What is cumulative layout shift (CLS)?

This stable Core Web Vital metric is an important, user-centric measurement of visual stability. It helps quantify how often users experience unexpected layout shifts.

Cumulative Layout Shift (CLS): Understanding and Improving Your Website's Visual Stability

Cumulative Layout Shift, abbreviated as CLS, is a vital metric in web design and user experience. In essence, CLS measures how much the elements on your webpage visually shift around as the page loads. It's a Core Web Vital, a set of metrics Google uses to assess and rank the quality of a user's experience on your site.

Why CLS Matters

Imagine reading an article online, and suddenly, the text jumps around, causing you to lose your place. Or you're about to click a button, and it shifts at the last second, leading to an accidental click. These unexpected shifts are precisely what CLS tracks.

A high CLS score indicates that your website suffers from frequent and jarring visual instability. This not only frustrates visitors but can lead to:

  • Decreased User Engagement: Visitors are more likely to leave your site quickly if it's visually unstable.
  • Lower Conversion Rates: Unexpected shifts can deter users from completing desired actions (filling out a form, making a purchase).
  • Negative SEO Impact: Google considers CLS as a ranking factor, so a poor score could affect your search visibility.

Causes of CLS

Common causes of layout shifts include:

  • Images Without Dimensions: Images that load without specified width and height attributes can force content to move around as they are rendered.
  • Dynamically Injected Content: Ads, embeds, or other elements that are added to the page after the initial content can cause shifts.
  • Web Fonts Causing FOIT/FOUT: Flash of Invisible Text (FOIT) or Flash of Unstyled Text (FOUT) can occur as web fonts load, causing layout changes.
  • Actions Waiting for a Network Response Before Updating DOM: Elements that adjust their size or position based on user interactions or network requests can lead to shifts.

How CLS is Measured

CLS is calculated based on two factors:

  1. Impact Fraction: The proportion of the viewport affected by the layout shift.
  2. Distance Fraction: The distance the affected elements move relative to the viewport size.

The total CLS score is the sum of all individual layout shift scores that occur during the entire lifespan of the page.

Improving Your CLS Score

Here are some key strategies to optimize your CLS:

  • Always Include Image Dimensions: Specify width and height attributes for images or reserve space for them in your CSS.
  • Optimize Font Loading: Use the font-display: optional CSS property to prevent layout shifts during font loading.
  • Reserve Space for Ads: Dynamically reserve the space where ads will be placed to avoid content reflows.
  • Minimize Dynamic Content Above the Fold: Avoid placing elements that often change above the fold, as these can cause shifts throughout the page load.
  • Prioritize Transform Animations: Use CSS transforms (e.g., translate or scale) for animations, as they are less likely to trigger layout changes.

Need Assistance with CLS Optimization?

If you're looking for expert guidance to improve your website's CLS score and enhance user experience, we're here to help! Contact us for a personalized consultation.

Remember: A smooth, visually stable website is crucial for keeping your audience engaged and improving your online performance. By optimizing CLS, you'll create a more positive and enjoyable user experience, leading to better results for your business.