Preventing Layout Shift with Proper CSS

Understanding Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) is a critical metric in web performance that measures visual stability. It quantifies unexpected layout shifts that occur during the lifespan of a page, which can result in poor user experience. These shifts happen when visible elements change their position or size abruptly, especially during loading.

Common Causes of Layout Shifts

• Images or videos without defined dimensions.
• Ads or third-party embeds loading asynchronously.
• Fonts loading late causing text reflow.
• Dynamically injected content without proper space reservation.

Using CSS to Prevent Layout Shifts

Proper CSS techniques play a crucial role in minimizing CLS. Two essential strategies are defining explicit sizing and utilizing the aspect-ratio property.

1. Defining Width and Height

Always specify width and height attributes on images and videos or define these explicitly using CSS. This helps the browser allocate the correct amount of space on the page before the resource loads, preventing content from shifting as media elements appear.

2. Using the aspect-ratio Property

The aspect-ratio CSS property allows you to define an element's width-to-height ratio without needing fixed height values. This is particularly useful for responsive images and containers that need to maintain a certain ratio while adjusting to different screen sizes.

img {
  width: 100%;
  aspect-ratio: 16 / 9;
  object-fit: cover;
}

By specifying an aspect ratio, browsers can reserve the right amount of space before the image fully loads, eliminating unexpected shifts.

3. Reserving Space for Dynamic Content

For ads, iframes, or any content injected asynchronously, use fixed size containers or CSS aspect ratios to reserve space. This reduces the chance of the surrounding content shifting when the dynamic content loads.

4. Avoid Layout Changes on Font Load

Use font-display strategies like font-display: swap; to prevent invisible text during font loading that causes reflow and shifts. Preload fonts to speed up availability.

Additional Tips

• Use placeholders or skeletons to notify users that content is loading.
• Test your pages with tools like Google Lighthouse to monitor CLS and identify problematic elements.
• Use the min-height or min-width properties to keep the layout stable during asynchronous content loading.

Conclusion

Preventing layout shifts is essential for delivering a smooth and user-friendly web experience. Utilizing CSS properties like explicit sizing and aspect-ratio helps allocate space efficiently and stabilize the layout during load times. By combining these strategies with good content loading practices, you can significantly reduce CLS and improve your site's performance and usability.