HTML and CSS Reference
188.8.131.52. Keyword property values
A property may have a keyword value that expresses action or dimen-
sion. For instance, the effects of underline and line-through are obvi-
ous property values. And you express property dimensions with such
keywords as small and xx-large . Some keywords are even relational:
bolder , for instance, is an acceptable value for the font-weight property.
Keyword values are not case sensitive: Underline, UNDERLINE , and under-
line are all acceptable keyword values.
184.108.40.206. Length property values
So-called length values (a term taken from the CSS2 standard) explicitly
set the size of a property. They are numbers, some with decimals, too.
Length values may have a leading + or - sign to indicate that the value
is to be added to or subtracted from the current value of the property.
Length values must be followed immediately by a two-letter unit abbre-
viation, with no intervening spaces.
There are three kinds of length-value units: relative, pixels, and abso-
lute. Relative units specify a size that is relative to the size of some oth-
er property of the content. Currently, there are only two relative units:
em , which is the width of the lowercase letter "m" in the current font;
and x-height, abbreviated ex , which is the height of the letter "x" in the
Pixels are the tiny dots of colored light that make up the onscreen text
and images on a computer monitor or TV image. The pixels unit, abbre-
viated px , is equal to the minute size of 1 pixel, so you may express the
size of some properties by how many pixels across or down they run.
Absolute property value units are more familiar to us all. They include
inches ( in ), centimeters ( cm ), millimeters ( mm ), points ( pt ; 1 / 72 of an
inch), and picas ( pc ; 12 points).