HTML and CSS Reference
In-Depth Information
Values, Units, and Colors
In this topic, we'll tackle features that are the basis for almost everything you can do
with CSS: the units that affect the colors, distances, and sizes of a whole host of prop-
erties, as well as the units that help to define those values. Without units, you couldn't
declare that an image should have 10 pixels of blank space around it, or that a heading's
text should be a certain size. By understanding the concepts put forth here, you'll be
able to learn and use the rest of CSS much more quickly.
Keywords, Strings, and Other Text Values
Of course, everything in a style sheet is text, but there are certain value types that directly
represent strings of text as opposed to, say, numbers or colors. Included in this category
are URLs and, interestingly enough, images.
Keywords
For those times when a value needs to be described with a word of some kind, there
are
keywords
. A very common example is the keyword
none
, which is distinct from
0
(zero). Thus, to remove the underline from links in an HTML document, you would
write:
a:link, a:visited {text-decoration: none;}
Similarly, if you want to force underlines on the links, then you would use the keyword
underline
.
If a property accepts keywords, then its keywords will be defined only for the scope of
that property. If two properties use the same word as a keyword, the behavior of the
keyword for one property will not necessarily be shared with the other. As an example,
normal
, as defined for
letter-spacing
, means something very different than the
normal
defined for
font-style
.
CSS3 defines two “global” keywords, one of which has fairly widespread support:
inherit
and
initial
.
