance fraction represents the greatest distance any unstable element has moved in the viewport. By combining these, Google creates a single, reliable number that reflects the visual instability of your page.
The Good, Needs Improvement, and Poor CLS Ranges
Google uses specific thresholds to categorize the stability of your web pages. Keeping your score within the “Good” range is essential for maintaining a high-quality user experience.
| Category | CLS Score Range | Performance Status |
| Good | 0.00 – 0.10 | Optimal Stability |
| Needs Improvement | 0.11 – 0.25 | Moderate Instability |
| Poor | Above 0.25 | High Instability |
Why Field Data and Lab Data Can Show Different Results
You might notice that your field data and lab data do not always match perfectly. This discrepancy is common because they measure performance under very different conditions.
Real-user measurements from Chrome User Experience Report
The field data comes directly from the Chrome User Experience Report. It reflects the actual experiences of real users on various devices, network speeds, and browser configurations.
Controlled testing with Lighthouse and PageSpeed Insights
In contrast, lab data is gathered in a controlled environment using tools like Lighthouse or PageSpeed Insights. While Lighthouse is excellent for debugging, it cannot replicate the infinite variety of real-world user interactions, caching states, or hardware limitations found in the wild.
Find the Elements Causing Layout Shifts
You cannot fix what you cannot see, which is why tracking down layout shifts is essential for performance. When elements jump around unexpectedly, it creates a frustrating experience for your visitors. Fortunately, modern diagnostic tools make it easier than ever to visualize exactly when and why your page layout breaks.
Use PageSpeed Insights to Identify Shifted Elements
PageSpeed Insights is a powerful starting point for any performance audit. By running your URL through this tool, you gain access to Lighthouse reports that specifically flag instances where content moved during the loading process. These reports provide a list of elements that contributed to the score, allowing you to focus your efforts on the most problematic areas.
Inspect Layout Shifts in Chrome DevTools Performance Panel
For a deeper look, the Chrome DevTools Performance panel is your best friend. You can record a page load and look for the “Experience” track, which highlights specific layout shifts in red. Clicking on these events reveals the exact DOM elements that moved, giving you a clear target for your optimization work.
Review the Layout Shift Regions Overlay
The Layout Shift Regions overlay provides a visual representation of the movement on your screen. By enabling this feature in the rendering settings, you can see a blue box highlighting the area that shifted. This visual feedback helps you understand the impact of the movement on the user’s field of view.
Correlating layout shifts with network requests and JavaScript tasks
To solve the mystery of why a shift occurred, you must look at the timing of your resources. Compare the shift timestamps with your network waterfall to see if a late-loading image or font is the culprit. Often, heavy JavaScript execution or delayed API responses trigger these movements, so check your main thread activity for long-running tasks.
Separate Initial-Load Shifts from Interaction-Based Shifts
It is important to distinguish between shifts that happen automatically and those triggered by the user. Initial-load layout shifts are usually caused by missing dimensions on media or late-injected content. Conversely, movements caused by user interactions, such as clicking a button, are often expected and may not count against your score if they occur within a specific timeframe.
Reserve Space for Images, Videos, and Other Media
You can easily stabilize your site by ensuring every image and video has a reserved space before it renders. When browsers know the dimensions of your media ahead of time, they can allocate the correct amount of room, which effectively prevents annoying layout shifts.
Set Explicit Width and Height Attributes on Images
Modern browsers use the width and height attributes to calculate the aspect ratio of an image before the file even finishes downloading. By including these attributes in your HTML, you provide the browser with the necessary information to create a stable container.
This simple practice ensures that the surrounding text does not jump when the image finally appears. Always define these values to match the intrinsic dimensions of your source files.
Use CSS aspect-ratio for Responsive Media Containers
For responsive media, setting fixed pixel dimensions is often not enough. The CSS aspect-ratio property allows you to maintain a consistent shape while the width of the container changes across different screen sizes.
By applying this property, you ensure that the browser maintains the correct height relative to the width. This technique is a powerful tool for keeping your layout stable on both mobile and desktop devices.
Prevent Video and iframe Embeds from Collapsing Before Load
Videos and iframes are notorious for causing sudden jumps because they often lack defined dimensions. You should always wrap these elements in a container that has a fixed height or a calculated aspect-ratio.
