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

6.1.2 Computed values
Specified values may be absolute (i.e., they are not specified relative to another
value, as in ’red’ or ’2mm’) or relative (i.e., they are specified relative to another
value, as in ’auto’, ’2em’, and ’12%’). For absolute values, no computation is
needed to find the computed value.
Relative values, on the other hand, must be transformed into computed values:
percentages must be multiplied by a reference value (each property defines
which value that is), values with relative units (em, ex, px) must be made absolute
by multiplying with the appropriate font or pixel size, ’auto’ values must be
computed by the formulas given with each property, certain keywords (’smaller’,
’bolder’, ’inherit’) must be replaced according to their definitions.
In most cases, elements inherit computed values. However, there are some
properties whose specified value may be inherited (e.g., the number value for the
’line-height’ property). In the cases where child elements do not inherit the
computed value, this is described in the property definition.
6.1.3 Actual values
A computed value is in principle ready to be used, but a user agent may not be
able to make use of the value in a given environment. For example, a user agent
may only be able to render borders with integer pixel widths and may therefore
have to approximate the computed width. The actual value is the computed value
after any approximations have been applied.
6.2 Inheritance
Some values are inherited by the children of an element in the document tree
[p. 30] . Each property defines [p. 14] whether it is inherited or not.
Suppose there is an H1 element with an emphasizing element (EM) inside:
The headline is important!
If no color has been assigned to the EM element, the emphasized "is" will
inherit the color of the parent element, so if H1 has the color blue, the EM
element will likewise be in blue.
To set a "default" style property for a document, authors may set the property
on the root of the document tree. In HTML, for example, the HTML or BODY
elements can serve this function. Note that this will work even if the author omits
the BODY tag in the HTML source since the HTML parser will infer the missing
tag.
Example(s):
For example, since the ’color’ property is inherited, all descendants of the
BODY element will inherit the color ’black’:
BODY { color: black; }
Specified percentage values are not inherited; computed values are.
Example(s):
For example, given the following style sheet:
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