CSS Cascading and Inheritance Level 3

Shortname: css-cascade
Level: 3
Status: ED
ED: http://dev.w3.org/csswg/css-cascade/
TR: http://www.w3.org/TR/css3-cascade/
Editor: Håkon Wium Lie, Opera Software, howcome@opera.com
Editor: fantasai, Mozilla, http://fantasai.inkedblade.net/contact
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
Previous Version: http://www.w3.org/TR/2013/WD-css3-cascade-20130103/
Previous Version: http://www.w3.org/TR/2005/WD-css3-cascade-20051215/
Abstract: This CSS module describes how to collate style rules and assign values to all properties on all elements. By way of cascading and inheritance, values are propagated for all properties on all elements.
Ignored Terms: auto, flex items

Introduction

One of the fundamental design principles of CSS is cascading, which allows several style sheets to influence the presentation of a document. When different declarations try to set a value for the same element/property combination, the conflicts must somehow be resolved.

The opposite problem arises when no declarations try to set a the value for an element/property combination. In this case, a value is be found by way of inheritance or by looking at the property's initial value.

The cascading and defaulting process takes a set of declarations as input, and outputs a specified value for each property on each element.

The rules for finding the specified value for all properties on all elements in the document are described in this specification. The rules for finding the specified values in the page context and its margin boxes are described in [[CSS3PAGE]].

Importing Style Sheets: the ''@import'' rule

The ''@import'' rule allows users to import style rules from other style sheets. Any ''@import'' rules must precede all other at-rules and style rules in a style sheet (besides ''@charset'', which must be the first thing in the style sheet if it exists), or else the ''@import'' rule is invalid. The syntax of ''@import'' is:

<import-rule> = @import [ <url> | <string> ] <media-query-list>? ;

If a <string> is provided, it must be interpreted as a url with the same value.

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");

If an ''@import'' rule refers to a valid stylesheet, user agents must treat the contents of the stylesheet as if they were written in place of the ''@import'' rule.

For example, declarations in style rules from imported stylesheets interact with the cascade as if they were written literally into the stylesheet at the point of the ''@import''. Similarly, style rules in a stylesheet imported into a scoped stylesheet are scoped in the same way.

Authors may also specify media-dependent ''@import'' rules. Such media-dependent imports include a comma-separated list of “media queries” after the URI. In the absence of any media query, the import is unconditional. (Specifying ''all'' for the medium has the same effect.) If the media query does not match, the rules in the imported stylesheet do not apply, exactly as if the imported stylesheet were wrapped in an ''@media'' block with the given media query. The evaluation and full syntax of the expressions after the URL is defined by the Media Queries specification [[!MEDIAQ]].

User agents may avoid fetching a media-dependent import until the media query matches.

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

@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
@import url("narrow.css") handheld and (max-width: 400px);

When the same style sheet is imported or linked to a document in multiple places, user agents must process (or act as though they do) each link as though the link were to a separate style sheet.

Note: This does not place any requirements on resource fetching, only how the style sheet is reflected in the CSSOM and used in specs such as this one. Assuming appropriate caching, it is perfectly appropriate for a UA to fetch a style sheet only once, even though it's linked or imported multiple times.

The origin of an imported style sheet is the same as the origin of the style sheet that imported it.

Content-Type of CSS Style Sheets

The processing of imported style sheets depends on the actual type of the linked resource. If the resource does not have Content-Type metadata, or the host document is in quirks mode and has the same origin as the imported style sheet, the type of the linked resource is text/css. Otherwise, the type is determined from its Content-Type metadata.

If the linked resource's type is text/css, it must be interpreted as a CSS style sheet. Otherwise, it must be interpreted as a network error.

Shorthand Properties

Some properties are shorthand properties, meaning that they allow authors to specify the values of several properties with a single property. A shorthand property sets all of its longhand sub-properties, exactly as if expanded in place.