This approach keeps the page layout steady while the external content fetches. Without this step, your page content will likely shift downward once the video player initializes.
Handling responsive YouTube and Vimeo embeds
When embedding content from platforms like YouTube, use a wrapper div with a specific percentage-based padding. This creates a reserved space that scales perfectly with your site design.
Many developers use a “16:9” ratio container to ensure the video player stays proportional. This prevents the rest of your page from moving when the embed loads.
Manage Lazy-Loaded Images Without Moving Existing Content
Lazy loading is great for performance, but it can trigger layout shifts if the browser does not know how much space to save. You must ensure that your lazy-loaded images have defined dimensions or placeholders.
If you do not reserve space, the page will “pop” as images load in. Always provide the browser with the expected size of the image to keep the user experience smooth.
Use Appropriate Placeholder Dimensions for Image Variants
When using responsive images with the srcset attribute, the browser might choose different files based on the screen width. It is vital to use placeholders that match the dimensions of the largest possible variant.
This ensures that the responsive media container remains consistent regardless of which image file the browser selects. Proper planning here eliminates the risk of unexpected movement during the loading process.
Prevent Web Fonts from Shifting Text and Page Layout
Typography plays a vital role in your site’s design, but it often becomes a hidden culprit behind unexpected layout shifts. When browsers struggle to render text, the visual experience can suffer significantly. Understanding how your site handles font loading is the first step toward a smoother, more stable interface.
Understand the Layout Effects of FOIT and FOUT
Two common issues, known as FOIT (Flash of Invisible Text) and FOUT (Flash of Unstyled Text), frequently disrupt the user experience. FOIT occurs when the browser hides text entirely until the custom font file finishes downloading. This leaves users staring at blank spaces, which can feel like the page is broken.
Conversely, FOUT displays the system fallback font immediately. While this is often better for perceived performance, the fallback font usually has different dimensions than your chosen web font. Once the custom font loads, the text swaps, causing the entire block to resize and push surrounding content around the page.
Preload Critical Fonts Without Creating New Performance Problems
Preloading your primary font files can help minimize these delays. By using the rel=”preload” hint in your HTML head, you tell the browser to fetch the font early in the loading process. However, you must be careful not to overdo it.
If you preload too many files, you create competition for bandwidth with other essential assets like images or scripts. Focus only on the fonts required for the “above-the-fold” content to ensure the most important text renders quickly without causing late-stage layout adjustments.
Match Fallback Fonts with CSS font-size-adjust and Metric Overrides
Modern CSS provides powerful tools to bridge the gap between fallback fonts and your custom web fonts. By aligning their metrics, you can ensure that the text occupies nearly the same space regardless of which font is currently active.
Using size-adjust, ascent-override, descent-override, and line-gap-override
These properties allow you to fine-tune the dimensions of your fallback font to match your primary font’s characteristics. By adjusting the ascent, descent, and line gap, you can force the fallback font to mimic the visual footprint of your custom design.
| CSS Property | Primary Function | Impact on Stability |
| size-adjust | Scales the glyphs | High |
| ascent-override | Sets the height above baseline | Medium |
| descent-override | Sets the height below baseline | Medium |
| line-gap-override | Controls spacing between lines | High |
Limit Font Weights and Styles That Must Load During Rendering
Every additional font weight or style you include adds to the browser’s rendering workload. Loading multiple variations simultaneously increases the risk of FOIT and FOUT, as the browser must manage more requests. Stick to the essential weights needed for your core content to keep the page stable.
If you need decorative styles, consider loading them asynchronously after the main content has settled. This strategy keeps your initial layout clean and prevents unnecessary movement while the user interacts with your site. Prioritizing simplicity in your typography stack is a proven way to maintain a high-quality user experience.
Stabilize Ads, Banners, and Third-Party Embeds
You can significantly improve your Core Web Vitals by stabilizing how ads and widgets load on your pages. When dynamic elements push content around after the initial render, users often lose their place, leading to a frustrating experience. Taking control of these elements is a vital step toward a smoother, more professional website.
Allocate Fixed or Minimum Heights for Ad Slots
Many ads start with zero height and expand once the bidding process finishes. This sudden expansion is a primary cause of layout shifts. You should always define a fixed or minimum height for your ad containers to ensure the page layout remains steady during the loading process.
