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A factor contributing to the legibility of fonts at different sizes is the relationship between the height of the 'font-size' and the height of the font's x-height (the height of its lowercase letter 'x'.) The ratio between these two factors ('font-size' divided by x-height) is called a font's "aspect value." If an aspect value ratio is large for a font, it is more likely to be legible at a given font-size than a font with a smaller aspect value.

The font-size-adjust property allows authors to specify the "aspect value" that they wish to maintain. It becomes helpful when a specified font is unavailable and the system needs hints to determine the most suitable substitute.


h5.med  { font-size-adjust: number | none }

Legal Values


  • Explicitly sets the value of this property to that of the parent.


  • Do not preserve the current specified font's x-height if the font is not available.


  • Specifies the Aspect Value ratio for the font. The formula for determining the appropriate font size for an available font is:
 ['font-size']*(['font-size-adjust']/[aspect value of available font]) = [font size to use]

Mozilla Recommended Values

Usage Examples

Ext/Doc: h5.med  { font-size-adjust: 0.58 }
In-Line:  <h5 STYLE="font-size-adjust: 0.58">text</h5>


  • Child elements inherit 'un-adjusted' font-size values.
  • The CSS2 spec states that a 'font-size-adjust' number value indicates the aspect value of the first-choice font. No mention is made of what should happen if the actual aspect value of the indicated font conflicts with the given aspect value.

Specification Conformance

Browser Compatibility

See Also