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

6 Assigning property values, Cascading,
and Inheritance
Contents
6.1 Specified, computed, and actual values . . . . . . . 69 .
6.1.1 Specified values . . . . . . . . . . . 69 .
6.1.2 Computed values . . . . . . . . . . . 70 .
6.1.3 Actual values . . . . . . . . . . . . 70 .
6.2 Inheritance . . . . . . . . . . . . . . 70 .
6.2.1 The ’inherit’ value . . . . . . . . . . . 71 .
6.3 The @import rule . . . . . . . . . . . . 71 .
6.4 The cascade . . . . . . . . . . . . . . 72 .
6.4.1 Cascading order . . . . . . . . . . . 73 .
6.4.2 !important rules . . . . . . . . . . . 73 .
6.4.3 Calculating a selector’s specificity . . . . . . . 74 .
6.4.4 Precedence of non-CSS presentational hints . . . . 74 .
6.1 Specified, computed, and actual values
Once a user agent has parsed a document and constructed a document tree
[p. 30] , it must assign, for every element in the tree, a value to every property
that applies to the target media type [p. 77] .
The final value of a property is the result of a three-step calculation: the value
is determined through specification (the "specified value"), then resolved into an
absolute value if necessary (the "computed value"), and finally transformed
according to the limitations of the local environment (the "actual value").
6.1.1 Specified values
User agents must first assign a specified value to a property based on the following
mechanisms (in order of precedence):
1. If the cascade [p. 72] results in a value, use it.
2. Otherwise, if the property is inherited [p. 70] , use the value of the parent
element, generally the computed value.
3. Otherwise use the property’s initial value. The initial value of each property is
indicated in the property’s definition.
Since it has no parent, the root of the document tree [p. 30] cannot use values
from the parent element; in this case, the initial value is used if necessary.
69