6 Assigning property values, Cascading, and Inheritance
Once a user agent has parsed a document and constructed a document tree, it must assign, for
every element in the tree, a value to every property that applies to the
target media type.
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").
User agents must first assign a specified value to a property based
on the following mechanisms (in order of precedence):
- If the cascade results in a value,
use it.
- Otherwise, if the property is inherited, use the value of the parent
element, generally the computed value.
- 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 cannot use values from
the parent element; in this case, the initial value is used if
necessary.
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.
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.
Some values are inherited by the children of an element in
the document tree. Each property defines whether it is inherited or
not.
Suppose there is an H1 element with an emphasizing element (EM)
inside:
<H1>The headline <EM>is</EM> important!</H1>
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:
BODY { font-size: 10pt }
H1 { font-size: 120% }
and this document fragment:
<BODY>
<H1>A <EM>large</EM> heading</H1>
</BODY>
the 'font-size' property
for the H1 element will have the computed value '12pt' (120% times
10pt, the parent's value). Since the computed value of 'font-size' is inherited, the EM
element will have the computed value '12pt' as well. If the
user agent does not have the 12pt font available, the
actual value of 'font-size'
for both H1 and EM might be, for example, '11pt'.
Each property may also have a specified value of 'inherit', which
means that, for a given element, the property takes the same computed value as the property for the
element's parent. The inherited value, which is normally only used as
a fallback value, can be strengthened by setting 'inherit' explicitly.
Example(s): In the example below, the 'color' and 'background' properties are set on
the BODY element. On all other elements, the 'color' value will be
inherited and the background will be transparent. If these rules are
part of the user's style sheet, black text on a white background will
be enforced throughout the document.
BODY {
color: black !important;
background: white !important;
}
* {
color: inherit !important;
background: transparent;
}
The '@import' rule allows users to
import style rules from other style sheets. Any @import rules must
precede all rule sets in a style sheet. The '@import' keyword must be
followed by the URI of the style sheet to include. A string is also
allowed; it will be interpreted as if it had url(...) around it.
Example(s):
The following lines are equivalent in meaning and illustrate both
'@import' syntaxes (one with "url()" and one with a bare string):
@import "mystyle.css";
@import url("mystyle.css");
So that user agents can avoid retrieving resources for unsupported
media types, authors may specify
media-dependent @import rules. These conditional
imports specify comma-separated media types after the URI.
Example(s):
The following rules have the same effect as if the imported style
sheet were wrapped in an @media rule for the same media, but it may
save the UA a fruitless download.
@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
In the absence of any media types, the import is
unconditional. Specifying 'all' for the medium has the same effect.
Style sheets may have three different origins: author, user, and
user agent.
- Author. The author specifies style sheets
for a source document according to the conventions of the document
language. For instance, in HTML, style sheets may be included in the
document or linked externally.
- User: The user may be able to specify style
information for a particular document. For example, the user may
specify a file that contains a style sheet or the user agent may
provide an interface that generates a user style sheet (or behave as
if it did).
- User agent: Conforming user agents must apply
a default style sheet
(or behave as if they did) prior to all other style sheets for a
document. A user agent's default style sheet should present the
elements of the document language in ways that satisfy general
presentation expectations for the document language (e.g., for visual
browsers, the EM element in HTML is presented using an italic
font). See "A sample style sheet for HTML
4.0" for a recommended default style sheet for HTML 4.0 documents.
Note that the default style sheet may change if system settings are
modified by the user (e.g., system colors). However, due to
limitations in a user agent's internal implementation, it may be
impossible to change the values in the default style sheet.
Style sheets from these three origins will overlap in scope, and
they interact according to the cascade.
The CSS cascade
assigns a weight to each style rule. When several rules apply, the one
with the greatest weight takes precedence.
By default, rules in author style sheets have more weight than
rules in user style sheets. Precedence is reversed, however, for
"!important" rules. All rules user and author rules have more weight
than rules in the UA's default style sheet.
Imported style sheets also cascade and their weight depends on
their import order. Rules specified in a given style sheet override
rules imported from other style sheets. Imported style sheets can
themselves import and override other style sheets, recursively, and
the same precedence rules apply.
To find the value for an element/property combination, user agents
must apply the following sorting order:
-
Find all declarations that apply to the element and property in
question, for the target media type.
Declarations apply if the associated selector matches the element in question.
