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Maximize App Performance by Optimizing Web Fonts

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You've just landed on a web page and you try to click a certain element, but just before you do, an ad loads on top of it and you end up clicking that thing instead.

That…that’s a layout shift. Everyone, developers and users alike, know that layout shifts are bad. And the later they happen, the more disruptive they are to users. In this workshop we're going to look into how web fonts cause layout shifts and explore a few strategies of loading web fonts without causing big layout shifts.

Table of Contents:

What’s CLS and how it’s calculated?

How fonts can cause CLS?

Font loading strategies for minimizing CLS

Recap and conclusion

This workshop has been presented at Vue.js London 2023, check out the latest edition of this JavaScript Conference.

FAQ

Cumulative Layout Shift (CLS) is a measure of visual stability, quantifying unexpected layout shifts during a webpage's lifespan. It's calculated using the impact fraction and the viewport distance fraction, both decimal values from 0 to 1, representing the percentage of the viewport impacted and the distance unstable elements have traveled, respectively.

To minimize CLS caused by web fonts, use efficient loading strategies like hosting your own fonts, using fewer font files, selecting the appropriate font formats such as WOFF2 for better compression, and utilizing the 'font-display' CSS property to control text visibility during font loading.

FOIT (Flash of Invisible Text) occurs when text remains invisible while the web font loads. FOUT (Flash of Unstyled Text) happens when a fallback or system font is displayed temporarily until the web font fully loads, potentially causing layout shifts.

A good CLS score ranges from 0.0 to 0.1, indicating minimal layout shifts. Scores from 0.1 to 0.25 need improvement, and scores above 0.25 are considered poor, leading to a bad user experience due to significant visual instability.

Hosting your own web fonts can enhance performance by reducing the number of HTTP roundtrips required for font files, ensuring faster loading times and greater reliability compared to relying on external services like Google Fonts.

The 'font-display' CSS property controls how text is displayed during font loading. Options like 'swap', 'fallback', and 'optional' dictate whether and when to show fallback text, directly influencing the occurrence and magnitude of layout shifts.

The CLS score can vary due to differences in device capabilities, browser behavior, and network speeds. Slower devices or networks might delay font loading, increasing the likelihood and severity of layout shifts.

Tools like Sentry can be used to monitor CLS in real-time production environments, providing insights into performance issues and their impact on user experience across different sessions and conditions.

performance optimization

Lazar Nikolov

49 min

10 May, 2023

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