CLS, or Cumulative Layout Shift, is a core web vital that measures how visually stable a website is as it loads. In simpler terms, CLS tracks how much things move around unexpectedly while a page is rendering. You’ve probably experienced it—just when you go to click a button, it shifts, and you end up clicking something else. That frustrating moment is usually caused by a high CLS score. This metric plays a major role in your SEO and user experience, so it’s important for web designers and developers to understand how to minimize it.

Why CLS matters in web design

CLS isn’t just an annoying glitch—it affects user trust, interaction, and how your site performs in search rankings. A stable layout improves accessibility and usability, especially on mobile devices where space is limited. Search engines like Google consider CLS a ranking factor, so having a poor score can actually push your site lower in search results. Keeping your CLS low helps build a smoother, more enjoyable browsing experience that keeps visitors engaged and reduces bounce rates.

What causes layout shifts on a website

Layout shifts usually happen when page elements load at different times or sizes than expected. Common causes include images without defined dimensions, ads or embeds that resize themselves, and custom fonts that load late and reflow text. Sloppy coding or a lack of planning in how content loads can also lead to these sudden jumps. The more your content moves around as the page loads, the higher your CLS score.

How to improve your CLS score

Improving your CLS score starts with planning for stability. Here are some easy fixes:

• Always include width and height attributes for your images and videos.
• Avoid injecting content above existing elements unless it’s in response to user interaction.
• Preload fonts to prevent late swapping that affects layout.
• Use CSS aspect ratio boxes for responsive media so the browser reserves the right amount of space before loading.

Testing your site using tools like Google Lighthouse or PageSpeed Insights can help identify exactly what’s causing shifts and how to fix them.

CLS and its role in SEO and user experience

CLS directly affects both your technical SEO performance and how users feel when interacting with your site. A high CLS score creates frustration and distrust, especially on mobile where layout shifts are more noticeable. Meanwhile, Google uses CLS as part of its ranking algorithm, so a poor score could hurt your site’s visibility. A low CLS improves perceived quality, which encourages users to stay longer, browse more, and convert. If you're building or redesigning a site, it's critical to bake layout stability into your process.

FAQs about CLS

What is considered a good CLS score?

A good CLS score is less than 0.1. Anything between 0.1 and 0.25 needs improvement, and anything above 0.25 is considered poor. Google’s web vitals guidelines are clear about these thresholds, so it’s important to run regular tests and optimize accordingly.

How do I test for CLS on my site?

You can use tools like Google PageSpeed Insights, Lighthouse, or the Chrome User Experience Report. These tools highlight which elements are shifting and provide practical suggestions on how to improve layout stability.

Does CLS impact mobile and desktop the same way?

It can impact both, but mobile users often notice it more because of smaller screen sizes. Mobile layouts are more sensitive to late-loading elements, so ensuring mobile responsiveness and design consistency is key.

Can a high CLS score affect conversions?

Yes, absolutely. When elements move unexpectedly, it creates a frustrating user experience, especially when people are trying to click or read something. This can lead to missed clicks, early exits, and reduced trust in your site’s quality.

Is CLS only important for Google rankings?

Not at all. While it does affect rankings, the bigger picture is about user experience. People expect a stable, smooth-loading website—so improving CLS helps build credibility and keeps users coming back.

Build more stable websites with the right tools

Avoiding layout shifts isn’t just a nice-to-have—it’s a must for a high-performing, user-friendly website. Whether you're optimizing your current site or starting from scratch, using a smart website builder can help you design pages that look great and stay stable. B12’s platform helps ensure elements load correctly, providing a smoother experience across devices. Get started today with B12 and give your visitors the seamless experience they expect.