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font-size-adjust

The font-size-adjust CSS property sets the size of lower-case letters relative to the current font size (which defines the size of upper-case letters).

/* Use the specified font size */
font-size-adjust: none;

/* Use a font size that makes lowercase
   letters half the specified font size */
font-size-adjust: 0.5;

/* Two values - added in the Level 5 spec */
font-size-adjust: ex-height 0.5;

/* Global values */
font-size-adjust: inherit;
font-size-adjust: initial;
font-size-adjust: revert;
font-size-adjust: revert-layer;
font-size-adjust: unset;

The property is useful since the legibility of fonts, especially at small sizes, is determined more by the size of lowercase letters than by the size of capital letters. Legibility can become an issue when the first-choice font-family is unavailable and its replacement has a significantly different aspect ratio (the ratio of the size of lowercase letters to the size of the font).

To use this property in a way that is compatible with browsers that do not support font-size-adjust, it is specified as a number that the font-size property is multiplied by. This means the value specified for the property should generally be the aspect ratio of the first choice font. For example, consider this style sheet:

font-size: 14px;
font-size-adjust: 0.5;

It is really specifying that the lowercase letters of the font should be 7px high (0.5 × 14px). This will still produce reasonable results in browsers that do not support font-size-adjust, where a 14px font will be used.

Syntax

Values

none

Choose the size of the font based only on the font-size property.

ex-height | cap-height | ch-width | ic-width | ic-height

Specifies the font metric to normalize on. Defaults to ex-height. One of:

ex-height: Normalize the aspect value of the fonts, using the x-height divided by the font size.
cap-height: Normalize the cap-height of the fonts, using the cap-height by the font size.
ch-width: Normalize the horizontal narrow pitch of the fonts, using the advance width of "0" (ZERO, U+0030) divided by the font size.
ic-width: Normalize the horizontal wide pitch of the font, using the advance width of "水" (CJK water ideograph, U+6C34) divided by the font size.
ic-height: Normalize the vertical wide pitch of the font, using the advance height of "水" (CJK water ideograph, U+6C34) divided by the font size.

<number>

Choose the size of the font so that its lowercase letters (as determined by the x-height of the font) are the specified number times the font-size.

The number specified should generally be the aspect ratio (ratio of x-height to font size) of the first choice font-family. This means that the first-choice font, when available, will appear the same size in browsers, whether or not they support font-size-adjust.

0 yields text of zero height (hidden text).

Formal definition

Initial value	`none`
Applies to	all elements. It also applies to `::first-letter` and `::first-line`.
Inherited	yes
Computed value	as specified
Animation type	a number

Formal syntax

font-size-adjust = 
  none            |
  <number [0,∞]>

Examples

Adjusting lower-case letter sizes

HTML

<p class="times">
  This text uses the Times font (10px), which is hard to read in small sizes.
</p>
<p class="verdana">
  This text uses the Verdana font (10px), which has relatively large lowercase
  letters.
</p>
<p class="adjtimes">
  This is the 10px Times, but now adjusted to the same aspect ratio as the
  Verdana.
</p>

CSS

.times {
  font-family: Times, serif;
  font-size: 10px;
}

.verdana {
  font-family: Verdana, sans-serif;
  font-size: 10px;
}

.adjtimes {
  font-family: Times, serif;
  font-size-adjust: ex-height 0.58;
  font-size: 10px;
}

Results

Specifications

Specification
CSS Fonts Module Level 5 # font-size-adjust-prop

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`font-size-adjust`	No	No	3 Before Firefox 40, `font-size-adjust: 0` was incorrectly interpreted as `font-size-adjust: none` (bug 1144885). 1–3 Only supported on Windows.	No	No	16.4	No	No	4 Before Firefox 40, `font-size-adjust: 0` was incorrectly interpreted as `font-size-adjust: none` (bug 1144885).	No	16.4	No
`two-values`	No	No	92	No	No	No	No	No	92	No	No	No