For example, the 'font' property is a shorthand property for setting font-style, font-variant, font-weight, 'font-size', 'line-height', and font-family all at once. The multiple declarations of this example:

h1 {
	font-weight: bold;
	font-size: 12pt;
	line-height: 14pt;
	font-family: Helvetica;
	font-variant: normal;
	font-style: normal;
}

can therefore be rewritten as

h1 { font: bold 12pt/14pt Helvetica }

When values are omitted from a shorthand form, unless otherwise defined, each “missing” sub-property is assigned its initial value.

This means that a shorthand property declaration always sets all of its sub-properties, even those that are not explicitly set. Carelessly used, this might result in inadvertently resetting some sub-properties. Carefully used, a shorthand can guarantee a “blank slate” by resetting sub-properties inadvertently cascaded from other sources.

For example, writing ''background: green'' rather than ''background-color: green'' ensures that the background color overrides any earlier declarations that might have set the background to an image.

In some cases, a shorthand might have different syntax or special keywords that don't directly correspond to values of its sub-properties. (In such cases, the shorthand will explicitly define the expansion of its values.)

If a shorthand is specified as one of the CSS-wide keywords [[!CSS3VAL]], it sets all of its sub-properties to that keyword. (Note that these keywords cannot be combined with other values in a single declaration, not even in a shorthand.)

Declaring a shorthand property to be ''!important'' is equivalent to declaring all of its sub-properties to be ''!important''.

Resetting All Properties: the ''all'' property

Name:	all
Value:	initial \| inherit \| unset
Initial:	See individual properties
Applies to:	See individual properties
Inherited:	See individual properties
Percentages:	See individual properties
Media:	See individual properties
Computed value:	See individual properties
Animatable:	See individual properties

The ''all'' property is a shorthand that resets all CSS properties except 'direction' and 'unicode-bidi'. It only accepts the CSS-wide keywords.

The excepted properties are actually markup-level features, and should not be set in the author's style sheet. (They exist as CSS properties only to style document languages not supported by the UA.) Authors should use the appropriate markup, such as HTML's dir attribute, instead. [[!CSS3-WRITING-MODES]]

For example, if an author specifies ''all: initial'' on an element it will block all inheritance and reset all properties, as if no rules appeared in the author, user, or user-agent levels of the cascade.

This can be useful for the root element of a "widget" included in a page, which does not wish to inherit the styles of the outer page. Note, however, that any "default" style applied to that element (such as, e.g. ''display: block'' from the UA style sheet on block elements such as <div>) will also be blown away.

Value Processing

Once a user agent has parsed a document and constructed a document tree, it must assign, to every element in the tree, and correspondingly to every box in the formatting structure, a value to every property that applies to the target media type.

The final value of a CSS property for a given element or box is the result of a multi-step calculation:

First, all the declared values applied to an element are collected, for each property on each element. There may be zero or many declared values applied to the element.
Cascading yields the cascaded value. There is at most one cascaded value per property per element.
Defaulting yields the specified value. Every element has exactly one specified value per property.
Resolving value dependencies yields the computed value. Every element has exactly one computed value per property.
Formatting the document yields the used value. An element only has a used value for a given property if that property applies to the element.
Finally, the used value is transformed to the actual value based on constraints of the display environment. As with the used value, there may or may not be an actual value for a given property on an element.

Declared Values

Each property declaration applied to an element contributes a declared value for that property associated with the element. See Filtering Declarations for details.

These values are then processed by the cascade to choose a single “winning value”.

Cascaded Values

The cascaded value represents the result of the cascade: it is the declared value that wins the cascade (is sorted first in the output of the cascade). If the output of the cascade is an empty list, there is no cascaded value.

Specified Values

The specified value the value of a given property that the style sheet authors intended for that element. It is the result of putting the cascaded value through the defaulting processes, guaranteeing that a specified value exists for every property on every element.