Keep Consent Banners and Promotional Bars from Pushing Content
Consent banners and promotional bars often appear at the top of the viewport, forcing existing content downward. To prevent this, consider using an overlay approach that sits on top of the content rather than pushing it. If the banner must be part of the flow, ensure the space is reserved space before the page finishes rendering.
Design Empty and Filled Ad States with Matching Dimensions
It is common for inventory to be unavailable, leaving an empty container on your page. You should design your empty states to match the dimensions of your filled states. This consistency prevents the page from collapsing or expanding unexpectedly when an ad fails to load.
Load Social Widgets and External Embeds Inside Reserved Containers
Third-party embeds like social media feeds or video players are notorious for shifting content. By wrapping these items in a container with a defined aspect ratio, you provide the browser with the necessary information to calculate the layout before the content arrives.
Managing unpredictable creative sizes and collapsed ad inventory
When dealing with unpredictable creative sizes, use CSS to set a maximum height that accommodates the largest possible ad. If an ad slot remains empty, you can use a placeholder image or a background color to maintain the layout. This strategy ensures that your site remains stable even when external scripts behave in unexpected ways.
| Element Type | Stability Strategy | Benefit |
| Ad Slots | Set min-height | Prevents sudden expansion |
| Consent Banners | Use CSS overlays | Avoids content reflow |
| Social Widgets | Reserved containers | Maintains visual rhythm |
| Promotional Bars | Fixed height blocks | Ensures consistent layout |
Control Dynamic Content That Appears After the Page Loads
Modern websites rely heavily on dynamic content, but this flexibility often comes at the cost of layout stability. When elements load after the initial page render, they frequently cause frustrating layout shifts that hurt your search rankings and overall user experience. By planning how these components appear, you can maintain a smooth, professional interface for every visitor.
Reserve Space for Personalized Recommendations and Related Content
Personalization modules and related article widgets are common culprits for unexpected movement. If these sections load asynchronously, they can push the entire page downward once the data arrives.
To prevent this, you should always reserve space for these containers before the content finishes loading. Using a CSS min-height or a skeleton screen ensures the layout remains locked in place, providing a seamless transition from empty to populated states.
Handle Cookie Notices, App Banners, and Account Prompts
Cookie notices and promotional banners are essential, but they are notorious for disrupting the flow of a page. If these elements are injected at the top of the document, they force all existing content to jump down instantly.
Consider these strategies to keep your layout stable:
- Position banners at the bottom of the viewport to minimize impact.
- Use fixed-height containers for promotional bars.
- Ensure account prompts are triggered by user interaction rather than automatic page-load scripts.
Update Existing Content Instead of Inserting Above It
A common mistake is injecting new elements above existing content, which forces the browser to recalculate the entire page geometry. Instead of pushing content down, try updating the data within an existing, pre-allocated region.
Using overlays when a message must appear immediately
When a message requires immediate attention, such as a critical system alert, overlays are your best friend. By using a modal or a floating notification, you can display information without altering the document flow or causing layout shifts.
Coordinate Client-Side Rendering with Server-Rendered Markup
Many modern frameworks use client-side rendering to hydrate the page after the initial HTML is delivered. If the server-rendered markup does not match the client-side output, the browser may replace elements, causing a sudden, jarring shift.
You must ensure that your server-rendered markup and client-side components share identical dimensions and structures. This alignment prevents the browser from having to re-render the page, keeping the user experience consistent and stable throughout the entire loading process.
Fix CSS and JavaScript Patterns That Trigger Layout Shifts
You can significantly reduce layout shifts by adopting smarter CSS and JavaScript practices that respect the browser’s rendering pipeline. When code executes inefficiently, it often forces the browser to recalculate the entire page geometry, which negatively impacts your Core Web Vitals.
Avoid Injecting Content Above Visible Elements
Inserting banners, alerts, or promotional components above existing content is a common cause of visual instability. When you inject these elements dynamically, the browser must push everything else down to make room, which creates a jarring experience for the user.
This sudden change in geometry is a primary driver of poor stability scores. Instead of inserting content at the top, consider reserving space for these elements in your initial HTML structure to ensure the layout remains consistent.
