By Shad Super in Questions —

What Impact Do Web Fonts Have on Core Web Vitals, and How Can I Optimize Their Loading and Rendering to Reduce CLS and LCP Times?

Summary

Web fonts significantly impact Core Web Vitals, particularly the Cumulative Layout Shift (CLS) and the Largest Contentful Paint (LCP) metrics. By optimizing the loading and rendering of web fonts, CLS and LCP times can be reduced, improving overall user experience. Optimization strategies involve establishing early connections to font providers, preventing invisible text during font loading, and loading only necessary font subsets or characters.

Impact of Web Fonts on Core Web Vitals

Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) is a Core Web Vital that measures visual stability. Web fonts can impact CLS if they cause elements on the page to move around during loading [Cumulative Layout Shift (CLS), 2020].

Largest Contentful Paint (LCP)

Largest Contentful Paint (LCP) measures when the most significant content element in the viewport becomes visible. When a web font loads slower, it can delay the LCP metric [Largest Contentful Paint (LCP) 2021].

Optimizing Web Fonts

Preconnect

Utilize the <link rel="preconnect"> directive to establish early connections to font providers. This practice can significantly speed up the time it takes for fonts to load [Use rel=preconnect to improve load speed, 2020].

Font Display

Avoid having invisible text while waiting for web fonts to load by using the font-display CSS feature. The most recommended value is <font-display: swap;>, which uses the fallback font until the custom font is ready to display [Controlling Font Performance with font-display, 2020].

Subsetting

To optimize the loading of web fonts, consider only loading the required font subsets or characters that you need for your site. This strategy significantly reduces font file sizes [Optimize Webfont Loading, 2020].

Conclusion

Proper optimization of web fonts can reduce CLS and LCP times, significantly improving the performance and user experience of a website. It's crucial to establish early connections to font providers, prevent invisible text, and load only necessary font subsets.

References