Best practices for fonts

Optimize web fonts for Core Web Vitals.

Katie Hempenius

Barry Pollard

This document discusses performance best practices for fonts. There are a variety of ways in which web fonts impact performance:

Delayed text rendering: If a web font has not loaded, browsers typically delay text rendering. In many situations, this delays First Contentful Paint (FCP). In some situations, this delays Largest Contentful Paint (LCP).
Layout shifts: The practice of font swapping has the potential to cause layout shifts and impact Cumulative Layout Shift (CLS). These layout shifts occur when a web font and its fallback font take up different amounts of space on the page.

This document has three sections: font loading, font delivery, and font rendering. Each section explains how that particular aspect of the font lifecycle works and provides corresponding best practices.

Font loading

Fonts are important resources. Without them, the user may be unable to view page content. Thus, best practices for font loading generally focus on making sure that fonts get loaded as early as possible. Particular care should be given to fonts loaded from third-party sites as downloading these font files requires separate connection setups.

If you're unsure if your page's fonts are being requested in time, check the Timing tab within the Network panel in Chrome DevTools for more information.

The Timing tab in DevTools.

Understand `@font-face`

Before diving into best practices for font loading it's important to understand how @font-face works and how this impacts font loading.

The @font-face declaration is an essential part of working with any web font. At a minimum, it declares the name that's used to refer to the font and indicates the location of the corresponding font file.

@font-face {
  font-family: "Open Sans";
  src: url("/fonts/OpenSans-Regular-webfont.woff2") format("woff2");
}

A common misconception is that a font is requested when a @font-face declaration is encountered. This is false. By itself, @font-face declaration doesn't trigger font download. Rather, a font is downloaded only if it's referenced by styling that is used on the page. For example:

@font-face {
  font-family: "Open Sans";
  src: url("/fonts/OpenSans-Regular-webfont.woff2") format("woff2");
}

h1 {
  font-family: "Open Sans"
}

In this example, Open Sans would only be downloaded if the page contained a <h1> element.

Thus, when thinking about font optimization, it's important to give stylesheets just as much consideration as the font files themselves. Changing the contents or delivery of stylesheets can have a significant impact on when fonts arrive. Similarly, removing unused CSS and splitting stylesheets can reduce the number of fonts loaded by a page.

Inline font declarations

Most sites would strongly benefit from inlining font declarations and other critical styling in the <head> of the main document rather than including them in an external stylesheet. This allows the browser to discover the font declarations sooner as the browser doesn't need to wait for the external stylesheet to download.

<head>
  <style>
    @font-face {
        font-family: "Open Sans";
        src: url("/fonts/OpenSans-Regular-webfont.woff2") format("woff2");
    }

    body {
        font-family: "Open Sans";
    }

    ...etc.

  </style>
</head>

Inlining critical CSS can be a more advanced technique that not all sites can achieve. The performance benefits are clear, but it requires additional processes and build tools to ensure the necessarily CSS, and ideally only the critical CSS, is inlined correctly and that any additional CSS is delivered in a non-render blocking fashion.

Preconnect to critical third-party origins

If your site loads fonts from a third-party site, it's highly recommended that you use the preconnect resource hint to establish early connection(s) with the third-party origin. Resource hints should be placed in the <head> of the document. The following resource hint sets up a connection for loading the font stylesheet.

<head>
  <link rel="preconnect" href="https://fonts.com">
</head>

To preconnect the connection that is used to download the font file, add a separate preconnect resource hint that uses the crossorigin attribute. Unlike stylesheets, font files must be sent over a CORS connection.

<head>
  <link rel="preconnect" href="https://fonts.com">
  <link rel="preconnect" href="https://fonts.com" crossorigin>
</head>

When using the preconnect resource hint, keep in mind that a font provider may serve stylesheets and fonts from separate origins. For example, this is how the preconnect resource hint would be used for Google Fonts.

<head>
  <link rel="preconnect" href="https://fonts.googleapis.com">
  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
</head>

Be cautious when using `preload` to load fonts

Although preload is highly effective at making fonts discoverable early in the page load process, this comes at the cost of taking away browser resources from the loading of other resources.

Inlining font declarations and adjusting stylesheets may be a more effective approach. These adjustments come closer to addressing the root cause of late-discovered fonts, rather than just providing a workaround.