Animate transform and opacity Instead of Layout-Affecting Properties
Many developers accidentally trigger reflows by animating properties like height, width, or margin. These properties force the browser to recalculate the position of every surrounding element on the page.
To maintain smooth performance, you should prefer using transform and opacity for your animations. These properties are handled by the browser’s compositor thread, which avoids expensive layout recalculations and keeps your interface feeling snappy.
Use CSS Containment to Limit Unnecessary Recalculation
CSS containment is a powerful tool that allows you to isolate specific parts of your page from the rest of the document. By using the contain property, you tell the browser that the internal layout of an element does not affect the surrounding content.
This technique is particularly useful for complex widgets or dynamic sidebars. It prevents a local change from triggering a global reflow, which is essential for maintaining a stable score.
Prevent Late-Loading Stylesheets from Reflowing the Page
When a browser discovers a new stylesheet mid-render, it often pauses to download and parse the file, which can cause the page to jump. This behavior is a frequent culprit behind unexpected movement during the initial load.
Inlining critical CSS and loading noncritical styles safely
To avoid this, you should inline your critical CSS directly into the document head. This ensures that the most important styles are available immediately, while noncritical styles can be loaded asynchronously without disrupting the user experience.
Batch DOM Updates to Reduce Rendering Disruptions
Frequent JavaScript calls that modify the DOM one by one can lead to multiple layout recalculations in a single frame. It is much more efficient to batch these updates together so the browser only performs a single reflow.
By grouping your changes, you minimize the work the browser needs to do to keep the page stable. This simple optimization strategy helps ensure that your site remains performant and user-friendly.
| Property Type | Performance Impact | Recommended Usage |
| Layout Properties (Width, Top) | High (Triggers Reflow) | Avoid for animations |
| Paint Properties (Color, Shadow) | Medium (Triggers Repaint) | Use sparingly |
| Compositor Properties (Transform) | Low (GPU Accelerated) | Best for animations |
Reduce CLS in Common WordPress and JavaScript Framework Setups
Modern web development platforms offer powerful tools, but they can also introduce unexpected layout instability. Whether you are building a site on a popular CMS or a custom application, understanding how your stack handles rendering is essential for a stable user experience.
Audit WordPress Themes, Page Builders, and Plugin-Generated Elements
Many WordPress users rely on page builders and plugins to add functionality quickly. However, these tools often inject dynamic content after the initial page load, which frequently triggers layout shifts.
You should audit your theme settings to ensure that containers for dynamic widgets have defined heights. Carefully review your plugin list to identify any tools that load content asynchronously without reserving space, as these are common culprits for poor stability scores.
Stabilize React, Next.js, Vue, and Angular Components
Developers working with modern frameworks like React, Next.js, Vue, and Angular must be particularly mindful of how components mount. When a component renders, it can push existing content down if the browser does not know its final dimensions beforehand.
Preserving consistent server-side and client-side markup
A major cause of instability in these frameworks is the mismatch between server-rendered HTML and client-side hydration. If the client-side JavaScript logic changes the DOM structure during hydration, the page will visibly jump.
Ensure that your server-side rendering (SSR) output matches the initial client-side state exactly. This prevents unnecessary re-renders that force the browser to recalculate the layout of your entire page.
Review Tag Managers and Marketing Scripts for Late Insertions
Marketing scripts and tag managers are notorious for injecting banners or ads long after the main content has appeared. These scripts often lack fixed dimensions, causing the page to reflow unexpectedly.
Audit your tag manager configurations to ensure that all injected elements have a pre-allocated space. If a script must insert a banner, use CSS to reserve that height in the layout before the script executes.
Replace Unstable Third-Party Components with Predictable Alternatives
Sometimes, an external widget is simply too unstable to fix through CSS alone. If a third-party component repeatedly inserts variable-height content, it may be time to look for a more predictable alternative.
Prioritize components that allow for fixed-height containers or provide a loading state that matches the final content size. Choosing stable tools is a proactive way to maintain high performance and keep your layout rock-solid for every visitor.
Test CLS Improvements Across Devices and Page Templates
Achieving a stable user experience requires rigorous testing across your entire website. After applying technical fixes, you must verify that your changes effectively reduce the CLS score without introducing new performance bottlenecks.
