Specifies an aspect value for an element that will preserve the x-height of the first-choice font. If the aspect value is high, the font will be legible when it is set to a smaller size.
Do not preserve the font's x-height if the font is unavailable
Defines the aspect value ratio for the font
The formula to use:
font-size of first-choice font*(font-size-adjust/aspect value of available font) = font-size to apply to available font
If 14px Verdana (aspect value of 0.58) was unavailable, but an available font had an aspect value of 0.46, the font-size of the substitute would be 14 * (0.58/0.46) = 17.65px