In many cases, the specified value is the cascaded value. However, if there is no cascaded value at all, the specified value is defaulted. The ''initial'' and ''inherit'' keywords are handled specially when they are the cascaded value of a property,

Computed Values

The computed value is the result of resolving the specified value as defined in the “Computed Value” line of the property definition table, generally absolutizing it in preparation for inheritance.

The computed value is the value that is transferred from parent to child during inheritance. For historical reasons, it is not necessarily the value returned by the getComputedStyle() function.

A specified value can be either absolute (i.e., not relative to another value, as in ''red'' or ''2mm'') or relative (i.e., relative to another value, as in ''auto'', ''2em''). Computing a relative value generally absolutizes it:

values with relative units (''em'', ''ex'', ''vh'', ''vw'') must be made absolute by multiplying with the appropriate reference size
certain keywords (e.g., ''smaller'', ''bolder'') must be replaced according to their definitions
percentages on some properties must be multiplied by a reference value (defined by the property)
valid relative URLs must be resolved to become absolute.

See examples (f), (g) and (h) in the table below.

In general, the computed value resolves the specified value as far as possible without laying out the document or performing other expensive or hard-to-parallelize operations, such as resolving network requests or retrieving values other than from the element and its parent.

The computed value exists even when the property does not apply (as defined by the “Applies To” line). However, some properties may change how they determine the computed value based on whether the property applies to the element.

Used Values

The used value is the result of taking the computed value and completing any remaining calculations to make it the absolute theoretical value used in the layout of the document. If the property does not apply to this element, then the element has no used value for that property.

For example, a declaration of ''width: auto'' can't be resolved into a length without knowing the layout of the element's ancestors, so the computed value is ''auto'', while the used value is an absolute length, such as ''100px''. [[CSS21]]

As another example, a <div> might have a computed 'break-before' value of ''auto'', but acquire a used 'break-before' value of ''page'' by propagation from its first child. [[CSS3-BREAK]]

Lastly, if a property does not apply to an element, it has no used value; so, for example, the 'flex' property has no used value on elements that aren't flex items.

Actual Values

A used 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 used width. Also, the font size of an element may need adjustment based on the availability of fonts or the value of the 'font-size-adjust' property. The actual value is the used value after any such adjustments have been made.

By probing the actual values of elements, much can be learned about how the document is laid out. However, not all information is recorded in the actual values. For example, the actual value of the 'page-break-after' property does not reflect whether there is a page break or not after the element. Similarly, the actual value of 'orphans' does not reflect how many orphan lines there is in a certain element. See examples (j) and (k) in the table below.

Examples

	Property	Winning declaration	Cascaded value	Specified value	Computed value	Used value	Actual value
(a)	'text-align'	`text-align: left`	''left''	''left''	''left''	''left''	''left''
(b)	'border-top-width', 'border-right-width', 'border-bottom-width', 'border-left-width'	`border-width: inherit`	''inherit''	''4.2px''	''4.2px''	''4.2px''	''4px''
(c)	'width'	(none)	(none)	''auto'' (initial value)	''auto''	''120px''	''120px''
(d)	'list-style-position'	`list-style-position: inherit`	''inherit''	''inside''	''inside''	''inside''	''inside''
(e)	'list-style-position'	`list-style-position: initial`	''initial''	''outside'' (initial value)	''outside''	''outside''	''outside''
(f)	'font-size'	`font-size: 1.2em`	''1.2em''	''1.2em''	''14.1px''	''14.1px''	''14px''
(g)	'width'	`width: 80%`	''80%''	''80%''	''80%''	''354.2px''	''354px''
(h)	'width'	`width: auto`	''auto''	''auto''	''auto''	''134px''	''134px''
(i)	'height'	`height: auto`	''auto''	''auto''	''auto''	''176px''	''176px''
(j)	'page-break-after'	(none)	(none)	''auto'' (initial value)	''auto''	''auto''	''auto''
(k)	'orphans'	`orphans: 3`	''3''	''3''	''3''	''3''	''3''

Filtering

In order to find the declared values, implementations must first identify all declarations that apply to each element. A declaration applies to an element if:

It belongs to a style sheet that currently applies to this document.
It is not qualified by a conditional rule [[!CSS3-CONDITIONAL]] with a false condition.
It belongs to a style rule whose selector matches the element. [[!SELECT]] (Taking scoping into account, if necessary.)
It is syntactically valid: the declaration's property is a known property name, and the declaration's value matches the syntax for that property.