-
The primary sort of the declarations is by weight and origin: for normal
declarations, author style sheets override
user style sheets which override the default style sheet. For
"!important"
declarations, user style sheets override author style sheets
which override the default style sheet.
"!important" declaration override normal declarations.
An imported style sheet has the same
origin as the style sheet that imported it.
-
The secondary sort is by specificity
of selector: more specific
selectors will override more general ones. Pseudo-elements and
pseudo-classes are counted as normal elements and classes,
respectively.
- Finally, sort by order specified: if two rules have the same
weight, origin and specificity,
the latter specified wins. Rules in imported style sheets are
considered to be before any rules in the style sheet itself.
Apart from the "!important" setting on individual declarations,
this strategy gives author's style sheets higher weight than those of
the reader. It is therefore important that the user agent give the
user the ability to turn off the influence of a certain style sheet,
e.g., through a pull-down menu.
CSS attempts to create a balance of power between author
and user style sheets. By default, rules in an author's style
sheet override those in a user's style sheet (see cascade
rule 3).
However, for balance, an "!important" declaration (the keywords "!"
and "important" follow the declaration) takes precedence over a normal
declaration. Both author and user style sheets may contain
"!important" declarations, and user "!important" rules override author
"!important" rules. This CSS feature improves accessibility
of documents by giving users with special requirements (large
fonts, color combinations, etc.) control over presentation.
Note.
This is a semantic change since CSS1. In CSS1, author "!important"
rules took precedence over user "!important" rules.
Declaring a shorthand property (e.g., 'background') to be "!important" is
equivalent to declaring all of its sub-properties to be "!important".
Example(s):
The first rule in the user's style sheet in the following example
contains an "!important" declaration, which overrides the corresponding
declaration in the author's styles sheet. The second declaration
will also win due to being marked "!important". However, the third
rule in the user's style sheet is not "!important" and will therefore
lose to the second rule in the author's style sheet (which happens to
set style on a shorthand property). Also, the third author rule will
lose to the second author rule since the second rule is
"!important". This shows that "!important" declarations have a
function also within author style sheets.
/* From the user's style sheet */
P { text-indent: 1em ! important }
P { font-style: italic ! important }
P { font-size: 18pt }
/* From the author's style sheet */
P { text-indent: 1.5em !important }
P { font: 12pt sans-serif !important }
P { font-size: 24pt }
A selector's specificity is calculated as follows:
- count the number of ID attributes in the selector (= a)
- count the number of other attributes and pseudo-classes
in the selector (= b)
- count the number of element names in the selector (= c)
- ignore pseudo-elements.
Concatenating the three numbers a-b-c (in a number system with a large
base) gives the specificity.
Example(s):
Some examples:
* {} /* a=0 b=0 c=0 -> specificity = 0 */
LI {} /* a=0 b=0 c=1 -> specificity = 1 */
UL LI {} /* a=0 b=0 c=2 -> specificity = 2 */
UL OL+LI {} /* a=0 b=0 c=3 -> specificity = 3 */
H1 + *[REL=up]{} /* a=0 b=1 c=1 -> specificity = 11 */
UL OL LI.red {} /* a=0 b=1 c=3 -> specificity = 13 */
LI.red.level {} /* a=0 b=2 c=1 -> specificity = 21 */
#x34y {} /* a=1 b=0 c=0 -> specificity = 100 */
In HTML, values of an element's "style" attribute are style sheet
rules. These rules have no selectors, but for the purpose of step 3 of
the cascade algorithm, they are considered to have an ID selector
(specificity: a=1, b=0, c=0). For the purpose of step 4, they are
considered to be after all other rules.
<HEAD>
<STYLE type="text/css">
#x97z { color: blue }
</STYLE>
</HEAD>
<BODY>
<P ID=x97z style="color: red">
</BODY>
In the above example, the color of the P element would be
red. Although the specificity is the same for both declarations, the
declaration in the "style" attribute will override the one in the
STYLE element because of cascading rule 4.
The UA may choose to honor presentational hints from other sources
than style sheets, for example the FONT element or the "align"
attribute in HTML. If so, the non-CSS presentational hints must be
translated to the corresponding CSS rules with specificity equal to
zero. The rules are assumed to be at the start of the author style sheet
and may be overridden by subsequent style sheet rules.
Note.
In a transition phase, this policy will make it easier for
stylistic attributes to coexist with style sheets.
Note.
In CSS1, the non-CSS presentational hints were given a specificity
equal to 1, not 0. The change is due to the introduction of the
universal selector, which has a specificity of 0.
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