W3C CSS2 Cascading Style Sheets, level 2 - instructional media + ...

late between table entries or round off to the closest one. The user agent
may have to extrapolate table values if the numerical value goes beyond the
keywords.
A length value specifies an absolute font size (that is independent of the
user agent’s font table). Negative lengths are illegal.
A percentage value specifies an absolute font size relative to the parent
element’s font size. Use of percentage values, or values in ’em’s, leads to
more robust and cascadable style sheets.
The actual value [p. 70] of this property may differ from the computed value
[p. 70] due a numerical value on ’font-size-adjust’ and the unavailability of certain
font sizes.
Child elements inherit the computed ’font-size’ value (otherwise, the effect of
’font-size-adjust’ would compound).
Example(s):
P { font-size: 12pt; }
BLOCKQUOTE { font-size: larger }
EM { font-size: 150% }
EM { font-size: 1.5em }
’font-size-adjust’
Value: | none | inherit
Initial: none
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
In bicameral scripts, the subjective apparent size and legibility of a font are
less dependent on their ’font-size’ value than on the value of their ’x-height’, or,
more usefully, on the ratio of these two values, called the aspect value (font size
divided by x-height). The higher the aspect value, the more likely it is that a font
at smaller sizes will be legible. Inversely, faces with a lower aspect value will
become illegible more rapidly below a given threshold size than faces with a
higher aspect value. Straightforward font substitution that relies on font size
alone may lead to illegible characters.
For example, the popular font Verdana has an aspect value of 0.58; when
Verdana’s font size 100 units, its x-height is 58 units. For comparison, Times
New Roman has an aspect value of 0.46. Verdana will therefore tend to remain
legible at smaller sizes than Times New Roman. Conversely, Verdana will often
look ’too big’ if substituted for Times New Roman at a chosen size.
This property allows authors to specify an aspect value for an element that will
preserve the x-height of the first choice font in the substitute font. Values have
the following meanings:
none
Do not preserve the font’s x-height.
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