In addition, using preload as a font-loading strategy should also be used carefully as it bypasses some of the browser's built-in content negotiation strategies. For example, preload ignores unicode-range declarations, and if used prudently, should only be used to load a single font format.

However, when using external stylesheets, preloading the most important fonts can be very effective since the browser won't otherwise discover whether the font is needed until much later.

Font delivery

Faster font delivery yields faster text rendering. In addition, if a font is delivered early enough, this can help eliminate layout shifts resulting from font swapping.

Use self-hosted fonts

On paper, using a self-hosted font should deliver better performance as it eliminates a third-party connection setup. In practice, the performance differences between these two options is less clear cut. For example, the Web Almanac found that sites using third-party fonts had a faster render than fonts that used first-party fonts.

If you are considering using self-hosted fonts, confirm that your site uses a Content Delivery Network (CDN) and HTTP/2. Without use of these technologies, it's much less likely that self-hosted fonts deliver better performance.

If you use a self-hosted font, it's recommended that you also apply some of the font file optimizations that third-party font providers typically provide automatically. For example, font subsetting and WOFF2 compression. The amount of effort required to apply these optimizations depends somewhat on the languages that your site supports. In particular, be aware that optimizing fonts for CJK languages can be particularly challenging.

Use WOFF2

Of the modern font fonts, WOFF2 is the newest, has the widest browser support, and offers the best compression. Because it uses Brotli, WOFF2 compresses 30% better than WOFF, leading to less data to download and therefore faster performance.

Given the browser support, experts now recommend only using WOFF2:

In fact, we think it is also time to proclaim: Use only WOFF2 and forget about everything else.

This will simplify your CSS and workflow massively and also prevents any accidental double or incorrect font downloads. WOFF2 is now supported everywhere. So, unless you need to support really ancient browsers, just use WOFF2. If you can't, consider not serving any web fonts to those older browsers at all. This won't be a problem if you have a robust fallback strategy in place. Visitors on older browsers will see your fallback fonts.
Bram Stein, from the 2022 Web Almanac

Subset fonts

Font files typically include a large number of glyphs for all the various characters they support. But you may not need all the characters on your page and can reduce the size of font files by subsetting fonts.

The unicode-range descriptor in the @font-face declaration informs the browser which characters a font can be used for.

@font-face {
    font-family: "Open Sans";
    src: url("/fonts/OpenSans-Regular-webfont.woff2") format("woff2");
    unicode-range: U+0025-00FF;
}

A font file is downloaded if the page contains one or more characters matching the unicode range. unicode-range is commonly used to serve different font files depending on the language used by page content.

unicode-range is often used in conjunction with the technique of subsetting. A subset font includes a smaller portion of the glyphs that were contained in the original font file. For example, rather than serve all characters to all users, a site might generate separate subset fonts for Latin and Cyrillic characters.

The number of glyphs per font varies wildly:

Latin fonts are usually on the magnitude of 100 to 1000 glyphs per font.
CJK fonts may have over 10,000 characters.

Removing unused glyphs can significantly reduce the filesize of a font.

Some font providers may provide different versions of fonts files with different subsets automatically. For example, Google Fonts does this by default:

/* devanagari */
@font-face {
  font-family: 'Poppins';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/poppins/v20/pxiEyp8kv8JHgFVrJJbecnFHGPezSQ.woff2) format('woff2');
  unicode-range: U+0900-097F, U+1CD0-1CF6, U+1CF8-1CF9, U+200C-200D, U+20A8, U+20B9, U+25CC, U+A830-A839, U+A8E0-A8FB;
}
/* latin-ext */
@font-face {
  font-family: 'Poppins';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/poppins/v20/pxiEyp8kv8JHgFVrJJnecnFHGPezSQ.woff2) format('woff2');
  unicode-range: U+0100-024F, U+0259, U+1E00-1EFF, U+2020, U+20A0-20AB, U+20AD-20CF, U+2113, U+2C60-2C7F, U+A720-A7FF;
}
/* latin */
@font-face {
  font-family: 'Poppins';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/poppins/v20/pxiEyp8kv8JHgFVrJJfecnFHGPc.woff2) format('woff2');
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
}

When moving to self-hosting, this is an optimization that can be missed and lead to larger font files locally.

You can manually subset fonts if your font provider allows this, either with an API (Google Fonts supports this by providing a text parameter), or by manually editing the font files and then self-hosting. Tools for generating font subsets include subfont and glyphanger.

