Tomi Adekanye An organism that makes code out of caffeine.

CSS font-size-adjust: How to auto-adjust your font size

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CSS 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:

How CSS font-size-adjust works

Developers 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.

An example using Open Sans font for the heading and 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.

A demonstration of how font-size-adjust works

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.

Browser compatibility for font-size-adjust

Before 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.

Browser support for font-size-adjust
Source: MDN Docs

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.

The CSS font-size property

The font-size CSS property sets the size of the font overall. There are some key things to note about the font-size property:

  • When the font-size property is set to a fixed value in em, the size is calculated based on the font size of the parent element
  • If other elements are stated in em, changing the font size of one element may impact the font size of others
  • When font size is provided as a percentage, it is calculated relative to the font size of the parent element

Here 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;

The CSS font-size-adjust property

As 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 number
  • None: This is the default value
  • Initial: Sets the value of this property to its default
  • Inherit: The font-size-adjust property is passed down from parent to child

Let’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
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;

You can set the font-size property and multiply it by the font-size-adjust prop for browsers that don't support the functionality

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

The same example as above, except using none, demonstrates the effect font-size-adjust has

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.

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Tomi Adekanye An organism that makes code out of caffeine.

One Reply to “CSS font-size-adjust: How to auto-adjust your font size”

  1. 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?

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