Assigning property values, Cascading, and Inheritance

# 6 Assigning property values, Cascading, and Inheritance

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## 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 and the element is not the root of the document tree, use the computed value of the parent element.
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 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.

When the specified value is not 'inherit', the computed value of a property is determined as specified by the Computed Value line in the definition of the property. See the section on inheritance for the definition of computed values when the specified value is 'inherit'.

The computed value exists even when the property doesn't apply, as defined by the 'Applies To' [add reference] line. However, some properties may define the computed value of a property for an element to depend on whether the property applies to that element.

### 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,as described above. 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.

When inheritance occurs, elements inherit computed values. The computed value from the parent element becomes both the specified value and the computed value on the child.

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'.

### 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 computed value as the property for the element's parent. The 'inherit' value can be used to strengthen inherited values, and it can also be used on properties that are not normally inherited.

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 !important;
    }

## 6.3 The @import rule

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 illustrate how @import rules can be made media-dependent:

    @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.

- 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 behaves 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 for a recommended default style sheet for HTML 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 user and author rules have more weight than rules in the UA's default style sheet.

Rules specified in a given style sheet override rules of the same weight 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 sorting order:

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.
2. Sort by weight (normal or important) and origin (author, user, or user agent). In ascending order:
   1. user agent style sheets
   2. user normal style sheets
   3. author normal style sheets
   4. author important style sheets
   5. user important style sheets
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. 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.

### 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. This CSS feature improves accessibility of documents by giving users with special requirements (large fonts, color combinations, etc.) control over presentation.

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 style 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:

- count 1 if the selector is a 'style' attribute rather than a selector, 0 otherwise (= a) (In HTML, values of an element's "style" attribute are style sheet rules. These rules have no selectors, so a=1, b=0, c=0, and d=0.)
- count the number of ID attributes in the selector (= b)
- count the number of other attributes and pseudo-classes in the selector (= c)
- count the number of element names and pseudo-elements in the selector (= d)

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

Example(s):

Some examples:

     *             {}  /* a=0 b=0 c=0 d=0 -> specificity = 0,0,0,0 */
     li            {}  /* a=0 b=0 c=0 d=1 -> specificity = 0,0,0,1 */
     li:first-line {}  /* a=0 b=0 c=0 d=1 -> specificity = 0,0,0,2 */
     ul li         {}  /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */
     ul ol+li      {}  /* a=0 b=0 c=0 d=3 -> specificity = 0,0,0,3 */
     h1 + *[rel=up]{}  /* a=0 b=0 c=1 d=1 -> specificity = 0,0,1,1 */
     ul ol li.red  {}  /* a=0 b=0 c=1 d=3 -> specificity = 0,0,1,3 */
     li.red.level  {}  /* a=0 b=0 c=2 d=1 -> specificity = 0,0,2,1 */
     #x34y         {}  /* a=0 b=1 c=0 d=0 -> specificity = 0,1,0,0 */
     style=""          /* a=1 b=0 c=0 d=0 -> specificity = 1,0,0,0 */

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

In the above example, the color of the P element would be green. The declaration in the "style" attribute will override the one in the STYLE element because of cascading rule 3, since it has a higher specificity.

Note: The specificity is based only on the form of the selector. In particular, a selector of the form "[id=p33]" is counted as an attribute selector (a=0, b=0, c=1, d=0), even if the id attribute is defined as an "ID" in the source document's DTD.

### 6.4.4 Precedence of non-CSS presentational hints

The UA may choose to honor presentational attributes in the source document. If so, these attributes are translated to the corresponding CSS rules with specificity equal to 0, and are treated as if they were inserted at the start of the author style sheet. They may therefore be overridden by subsequent style sheet rules. In a transition phase, this policy will make it easier for stylistic attributes to coexist with style sheets.

For HTML, any attribute that is not in the following list should be considered presentational: abbr, accept-charset, accept, accesskey, action, alt, archive, axis, charset, checked, cite, class, classid, code, codebase, codetype, colspan, coords, data, datetime, declare, defer, dir, disabled, enctype, for, headers, href, hreflang, http-equiv, id, ismap, label, lang, language, longdesc, maxlength, media, method, multiple, name, nohref, object, onblur, onchange, onclick, ondblclick, onfocus, onkeydown, onkeypress, onkeyup, onload, onload, onmousedown, onmousemove, onmouseout, onmouseover, onmouseup, onreset, onselect, onsubmit, onunload, onunload, profile, prompt, readonly, rel, rev, rowspan, scheme, scope, selected, shape, span, src, standby, start, style, summary, title, type, usemap, value, valuetype, version.

For XHTML and other languages written in XML, no attribute should be considered presentational. The styling of elements and non-presentational attributes should be handled in the user agent stylesheet.

Example(s):

The following user stylesheet would override the font weight of 'b' elements in all documents, and the color of 'font' elements with color attributes in XML documents. It would not affect the color of any 'font' elements with color attributes in HTML documents:

    b { font-weight: normal; }
    font[color] { color: orange; }

The following, however, would override the color of font elements in all documents:

    font[color] { color: orange ! important; }