Always check the font licenses to confirm they allow subsetting and self-hosting.

Use fewer web fonts

The fastest font to deliver is a font that isn't requested in the first place. System fonts and variable fonts are two ways to potentially reduce the number of web fonts used on your site.

A system font is the default font used by the user interface of a user's device. System fonts typically vary by operating system and version. Because the font is already installed, the font does not need to be downloaded. System fonts can work particularly well for body text.

To use the system font in your CSS, list system-ui as the font-family:

font-family: system-ui

The idea behind variable fonts is that a single variable font can be used as a replacement for multiple font files. Variable fonts work by defining a "default" font style and providing "axes" for manipulating the font. For example, a variable font with a Weight axis could be used to implement lettering that would previously require separate fonts for light, regular, bold, and extra bold.

Not everyone benefits from switching to variable fonts. Variable fonts contain many styles, so typically have larger file sizes than individual non-variable fonts that only contain one style. Sites that will see the largest improvement from using variable fonts are those that use (and need to use) a variety of font styles and weights.

Font rendering

When faced with a web font that has not yet loaded, the browser is faced with a dilemma: should it hold off on rendering text until the web font has arrived? Or should it render the text in a fallback font until the web font arrives?

Different browsers handle this scenario differently. By default, Chromium-based and Firefox browsers block text rendering for up to 3 seconds if the associated web font has not loaded. Safari blocks text rendering indefinitely.

This behavior can be configured by using the font-display attribute. This choice can have significant implications: font-display has the potential to impact LCP, FCP, and layout stability.

Choose an appropriate `font-display` strategy

font-display informs the browser how it should proceed with text rendering when the associated web font has not loaded. It's defined per font-face.

@font-face {
  font-family: Roboto, Sans-Serif
  src: url(/fonts/roboto.woff) format('woff'),
  font-display: swap;
}

There are five possible values for font-display:

Value	Block period	Swap period
Auto	Varies by browser	Varies by browser
Block	2-3 seconds	Infinite
Swap	0ms	Infinite
Fallback	100ms	3 seconds
Optional	100ms	None

Block period: The block period begins when the browser requests a web font. During the block period, if the web font isn't available, the font is rendered in an invisible fallback font and thus the text is invisible to the user. If the font isn't available at the end of the block period, it's rendered in the fallback font.
Swap period: The swap period comes after the block period. If the web font becomes available during the swap period, it's "swapped" in.

font-display strategies reflect different viewpoints about the tradeoff between performance and aesthetics. As such, it's difficult to recommend an approach as it depends on individual preferences, how important the web font is to the page and brand, and how jarring a late-arriving font can be when swapped in.

For most sites, these are the three most applicable strategies, based on your top priority:

Performance: Use font-display: optional. This is the most "performant" approach: text render is delayed for no longer than 100ms and there's assurance that there isn't font-swap related layout shifts. The downside is that the web font won't be used if it arrives late.
Quickly display text and still use a web-font: Use font-display: swap but make sure to deliver the font early enough that it does not cause a layout shift. The downside of this option is the jarring shift when the font arrives late.
Text is displayed in a web font: Use font-display: block but make sure to deliver the font early enough that it minimizes the delay of the text. The initial text display is delayed. Despite this delay, it can still cause a layout shift as the text is actually drawn invisible, and the fallback font space is therefore used to reserve the space. Once the web font loads, this may require difference space, hence a shift. This may be a less jarring shift than font-display: swap, as the text itself won't be seen to shift.

Also keep in mind that these two approaches can be combined: for example, use font-display: swap for branding and other visually distinctive page elements. Use font-display: optional for fonts used in body text.

Icon fonts

The font-display strategies that work well for conventional web fonts don't work as well for icon fonts. The fallback font for an icon font typically looks significantly different than the icon font, and its characters may convey a completely different meaning. As a result, icon fonts are more likely to cause significant layout shifts.

In addition, using a fallback font may not be practical. When possible, replace icon fonts with SVGs, which is also better for accessibility. Newer versions of popular icon fonts typically support SVG. For more information on switching to SVGs, see Font Awesome and Material Icons.

Reduce the shift between your fallback font and your webfont

To reduce the CLS impact, you can use the size-adjust attributes.

Conclusion

Web fonts are still a performance bottleneck, but we have an ever-growing range of options to allow us to optimize them to reduce this bottleneck as much as possible.