font-size-adjust: How to auto-adjust your font size
In this article, we’ll take a look at how the font-size-adjust attribute helps you auto-adjust your font size. We’ll cover the following topics:
font-size-adjust attribute worksfont-size-adjustfont-size CSS attribute compares to font-size-adjustThe Replay is a weekly newsletter for dev and engineering leaders.
Delivered once a week, it's your curated guide to the most important conversations around frontend dev, emerging AI tools, and the state of modern software.
font-size-adjust worksDevelopers often employ numerous font families and font sizes on a single webpage for style and readability purposes. A developer might, for example, use the Open Sans typeface for headlines and the Roboto Mono font for the body.
When a font cannot be accessed, the browser falls back on the second font provided, which may cause a significant shift in font size, and, in turn, shift around other parts of our UI. To give a better depiction of this as it would appear when implemented, take a look at the below illustration.
This example compares two typefaces with differing x-heights and demonstrates how the x-height of one may be adjusted to match the x-height of the other using font-size-adjust. Though both examples use the same two fonts, in the first line without font-size-adjust applied, all of the lowercase letters in the first font are much taller than those in the second font.
This is where font-size-adjust comes in handy — in the second line, the attribute adjusts the size of the letters in the second font to match the x-height of the first font’s letters.
font-size-adjust is useful because it determines font readability based on the size of lowercase letters, rather than the size of capital letters, and adjusts the size of lowercase letters to the size of those in the font currently in use.
Using the font-size-adjust attribute can help prevent this from happening by giving you more control over the font size. However, not all typefaces are supported by all browsers, and using unsupported typefaces may make your website look weird, as previously discussed. Let’s take a look at browser compatibility in the next section.
font-size-adjustBefore we move into the details, let’s review browser support for the font-size-adjust property. As of writing of this article, the CSS font-size-adjust property is currently only supported by Firefox by default.
Chrome and Opera support this property behind the “Experimental Web Platform Features” flag, which may be activated in chrome:/flags, and ranges from version 43 to 30. The CSS font-size-adjust attribute isn’t currently supported by Edge or Safari.
Source: MDN Docs
Now, let’s take a brief look at the CSS font-size property more broadly before diving into the CSS font-size-adjust property.
font-size propertyThe font-size CSS property sets the size of the font overall. There are some key things to note about the font-size property:
font-size property is set to a fixed value in em, the size is calculated based on the font size of the parent elementem, changing the font size of one element may impact the font size of othersHere is a sample of the font-size syntax:
/ absolute-size / font-size: medium | xx-small | x-small | small | large | x-large | xx-large; / relative-size / font-size: smaller | larger; / percentage / font-size: 10%; / length / font-size: 5px;
font-size-adjust propertyAs we mentioned before, the CSS attribute docs specify that the element’s font size should be adjusted based on the height of lowercase letters, rather than the height of uppercase letters.
When the primary font type cannot be accessed, the CSS font-size-adjust property allows developers to control font size at the component level. In such cases, a font backup is referenced, and the browser switches to display the secondary font.
But if there is a variation in the aspect ratio of the desired original and current fonts, this may cause issues with legibility and accessibility. The CSS font-size-adjust property may be used in situations where we need to retain the text’s legibility while maintaining its aesthetic.
The font-size-adjust syntax is as follows:
number | none | initial | inherit;
Number: The font-size-adjust property is set to a numberNone: This is the default valueInitial: Sets the value of this property to its defaultInherit: The font-size-adjust property is passed down from parent to childLet’s look at an example. Say that the font-size-adjust property receives a value of 0.5, which will return a font size that is half the given font size for lowercase letters.
See the Pen
font-size-adjust by Adekanye Oluwatomiyin (@Adetomi)
on CodePen.
We can also set this property as a number and multiply it by the font-size property to make it compatible with browsers that do not support font-size-adjust. Here’s another example:
font-size: 20px; font-size-adjust: 0.5;
This sets the x-height of the font’s lowercase letters to 20 * 0.5, which is equal to 10px.
In browsers that do not support font-size-adjust, a 20px font will still be executed when using this method.
Based on the image above, the difference between the two fonts isn’t very obvious. Whereas, if you use none, the predefined font, and its given size:
font-size-adjust: none
You can more clearly see that there is a clear difference between these two fonts. The size of both sentences is based on the font-size property, using the predefined font-size.
You should now have a better understanding of what the CSS font-size-adjust property does, why it’s important, and how to include it in your CSS styles.
You can start using this property in your web applications in order to improve legibility and aesthetics. Given that it is only supported by one browser, to avoid size difference due to browser incompatibility, it’s probably still a best practice to pick typefaces with closely comparable ratios.
Thank you, and happy styling.
As web frontends get increasingly complex, resource-greedy features demand more and more from the browser. If you’re interested in monitoring and tracking client-side CPU usage, memory usage, and more for all of your users in production, try LogRocket.
LogRocket lets you replay user sessions, eliminating guesswork around why bugs happen by showing exactly what users experienced. It captures console logs, errors, network requests, and pixel-perfect DOM recordings — compatible with all frameworks.
LogRocket's Galileo AI watches sessions for you, instantly identifying and explaining user struggles with automated monitoring of your entire product experience.
Modernize how you debug web and mobile apps — start monitoring for free.
Learn more →
children correctly in TypeScriptLearn modern best practices for typing React children in TypeScript, including ReactNode, PropsWithChildren, ComponentProps, and why React.FC is no longer recommended.
Learn how to internationalize Next.js apps with Lingui and next-intl, covering App Router, RSC, routing, locale detection, and dynamic language switching.
Vite vs Webpack in 2025: a senior engineer’s take on performance, developer experience, build control, and when each tool makes sense for React apps.
Learn how Vitest 4 makes migrating from Jest painless, with codemods, faster tests, native ESM, browser testing, and a better DX.
Hey there, want to help make our blog better?
Join LogRocket’s Content Advisory Board. You’ll help inform the type of content we create and get access to exclusive meetups, social accreditation, and swag.
Sign up now
One Reply to "CSS <code>font-size-adjust</code>: How to auto-adjust your font size"
It helped me a lot in setting the subtitles to display well. I always thought that you have to enter in PX and you can also enter in%. And can font family also use something to use the default from the page and not impose it?