font-size-adjust

The font-size-adjust property takes as its value the keyword none, one (<number> or from-font), or two (<font-metric> and either <number> or from-font) values.

none

No adjustment is applied to the font-size value for the fallback font.

<font-metric> Optional

Specifies the first-choice font metric to use for adjusting the font size of the fallback font. This parameter accepts one of the keywords listed below. It is an optional parameter, and ex-height is used if no <font-metric> is specified.

ex-height

Uses the ratio of x-height (height of lowercase "x" in a font) to font size (aspect value) to adjust the fallback font size. This keyword value is used to normalize lowercase letters across fonts.

cap-height

Uses the ratio of cap-height (height of uppercase letters) to font size to adjust fallback font size. This keyword value is used to normalize uppercase letters across fonts.

ch-width

Uses the ratio of the advance width (horizontal space taken up by a character in a font) of the character "0" (ZERO, U+0030) to font size. This keyword value is used to normalize horizontal narrow pitch of fonts.

ic-width

Uses the ratio of the advance width of the character "水" (CJK water ideograph, U+6C34) to font size. This keyword value is used to normalize horizontal wide pitch of fonts, particularly those that include CJK (Chinese, Japanese, Korean) characters.

ic-height

Uses the ratio of the advance height (vertical space taken up by a character in a font) of the character "水" (CJK water ideograph, U+6C34) to font size. This keyword value is used to normalize vertical wide pitch of fonts, particularly those that include CJK characters.

<number>

Adjusts the font size used depending on the specified <font-metric>. When no <font-metric> is specified (in which case the default value ex-height is used), the <number> value adjusts the font size of the fallback font so that its x-height is the specified multiple of the font size. This value should generally match the aspect value (ratio of x-height to font size) of the first-choice font. This means that the first-choice font, when available, will display consistently across browsers, regardless of their support for font-size-adjust.

When a <font-metric> value is specified, the <number> value adjusts the font size as per the chosen <font-metric> to maintain a consistent appearance for the specified font metric across different fonts.

The <number> value accepts any number from 0 to infinity. 0 yields text of zero height (that is, the text is hidden). Negative values are invalid.

from-font

Uses the <number> value for the specified <font-metric> from the first available font.

To ensure compatibility with browsers that don't support font-size-adjust, this property is specified as a numeric multiplier of the font-size property. This number should generally match the aspect value of the first-choice font.

The adjusted font size is calculated using the formula u = ( m / m′ ) s, where:

• m is the ratio of the specified <font-metric> to the first-choice font size.

• m′ is the ratio of the corresponding <font-metric> to the fallback font size.

• s is the value of the font-size property.

• u is the new, adjusted font size for the fallback font.

Consider this example to see how the adjusted font size is calculated. A first-choice font has a font-size of 12px (s), and the ratio of cap-height to font size is 0.20 (m). The cap-height to font size ratio in the fallback font is 0.15 (m′). The font-size-adjust value has been specified as cap-height 0.20. If the primary font is unavailable, the adjusted font size of the fallback font will be calculated to be 16px ((0.20 / 0.15) * 12). This will ensure that the cap-height of the fallback font is similar to that of the first-choice font when displayed.

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

This example demonstrates how the font-size-adjust property can be used to retain the same aspect value across fonts. The Verdana font has a relatively high aspect value of 0.545, which means that the lowercase letters are relatively tall compared to uppercase letters. This makes the text in small font sizes appear legible. However, the Times font has a lower aspect value of 0.447, so the text is less legible at small sizes. If Verdana is the first-choice font and Times is the fallback font, specifying the font-size-adjust property can help to retain the same aspect value in Times. So if the font falls back to Times, the text will maintain a similar level of legibility as it would have with Verdana.

Similarly, the cap-height to font size ratio in Verdana is 0.73 and that in Times is 0.66. When font-size-adjust property is applied to Times to adjust its uppercase letters to match the ratio in Verdana, the Times font displays in adjusted font size ((0.73 / 0.66) * 14) 15.48px.

<p class="verdana">
  A: This text uses the Verdana font (14px), which has relatively large
  lowercase letters.
</p>
<p class="times">
  B: This text uses the Times font (14px), which is hard to read in small sizes.
</p>
<p class="times adj-times-ex-height">
  C: This text in 14px Times font is adjusted to the same aspect value as the
  Verdana font, so lowercase letters are normalized across the two fonts.
</p>
<p class="times adj-times-cap-height">
  D: This text in 14px Times font is adjusted to the same cap-height to font
  size ratio as the Verdana font, so uppercase letters are normalized across the
  two fonts.
</p>

.times {
  font-family: Times, serif;
  font-size: 14px;
}

.verdana {
  font-family: Verdana, sans-serif;
  font-size: 14px;
}

.adj-times-ex-height {
  font-size-adjust: 0.545;
}

.adj-times-cap-height {
  font-size-adjust: cap-height 0.73;
}

Without font-size-adjust in B, the switch from Verdana font to Times font could result in a noticeable decrease in legibility due to its lower aspect value. In C, notice that only one value is specified for the font-size-adjust property, so the default <font-metric> value ex-height is used. D shows how the font would look compared to A if its uppercase letter height is adjusted.

For a given font, the same content in two side-by-side <span> elements can be used to determine the font's aspect value. If the same font size is used for content in both spans, the spans will match when the font-size-adjust value in one span is accurate for the given font.

In the example below, there are three pairs of side-by-side <span> elements, each containing the letter "b". The goal is to adjust the font-size-adjust property for the right <span> in each pair until the borders around the two letters align. The resulting font-size-adjust value can be considered the aspect value for the font.

Starting at 0.6 in the first pair and adjusting to 0.5 in the second, we continue adjusting the font-size-adjust property value until the borders around the "b" letters align perfectly in the third pair. In this example, the aspect value is determined to be 0.482.

<div>
  <p><span>b</span><span class="adjust1">b</span></p>
  0.6
</div>

<div>
  <p><span>b</span><span class="adjust2">b</span></p>
  0.5
</div>

<div>
  <p><span>b</span><span class="adjust3">b</span></p>
  0.482
</div>

body {
  display: flex;
}

div {
  text-align: center;
}

p {
  margin: 0 30px 10px 30px;
}

body {
  display: flex;
}

div {
  text-align: center;
}

p {
  font-family: Futura;
  font-size: 50px;
}

span {
  border: solid 1px red;
}

.adjust1 {
  font-size-adjust: 0.6;
}

.adjust2 {
  font-size-adjust: 0.5;
}

.adjust3 {
  font-size-adjust: 0.482;
}

• font-size
• font-weight
• size-adjust @font-face descriptor
• Learn: Fundamental text and font styling