Understanding Cumulative Layout Shift: Why CLS Matters for Your Website

Cumulative Layout Shift (CLS) is becoming a buzzword in the realm of web development and digital marketing, but not everyone understands exactly what it is or why it is crucial. As websites become more dynamic, ensuring a seamless user experience is essential. CLS is a metric that helps quantify how often users experience unexpected layout shifts—a key component of that experience.

What Exactly Is Cumulative Layout Shift?

CLS is one of the core web vitals that Google uses to understand the stability of a webpage's content as it loads. A 'layout shift' occurs any time a visible element changes its position from one rendered frame to the next. Cumulative Layout Shift is the sum total of all individual layout shift scores for every unexpected layout shift that occurs during the entire lifespan of a page.

Essentially, it measures the sum of all layout shifts that aren't caused by direct user interactions. High CLS values indicate a poor user experience, while lower values suggest a more stable and pleasant browsing environment.

Why Is CLS Important?

Impact on User Experience

Unstable pages can be frustrating to interact with. Users might click on the wrong button because it moved, or they might lose their place in an article when a text block shifts. High CLS scores are indicative of such issues, which can degrade user experience and increase the likelihood of site abandonment.

SEO Implications

Google has made it clear that user experience is a ranking factor, and CLS is part of their Core Web Vitals initiative. Websites with better CLS scores are likely to rank higher in search results because they provide a better user experience.

Identifying Causes of High CLS

Common culprits behind high CLS include:

• Images without dimensions: When images don’t have dimensions specified, the browser doesn’t know how much space to allocate for them until they load, leading to shifts.
• Ads, embeds, and iframes without dimensions: Similar to images, these elements can cause shifts if their dimensions are not specified.
• Dynamically injected content: Content added to the page after initial load can push down other content already on the page.
• Web fonts causing FOIT/FOUT: Flash of Invisible Text (FOIT) or Flash of Unstyled Text (FOUT) can also cause shifts when the web fonts load late.

Strategies to Improve CLS

Reducing CLS is not just about fixing a number; it’s about enhancing the user experience. Here are practical strategies:

Set Size Attributes for Media

Defining width and height for all media elements ensures that the browser can allocate the correct amount of space while the media is loading, reducing shifts.

Use CSS Aspect Ratio Boxes

For responsive designs, where size attributes are not practical, using CSS aspect ratio boxes can help maintain layout stability without sacrificing responsiveness.

Reserve Space for Ads and Embeds

By statically reserving space for ads or embeds, you can prevent them from pushing content around as they load.

Preload Important Assets

Preloading fonts and other important assets can prevent them from loading too late in the page load process, reducing shifts.

Avoid Inserting Content Above Existing Content

Be mindful of dynamically injected content and try to introduce it below the current viewport or during natural breaks.

Conclusion

Cumulative Layout Shift is a critical metric for assessing the stability of your website's layout. By understanding what causes layout shifts and taking steps to minimize them, you can significantly enhance user experience and potentially improve your SEO rankings. Remember, a stable page means happy users, and happy users lead to better engagement and conversions.

By taking proactive steps to address CLS, you can ensure that your website not only ranks well but provides a smooth and enjoyable experience for all visitors.