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.
Allowed Values
inherit
[CSS2]
Type: Explicit
Description:
Explicitly sets the value of this property to that of the parent.
none
[CSS2]
Type: Explicit
Description:
Do not preserve the current specified font's x-height if the font is not available.
[number]
[CSS2]
Type: Explicit
Description:
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]
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.