What is CLS? The Secret to Keeping Your Website Stable and User-Friendly!

Have you ever been engrossed in an interesting article, about to click a link or button, when suddenly – "boom!" – the entire page content shifts, causing you to accidentally click somewhere else? If so, you've been a victim of an annoying problem called Cumulative Layout Shift (CLS). This isn't just a minor glitch; it's a critical factor impacting user experience and your website's SEO ranking.

What is CLS (Cumulative Layout Shift)?

CLS, or Cumulative Layout Shift, is one of the three crucial Core Web Vitals metrics introduced by Google to measure user experience on websites. It quantifies the degree of visual stability of a web page, meaning the total sum of all unexpected layout shifts of visible page content during the page's lifecycle.

• A higher CLS score indicates a less stable website.
• A website with a CLS score below 0.1 is considered to have a good user experience.
• Scores between 0.1 and 0.25 need improvement.
• Above 0.25 is poor.

Imagine building a house where the walls keep moving on their own; quite frustrating, isn't it?

Why is CLS So Important?

CLS is not just a dry technical number; it directly impacts:

• User Experience (UX): As in the example at the beginning, content jumping around causes frustration, annoyance, and can lead users to abandon the page. It hinders users' ability to interact, resulting in misclicks.
• SEO and Search Rankings: Google uses Core Web Vitals (including CLS) as a significant ranking factor. A low CLS score means a more user-friendly website, which can help your site rank higher in search results.

Main Culprits Causing CLS

So, what are the common reasons your website might be "shaking"? Here are the most common "suspects":

1. Images and Videos Without Dimensions

This is the most frequent error. When a browser loads an image or video without knowing its dimensions (width and height) beforehand, it cannot reserve space. Once the image loads, it "pushes" surrounding content to make room, causing a layout shift.

2. Ads, Embeds, and Iframes

Similar to images, third-party content like advertisements, social media embedded widgets, or iframes often load dynamically and don't have predefined dimensions. They can appear unexpectedly and push page content.

3. Dynamically Injected Content

Cookie banners, email signup notifications, or pop-ups that suddenly appear without reserved space are also common causes, especially if they are inserted above existing content.

4. Web Fonts Causing Text "Jumps" (FOIT/FOUT)

When the browser loads custom web fonts, it sometimes displays text using a fallback font first (Flash Of Unstyled Text - FOUT) or nothing at all (Flash Of Invisible Text - FOIT). Once the custom font finishes loading, it replaces the old one, and if the two fonts have different sizes or line heights, the text will shift.

5. Improper Animations and Transitions

Some developers use CSS properties like top, left, width, height to create animations. Changing these properties forces the browser to recalculate the entire page layout, causing CLS. Instead, CSS transform properties should be used.

How to Effectively "Cure" CLS?

Fortunately, there are many simple ways to fix and prevent CLS:

• Always specify dimensions for images and videos:
  Add width and height attributes to <img> and <video> tags. Alternatively, use the CSS aspect-ratio property to maintain the aspect ratio.

  <img src="example.jpg" alt="Description" width="800" height="600">

  /* Or use CSS aspect-ratio */img {  aspect-ratio: 4 / 3; /* Width / Height */  width: 100%;  height: auto;}

• Reserve space for ads and embedded content:
  Use CSS to define min-height, min-width, or specific size properties for containers holding ads, iframes, or third-party widgets. If the exact size is unknown, use an average or largest predictable size.
• Avoid inserting dynamic content above existing content:
  If you need to display cookie banners or pop-ups, reserve space for them or only show them after a specific user action (e.g., scrolling to a certain point). Consider placing them at the bottom of the page or as an overlay so they don't shift main content.
• Optimize Web Font loading:
  Use font-display: swap; in CSS to tell the browser to display a fallback font first, then swap to the custom font once it's loaded. This helps minimize FOIT/FOUT. You can also preload important fonts.

  @font-face {  font-family: 'MyCustomFont';  src: url('my-custom-font.woff2') format('woff2');  font-display: swap; /* Use swap to display fallback font first */}

• Use CSS transform properties for animations:
  Instead of directly changing size or position properties, use transform: scale(), transform: translate() to create motion effects. These properties do not trigger layout recalculations.

Conclusion

CLS is not just a technical metric; it's a direct measure of your attention to user experience. A stable website that doesn't "jump around" will provide comfort, professionalism, and keep readers engaged longer. Start "cleaning up" CLS today to ensure your website is always "stable" and user-friendly!