Run Lighthouse Tests Under Consistent Conditions
To get reliable results, you should use Lighthouse and PageSpeed Insights under controlled environments. Ensure that your testing conditions, such as network throttling and CPU simulation, remain identical for every audit.
Variations in cache states or browser extensions can skew your lab data. By keeping these variables constant, you ensure that your performance improvements are genuine and not just a result of testing fluctuations.
Compare Mobile and Desktop Layout Stability
Mobile and desktop environments often behave differently due to unique breakpoints and responsive design patterns. You must test both platforms separately to account for differences in image variants, font rendering, and ad placements.
A layout that appears stable on a wide desktop screen might shift significantly on a narrow mobile device. Checking both ensures your Core Web Vitals remain healthy for every visitor, regardless of their hardware.
Test Key Templates Instead of Only the Homepage
Many developers make the mistake of only optimizing the homepage. However, users often land on deep pages that have different structures and dynamic content requirements.
Prioritizing product pages, article pages, landing pages, and checkout flows
You should audit your most critical templates to ensure stability where it matters most. Focus your efforts on high-traffic areas like product pages and checkout flows, as these are vital for user retention and conversion.
| Template Type | Common Shift Risk | Testing Priority |
| Product Pages | Dynamic image galleries | High |
| Article Pages | Late-loading ads | Medium |
| Checkout Flows | Form validation messages | Critical |
Confirm That Fixes Do Not Harm Largest Contentful Paint or Interaction to Next Paint
While optimizing for layout stability, keep a close eye on other Core Web Vitals. Sometimes, aggressive CSS changes intended to fix shifts can inadvertently delay the rendering of your main content.
Always verify that your PageSpeed Insights report shows improvements in your CLS score without negatively impacting your Largest Contentful Paint or Interaction to Next Paint. A balanced approach ensures a fast, stable, and responsive site for all users.
Monitor Core Web Vitals After Publishing Fixes
Your work to improve site stability does not end the moment you hit the publish button. To maintain high rankings and a smooth user experience, you must keep a close eye on your Core Web Vitals metrics. Consistent oversight ensures that your site remains fast and reliable for every visitor.
Track Field CLS Data in Google Search Console
Google Search Console is an essential tool for tracking how your site performs in the real world. It groups your URLs based on field data, which allows you to see how specific templates or page types are behaving.
By reviewing these reports, you can quickly identify if stability problems are isolated to a single page or affecting your entire site. This insight is vital for maintaining your SEO health and ensuring that your layout remains predictable.
Use Real User Monitoring to Detect Regressions
Real User Monitoring (RUM) provides a deeper look into how different users experience your site. It helps you detect regressions caused by varying device types, regional network speeds, or browser updates.
Third-party scripts and new marketing campaigns can often introduce unexpected movement. RUM tools alert you to these changes immediately, allowing you to fix issues before they negatively impact your Cumulative Layout Shift (CLS) scores.
Set Performance Budgets for Images, Fonts, Ads, and Scripts
Establishing performance budgets is a proactive way to prevent future stability issues. By setting strict limits on the size and loading behavior of images, fonts, and ads, you create a framework for your development team.
These budgets act as a guardrail for your site’s performance. When you treat speed as a core requirement, you protect your Core Web Vitals from being compromised by heavy, unoptimized assets.
Create a Repeatable CLS Review Before Every Major Release
The best way to ensure long-term success is to integrate stability checks into your deployment pipeline. You should create a repeatable Cumulative Layout Shift (CLS) review process that occurs before every major site update.
This routine check confirms that new features do not break existing layouts. By making this a standard part of your workflow, you guarantee that your SEO efforts remain effective and your site stays user-friendly over time.
Conclusion
Achieving a stable interface is a journey that begins with identifying the root causes of movement. Improving your Cumulative Layout Shift (CLS) requires a precise approach where you locate the exact source of instability before applying a targeted fix.
Prioritize reserving dimensions for media and dynamic modules to keep your layout firm. Aligning font metrics and stabilizing ads or third-party embeds prevents unexpected jumps that frustrate your visitors. These steps ensure that your site remains predictable during the entire loading process.
Consistent monitoring of your CLS score remains vital for maintaining high standards. Use real user data alongside your standard Lighthouse testing to catch regressions early. A strong focus on these Core Web Vitals creates a smoother user experience that keeps your audience engaged.
