6 Assigning property values, Cascading, and Inheritance


6.1 Specified, computed, and actual values

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").

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 results in a value, use it.
  2. Otherwise, if the property is inherited, use the value of the parent element, usually the actual 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 cannot use values from the parent element; in this case, the initial value is used if necessary.

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 the referenced actual 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.

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.

In most cases, elements inherit actual values. However, there are some properties whose specified or computed values may be inherited (e.g., the 'font-size' property and the number value for the 'line-height' property). The definition of a property whose actual value is not what other elements inherit will clearly say so.

6.2 Inheritance

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.

For example, since the 'color' property is inherited, all descendants of the BODY element will inherit the color 'black':

BODY { color: black; }

The following example illustrates that specified percentage values are not inherited; only actual values are inherited. Consider the style sheet:

BODY { font-size: 10pt }
H1 { font-size: 120% }

and the document fragment:

  <H1>A <EM>large</EM> heading</H1>

The computed value of the 'font-size' property for the H1 element is 12pt (120% times 10pt). If the user agent has the appropriate 12pt font available, 12pt will also be the property's actual value and the EM will inherit that value for the 'font-size' property. However, if the user agent does not have the 12pt font available, it may assign an actual value of, for example, 11pt to the 'font-size' property of the H1 element. In that case, the EM will inherit a value of 11pt for the same property.

6.2.1 The 'inherit' value

Each property may also have a specified value of 'inherit', which means that, for a given element, the property takes the same actual 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.

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.

  color: black !important; 
  background: white !important;

* { 
  color: inherit !important; 
  background: transparent;

6.3 The @import rule

The '@import' rule allows users to import style rules from other style sheets. 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.). Any @import rules must precede all rule sets in a style sheet.

Two examples: both of these import rules are valid; the first one illustrates that the "url()" syntax may be replaced by a string in the @import rule:

@import "mystyle.css";
@import url(herstyle.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.

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.

6.4 The cascade

Style sheets may have three different origins: author, user, and user agent.

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 a user's personal style sheets have less weight than rules in the author's style sheets. Thus, if there are conflicts between the style sheets of an incoming document and the reader's personal sheets, the author's rules will be used. Both reader and author rules override 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.

6.4.1 Cascading order

To find the value for an element/property combination, user agents must apply the following algorithm:

  1. 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. If no declarations apply, terminate the algorithm.
  2. Sort the declarations 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. An imported style sheet has the same origin as the style sheet that imported it.
  3. Sort 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.
  4. Sort by order specified: if two rules have the same weight, the latter specified wins. Rules in imported style sheets are considered to be before any rules in the style sheet itself.

The search for the property value must be terminated when any of the above steps yields a rule that has a higher weight than the other rules that apply to the same element/property combination.

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 gives the user the ability to turn off the influence of a certain style sheet, e.g., through a pull-down menu.

6.4.2 !important rules

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.

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".

The first rule in the user's style sheet in the following example contains an "!important" declaration, which override 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 }

6.4.3 Calculating a selector's specificity

A selector's specificity is calculated as follows:

Concatenating the three numbers a-b-c (in a number system with a large base) gives the specificity.

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.

<STYLE type="text/css">
  #x97z { color: blue }
<P ID=x97z style="color: red">

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.

6.4.4 Precedence of non-CSS presentational hints

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.