The values of the declarations that apply form, for each property on each element, a list of declared values. The next section, the cascade, prioritizes these lists.

Cascading

The cascade takes a unordered list of declared values for a given property on a given element, sorts them by their declaration’s precedence as determined below, and outputs a single cascaded value.

The cascade sorts declarations according to the following criteria, in descending order of priority:

Origin and Importance

The origin of a declaration is based on where it comes from and its importance is whether or not it is declared ''!important'' (see below). The precedence of the various origins is, in descending order:

Transition declarations [[!CSS3-TRANSITIONS]]
Important user agent declarations
Important user declarations
Important override declarations [[!DOM-LEVEL-2-STYLE]]
Important author declarations
Animation declarations [[!CSS3-ANIMATIONS]]
Normal override declarations [[!DOM-LEVEL-2-STYLE]]
Normal author declarations
Normal user declarations
Normal user agent declarations

Declarations from origins earlier in this list win over declarations from later origins.

Scope

A declaration can be scoped to a subtree of the document so that it only affects its scoping element and that element's descendants. For example, [[HTML5]] defines scoped <style> elements, whose style sheets are scoped to the element's parent.

If the scoping elements of two declarations have an ancestor/descendant relationship, then for normal rules the declaration whose scoping element is the descendant wins, and for important rules the declaration whose scoping element is the ancestor wins.

In other words, for normal declarations the inner scope's declarations override, but for !important rules outer scope's override.

For the purpose of this step, all unscoped declarations are considered to be scoped to the root element. Normal declarations from style attributes are considered to be scoped to the element with the attribute, whereas important declarations from style attributes are considered to be scoped to the root element. [[!CSSSTYLEATTR]]

This odd handling of !important style attribute declarations is to match the behavior defined in CSS Levels 1 and 2, where style attributes simply have higher specificity than any other author rules. [[CSS21]]

Specificity

The Selectors module [[!SELECT]] describes how to compute the specificity of a selector. Each declaration has the same specificity as the style rule it appears in. For the purpose of this step, declarations that do not belong to a style rule (such as the contents of a style attribute) are considered to have a specificity higher than any selector. The declaration with the highest specificity wins.

Order of Appearance

The last declaration in document order wins. For this purpose:

Declarations from imported style sheets are ordered as if their style sheets were substituted in place of the ''@import'' rule.
Declarations from style sheets independently linked by the originating document are treated as if they were concatenated in linking order, as determined by the host document language.
Declarations from style attributes are ordered according to the document order of the element the style attribute appears on, and are all placed after any style sheets.

The output of the cascade is a (potentially empty) sorted list of declared values for each property on each element.

Cascading Origins

Each style rule has an origin, which determines where it enters the cascade. CSS defines three core origins:

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). 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 e.g. the HTML user agent style sheet. [[HTML5]]

Extensions to CSS define the following additional origins:

Override: DOM Level 2 Style [[!DOM-LEVEL-2-STYLE]] defines an interface for “override” styles, which enter the cascade at a higher level than other author rules.
Animation: CSS Animations [[CSS3-ANIMATIONS]] generate “virtual” rules representing their effects when running.
Transition: Like CSS Animations, CSS Transitions [[CSS3-TRANSITIONS]] generate “virtual” rules representing their effects when running.

Important Declarations: the ''!important'' annotation

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, which override those in the user-agent's default style sheet. To balance this, a declaration can be made important, which increases its weight in the cascade and inverts the order of precedence.