Investing time in these technical refinements pays off by boosting your overall SEO performance. Search engines favor sites that provide a seamless, flicker-free environment for every visitor. Keep refining your templates to ensure your digital presence stays fast, stable, and ready for growth.
FAQ
What exactly is Cumulative Layout Shift (CLS) and why does Google care about it?
A: Cumulative Layout Shift (CLS) is a Core Web Vital that measures the visual stability of a webpage. It calculates how often users experience unexpected movement of page content while it is still loading or being interacted with. Google prioritizes this metric because a high CLS score often leads to a frustrating user experience, such as a visitor clicking the wrong button or losing their place while reading. For SEO purposes, Google evaluates CLS at the 75th percentile of real-user visits to determine if a page provides a high-quality experience.
What is considered a “Good” CLS score?
To provide a great user experience, Google recommends a CLS score of 0.10 or lower. Scores between 0.10 and 0.25 are labeled as “Needs Improvement,” while any score above 0.25 is considered “Poor.” These scores are calculated using a formula that combines the impact fraction (how much space an unstable element takes up) and the distance fraction (how far the element moved).
Why do my scores differ between PageSpeed Insights and Google Search Console?
This happens because of the difference between Lab Data and Field Data. Lighthouse and PageSpeed Insights often use Lab Data, which are controlled tests in a simulated environment. Google Search Console uses Field Data from the Chrome User Experience Report (CrUX), which reflects real-world experiences from actual visitors. Factors like different devices, varying internet speeds, and specific user interactions can cause these two datasets to show different results.
How do explicit width and height attributes prevent layout shifts?
When you set explicit width and height attributes on images or videos, the browser can calculate the necessary space (the “display box”) before the actual file finishes downloading. This prevents the surrounding text or buttons from jumping once the media finally appears. For modern, responsive designs, using the CSS aspect-ratio property is an excellent way to maintain these proportions across different screen sizes.
What are FOIT and FOUT, and how do they affect my layout?
A: FOIT (Flash of Invisible Text) occurs when the browser hides text until a web font is fully loaded, while FOUT (Flash of Unstyled Text) shows a fallback font first. If the fallback font has different dimensions than the final font, the text will shift once the custom font swaps in. You can minimize this by using font-display: swap, preloading critical fonts, and utilizing CSS overrides like size-adjust, ascent-override, and descent-override to match the metrics of your fallback and primary fonts.
How can I stabilize third-party advertisements and YouTube embeds?
Ads and third-party widgets are notorious for causing shifts because they often load dynamically. To fix this, you should reserve space by setting a fixed or minimum height for the ad container. This ensures that even if the ad takes a few seconds to appear, the content below it stays in place. This same strategy applies to YouTube or Vimeo embeds; always wrap them in a container with a predefined aspect-ratio.
Can I use animations without ruining my CLS score?
Yes, but you must choose your properties wisely. Avoid animating properties that trigger layout changes, such as height, width, margin, or top. Instead, use transform: translate() for movement and opacity for fading effects. These properties are handled by the browser’s compositor and do not force a layout recalculation, keeping your visual stability intact.
How do I find the specific elements causing shifts on my site?
You can use the Chrome DevTools Performance panel to identify specific layout shift events. By checking the “Layout Shift Regions” overlay, you can see exactly which parts of the page are moving during the load sequence. PageSpeed Insights also provides a “Diagnostics” section that lists the specific DOM elements contributing most to your CLS score.
Do JavaScript frameworks like React or Next.js cause CLS?
They can if not managed properly. In frameworks like React, Next.js, Vue, or Angular, layout shifts often occur during hydration—the process where client-side JavaScript takes over server-rendered HTML. If the server-side markup doesn’t perfectly match the client-side rendering, elements may jump or be replaced. Ensuring consistent markup and using placeholders for dynamic components can help maintain stability.
Should I worry about CLS on mobile if my desktop score is fine?
Absolutely. Layouts often change drastically between desktop and mobile devices. A banner that looks fine on a wide screen might stack vertically and push content down significantly on a mobile viewport. Always test your templates—including product pages, article pages, and checkout flows—across multiple device types to ensure a stable experience for everyone.