CSS size-adjust for @font-face

As a web font loads, you can now adjust its scale, to normalize the document font sizes and prevent layout shift when switching between fonts

Adam Argyle

Consider the following demo where the font-size is a consistent 64px, and the only difference between each of these headers is the font-family. The examples on the left have not been adjusted and have an inconsistent final size. The examples on the right use size-adjust to ensure 64px is the consistent final size.

This post explores a new CSS font-face descriptor called size-adjust. It also demonstrates a few ways to correct and normalize font sizes for smoother user experience, consistent design systems and more predictable page layout. One important use case is optimizing web font rendering to prevent cumulative layout shift (CLS).

Here's an interactive demo of the problem space. Try changing the typeface with the dropdown and observe:

1. The height differences in the results.
2. Visually jarring content shifts.
3. Moving interactive target areas (the dropdown jumps around!).

How to scale fonts with size-adjust

An introduction to the syntax:

@font-face {
  font-family: "Adjusted Typeface";
  size-adjust: 150%;
  src: url(some/path/to/typeface.woff2) format('woff2');
}

1. Creates a custom typeface named Adjusted Typeface.
2. Uses size-adjust to scale up each glyph to 150% their default size.
3. Affects only the single imported typeface.

Use the above custom typeface like this:

h1 {
  font-family: "Adjusted Typeface";
}

Mitigating CLS with seamless font swapping

In the following gif, watch the examples on the left and how there's a shift when the font is changed. This is just a small example with a single headline element and the issue is very noticeable.

To improve font rendering, a great technique is font swapping. Render a quick-loading system font to show the content first, then swap that with a web font when the web font finishes loading. This gives you the best of both worlds: the content is visible as soon as possible, and you get a nicely styled page without sacrificing your user's time to content. The problem however, is that sometimes when the web font loads, it shifts the entire page around since it presents at a slightly different box height size.

By putting size-adjust in the @font-face rule, it sets a global glyph adjustment for the font face. Timing this right will lead to minimal visual change, a seamless swap. To create a seamless swap, hand adjust or try this size-adjust calculator by Malte Ubl.

At the beginning of this post, fixing the font size issue was done by trial and error. size-adjust was nudged in the source code until the same header in Cookie and Arial rendered the same 64px as Press Start 2P did naturally. I aligned two fonts to a target font size.

@font-face {
  font-family: 'Adjusted Arial';
  size-adjust: 86%;
  src: local(Arial);
}

@font-face {
  font-family: 'Cookie';
  size-adjust: 90.25%;
  src: url(...woff2) format('woff2');
}

Calibrating fonts

You as the author determine the calibration target(s) for normalizing font scale. You might normalize on Times, Arial, or a system font, then adjust custom fonts to match. Or, you might adjust local fonts to match what you download.

Strategies for size-adjust calibration:

1. Remote target:
   Adjust local fonts towards downloaded fonts.
2. Local target:
   Adjust downloaded fonts towards local target fonts.

Example 1: Remote target

Consider the following snippet which adjusts a locally available font to size match a remote src custom font. A remote custom font is the target for calibration, a brand font perhaps:

@font-face {
  font-family: "Adjusted Regular Arial For Brand";
  src: local(Arial);
  size-adjust: 90%;
}

@font-face {
  font-family: "Rad Brand";
  src: url(some/path/to/a.woff2) format('woff2');
}

html {
  font-family: "Rad Brand", "Adjusted Regular Arial For Brand";
}

In this example, local font Arial is adjusting in anticipation of a loaded custom font, for a seamless swap.

This strategy has an advantage of offering designers and developers the same font for sizing and typography. The brand is the calibration target. This is great news for design systems.

Having a brand typeface as the target is also how Malte's calculator works. Choose a Google Font and it will calculate how to adjust Arial to seamlessly swap with it. Here's an example of Roboto CSS from the calculator, where Arial is loaded and named "Roboto-fallback":

@font-face {
    font-family: "Roboto-fallback";
    size-adjust: 100.06%;
    src: local("Arial");
}

To create a fully cross platform adjustment, see the following example which provides 2 adjusted local fallback fonts in anticipation of a brand font.

@font-face {
    font-family: "Roboto-fallback-1";
    size-adjust: 100.06%;
    src: local("Arial");
}

@font-face {
    font-family: "Roboto-fallback-2";
    size-adjust: 99.94%;
    src: local("Arimo");
}

@font-face {
  font-family: "Roboto Regular";
  src: url(some/path/to/roboto.woff2) format('woff2');
}

html {
  font-family: "Roboto Regular", "Roboto-fallback-1", "Roboto-fallback-2";
}

Example 2: Local target

Consider the following snippet which adjusts a brand custom font to match Arial:

@font-face {
  font-family: "Rad Brand - Adjusted For Arial";
  src: url(some/path/to/a.woff2) format('woff2');
  size-adjust: 110%;
}

html {
  font-family: "Rad Brand - Adjusted For Arial", Arial;
}

This strategy has the advantage of rendering without any adjustments, then adjusting any incoming fonts to match.

Finer tuning with ascent-override, descent-override and line-gap-override

If generic scaling of glyphs isn't enough adjustment for your design or loading strategies, here are some finer tuning options that work along with size-adjust. The ascent-override, descent-override, and line-gap-override properties are currently implemented in Chromium 87+, and Firefox 89+.

ascent-override

The ascent-override descriptor defines the height above the baseline of a font.

@font-face {
  font-family: "Ascent Adjusted Arial Bold";
  src: local(Arial Bold);
  ascent-override: 71%;
}

The red headline (unadjusted) has space above it's capital letter A and O, while the blue headline has been adjusted so it's cap height fits snug against the overall bounding box.

descent-override

The descent-override descriptor defines the height below the baseline of the font.

@font-face {
  font-family: "Ascent Adjusted Arial Bold";
  src: local(Arial Bold);
  descent-override: 0%;
}

The red headline (unadjusted) has space below it's D and O baselines, while the blue headline has been adjusted so it's letters rest snug on the baseline.

line-gap-override

The line-gap-override descriptor defines the line-gap metric for the font. This is the font recommended line-gap or external leading.

@font-face {
  font-family: "Line Gap Adjusted Arial";
  src: local(Arial);
  line-gap-override: 50%;
}

The red headline (unadjusted) has no line-gap-override, essentially it's at 0%, while the blue headline has been adjusted up by 50%, creating space above and below the letters accordingly.

Putting it all together

Each of these overrides offer an additional way to trim excess from the web's safe text bounding box. You can tailor the text box for precise presentation.

Conclusion

The @font-face size-adjust CSS feature is an exciting way to customize the text bounding box of your web layouts to improve the font swapping experience thus avoiding layout shift for your users. To learn more, check out these resources:

• CSS Fonts Level 5 Spec
• Size Adjust on MDN
• Seamless swap @font-face generator
• Cumulative Layout Shift (CLS) on web.dev
• A New Way To Reduce Font Loading Impact: CSS Font Descriptors

Photo by Kristian Strand on Unsplash