A declaration is important if it has a !important annotation, i.e. if the last two (non-whitespace, non-comment) tokens in its value are the delimiter token ''!'' followed by the identifier token ''important''.

[hidden] { display: none !important; }

An important declaration takes precedence over a normal declaration. Author and user style sheets may contain ''!important'' declarations, with user ''!important'' declarations overriding author ''!important'' declarations. This CSS feature improves accessibility of documents by giving users with special requirements (large fonts, color combinations, etc.) control over presentation.

Important declarations from all origins take precedence over animations. This allows authors to override animated values in important cases. (Animated values normally override all other rules.) [[CSS3-ANIMATIONS]]

User agent style sheets may also contain ''!important'' declarations. These override all author and user declarations.

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 declaration in the second rule will also win due to being marked ''!important''. However, the third declaration 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 declaration 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: normal 12pt sans-serif !important }
p { font-size: 24pt }

Precedence of Non-CSS Presentational Hints

The UA may choose to honor presentational hints in a source document's markup, for example the bgcolor attribute or <s> element in [[HTML5]]. All document language-based styling must be translated to corresponding CSS rules and either enter the cascade at the user agent level or be treated as author level rules with a specificity of zero placed at the start of the author style sheet. A document language may define whether a presentational hint enters at the UA or author level of the cascade; if so, the UA must behave accordingly. For example, [[SVG11]] maps its presentation attributes into the author level.

Presentational hints entering the cascade at the UA level can be overridden by author or user styles. Presentational hints entering the cascade at the author level can be overridden by author styles, but not by non-!important user styles. Host languages should choose the appropriate level for presentational hints with these considerations in mind.

Defaulting

When the cascade does not result in a value, the specified value must be found some other way. Inherited properties draw their defaults from their parent element through inheritance; all other properties take their initial value. Authors can explicitly request inheritance or initialization via the ''inherit'' and ''initial'' keywords.

Initial Values

Each property has an initial value, defined in the property's definition table. If the property is not an inherited property, and the cascade does not result in a value, then the specified value of the property is its initial value.

Inheritance

Inheritance propagates property values from parent elements to their children. The inherited value of a property on an element is the computed value of the property on the element's parent element. For the root element, which has no parent element, the inherited value is the initial value of the property.

(Pseudo-elements inherit according to a fictional tag sequence described for each pseudo-element [[!SELECT]].)

Some properties are inherited properties, as defined in their property definition table. This means that, unless the cascade results in a value, the value will be determined by inheritance.

A property can also be explicitly inherited. See the ''inherit'' keyword.

Note that inheritance follows the document tree and is not intercepted by anonymous boxes, or otherwise affected by manipulations of the box tree.

Explicit Defaulting

Several CSS-wide property values are defined below; declaring a property to have these values explicitly specifies a particular defaulting behavior. As specified in CSS Values and Units Level 3 [[!CSS3VAL]], all CSS properties can accept these values.

Resetting a Property: the ''initial'' keyword

If the cascaded value is the ''initial'' keyword, the property's initial value becomes its specified value.

Explicit Inheritance: the ''inherit'' keyword

If the cascaded value of a property is the ''inherit'' keyword, the inherited value becomes the property's specified and computed values.

Erasing All Declarations: the ''unset'' keyword

If the cascaded value of a property is the ''unset'' keyword, then if it is an inherited property, this is treated as ''inherit'', and if it is not, this is treated as ''initial''. This keyword effectively erases all declared values occurring earlier in the cascade, correctly inheriting or not as appropriate for the property (or all longhands of a shorthand).

Changes

Changes Since the 3 January 2013 Working Draft

The following major changes were made to this specification since the 3 January 2013 Working Draft:

Dropped the ''default'' keyword.
Added the ''unset'' keyword.
Excluded 'unicode-bidi' and 'direction' from the 'all' shorthand.
Various clarifications.

Acknowledgments

David Baron, Simon Sapin, and Boris Zbarsky contributed to this specification.