By Shad Super in Questions —

How Does Preloading Critical Web Fonts Improve Mobile Layout Stability and Reduce "CLS Issue: More Than 0.1 (Mobile)"?

Overview

Preloading critical web fonts helps improve mobile layout stability and reduces issues related to Cumulative Layout Shift (CLS), which should ideally be less than 0.1 on mobile devices. Preloading can help achieve faster paint times, prevent font-related layout shifts, and contribute to a better overall user experience.

Understanding CLS and Web Fonts

Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) is a metric that quantifies how much a page's content visually shifts around [CLS, Web.dev, 2020]. A lower CLS contributes to a superior user experience.

The role of web fonts

Web fonts can play a significant role in CLS. When web fonts load on a page, they may cause the content to shift, contributing to a higher CLS score. Hence, it's crucial to manage your web font loading process efficiently.

The Impact of Preloading Web Fonts

What is font preloading?

Preloading is a technique that fetches necessary resources earlier in the critical rendering path, reducing the chance for unwanted layout shifts. Preloading fonts with <link rel="preload"> informs the browser to download the web font earlier [preload, Web.dev, 2020].

Benefits of preloading

Preloading web fonts contributes to improved layout stability and better CLS scores by:

  • Ensuring that fonts are loaded earlier, reducing the timeframe in which layout shifts can occur due to font loading.
  • Reducing potential for a Flash of Unstyled Text (FOUT) or a Flash of Invisible Text (FOIT), both of which can cause layout shifts [FOUT, FOIT, FOFT, CSS-Tricks, 2020].

How to Implement Font Preloading

Implement font preloading with the <link> HTML element, indicating the importance of the resource with "preload" and specifying the font's file type:

<link rel="preload" href="myfont.woff2" as="font" type="font/woff2" crossorigin>

Using font-display

Additionally, use the "font-display" property to control how fonts display based on whether and when they are downloaded and ready to use. For example, font-display:swap allows the text to be visible in the fallback font, then swap to the web font when it becomes available, reducing content shift [font-display Google Developers, 2016].

Conclusion

Efficient management of web font loading, especially through techniques like preloading, can significantly enhance website performance and user experience by reducing CLS and improving mobile layout stability.

References