Reduce Cumulative Layout Shift (CLS) for Better UX

Understanding Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) is a crucial aspect of web performance that measures the total amount of unexpected layout shifts that occur during the lifetime of a webpage. A high CLS can lead to a poor user experience, increased bounce rates, and even negatively impact SEO rankings. To minimize CLS, it's essential to identify and address the common causes of layout shifts.

Common Causes of CLS

Some common causes of CLS include:

• Images without defined dimensions
• Dynamically loaded content
• Web fonts that load after the initial render
• Third-party scripts that inject content into the page

To mitigate these issues, developers can use various techniques such as defining image dimensions, using lazy loading, and optimizing web font loading.

Optimizing Images for Better CLS

Images are one of the most common causes of CLS. To minimize the impact of images on CLS, developers can use tools like the image-resizer to define image dimensions and compress images. Additionally, using the svg-optimizer can help reduce the file size of SVG images, which can also contribute to CLS.

Comparison of Image Formats

The following table compares the characteristics of different image formats:

| Format | Supports Transparency | Lossy/Lossless | Browser Support | | --- | --- | --- | --- | | PNG | Yes | Lossless | All modern browsers | | JPEG | No | Lossy | All modern browsers | | WebP | Yes | Lossy/Lossless | Most modern browsers | | SVG | Yes | Lossless | All modern browsers |

By choosing the right image format and optimizing images, developers can reduce the impact of images on CLS.

Optimizing Web Fonts for Better CLS

Web fonts can also contribute to CLS, especially if they load after the initial render. To minimize the impact of web fonts on CLS, developers can use techniques such as font preloading and using the font-display property.

Example of Font Preloading

@font-face {
  font-family: 'Open Sans';
  src: url('https://example.com/open-sans.woff2') format('woff2');
  font-display: swap;
}

By using font preloading and the font-display property, developers can reduce the impact of web fonts on CLS.

Implementing Lazy Loading for Better CLS

Lazy loading is a technique that involves loading content only when it comes into view. This can help reduce the impact of dynamically loaded content on CLS.

Example of Lazy Loading

const images = document.querySelectorAll('img');

images.forEach((image) => {
  image.addEventListener('intersection', (event) => {
    if (event.isIntersecting) {
      image.src = image.dataset.src;
    }
  });
});

By implementing lazy loading, developers can reduce the impact of dynamically loaded content on CLS.

Next Steps

To get started with reducing CLS on your website, try using the image-resizer to define image dimensions and compress images. Additionally, consider implementing lazy loading and optimizing web font loading to further minimize CLS. By taking these steps, you can improve the performance and user experience of your website.