CR: CSS Cascading and Inheritance Level 3

CSS Cascading and Inheritance Level 3

http://www.w3.org/TR/2016/CR-css-cascade-3-20160519/

feedback due by: 2016-07-19

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.CSS

Status of the Document



This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document was produced by the CSS Working Group (part of the Style Activity) as a Candidate Recommendation. This document is intended to become a W3C Recommendation. This document will remain a Candidate Recommendation at least until 19 July 2016 in order to ensure the opportunity for wide review.

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Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 September 2015 W3C Process Document.

For changes since the last draft, see the Changes section.


Table of Contents 1 Introduction 2 Importing Style Sheets: the @import rule 2.1 Content-Type of CSS Style Sheets 3 Shorthand Properties 3.1 Resetting All Properties: the all property 4 Value Processing 4.1 Declared Values 4.2 Cascaded Values 4.3 Specified Values 4.4 Computed Values 4.5 Used Values 4.6 Actual Values 4.7 Examples 5 Filtering 6 Cascading 6.1 Cascading Origins 6.2 Important Declarations: the !important annotation 6.3 Precedence of Non-CSS Presentational Hints 7 Defaulting 7.1 Initial Values 7.2 Inheritance 7.3 Explicit Defaulting 7.3.1 Resetting a Property: the initial keyword 7.3.2 Explicit Inheritance: the inherit keyword 7.3.3 Erasing All Declarations: the unset keyword 8 Changes 8.1 Changes Since the 3 October 2013 Candidate Recommendation Acknowledgments Conformance Document conventions Conformance classes Requirements for Responsible Implementation of CSS Partial Implementations Implementations of Unstable and Proprietary Features Implementations of CR-level Features CR exit criteria Inde Terms defined by this specification Terms defined by reference References Normative References Informative References Property Index1. 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]. 2. Imorting Style Sheets: the @import rule The @import rule allows users to import style rules from other style sheets. 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. 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 [ <url> | <string> ] <media-query-list>? ; Where the <url> or <string> gives the URL of the style sheet to be imported, and the optional <media-query-list> (the import conditions) states the condition under which it applies. 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"); The import conditions allow the import to be media-dependent. In the absence of any import conditions, the import is unconditional. (Specifying all for the <media-query-list> has the same effect.) The evaluation and full syntax of the expressions after the URL is defined by the Media Queries specification [MEDIAQ]. 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. User agents may therefore avoid fetching a media-dependent import as long as the media query does not match. The following rules illustrate how @import rules can be made media-dependent: @import url("ineprint.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 an independent 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. The environment encoding of an imported style sheet is the encoding of the style sheet that imported it. [CSS3SYN] 2.1. 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 no 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. 3. 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. 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 guaranee 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 with background-image. For example, the CSS Level 1 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 } As more font sub-properties are introduced into CSS, the shorthand declaration resets those to their initial values as well. 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 expicitly define the expansion of its values.) In other cases, a property might be a reset-only sub-property of the shorthand: Like other sub-properties, it is reset to its initial value by the shorthand when unspecified, but the shorthand might not include syntax to set the sub-property to any of its other values. For example, the border shorthand resets border-image to its initial value of none, but has no syntax to set it to anything else. [CSS3BG] If a shorthand is specified as one of the CSS-wide keywords [CSS3VAL], it sets all of its sub-properties to that keyword, including any that are reset-only sub-properties. (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. 3.1. Resetting All Properties: the all property Name: all Value: initial | inherit | unset Initial: See individual properties Applies to: See individual properties nherited: 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. Note: 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&uot; 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. 4. 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 doument 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. 4.1. 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”. 4.2. 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. 4.3. 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 vlue 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, 4.4. 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. Note: 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 unit (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. Note: 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. 4.5. Used Values The used value is the result of taking the computed value and completing any remaining calculations to mke 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. 4.6. 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 approximat 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. Note: 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. 4.7. 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 (iitial 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 5. 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 declaraion’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. 6. 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. Note: 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 stle 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] Note: 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. 6.1. 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 ser 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. 6.2. Important Declarations: the !important annotation CSS attempts to create a balance of power between uthor 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, as defined by [CSS3SYN]. 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 animaions. 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 !imporant } 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 } 6.3. 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. Note: 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 overriddn 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. 7. 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. 7.1. 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. 7.2. 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 paent 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: Inheritance follows the document tree and is not intercepted by anonymous boxes, or otherwise affected by manipulations of the box tree. 7.3. 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. 7.3.1. Resetting a Property: the initial keyword If the cascaded value is the initial keyword, the property’s initial value becomes its specified value. 7.3.2. Explcit 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. 7.3.3. 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). 8. Changes 8.1. Changes Since the 3 October 2013 Candidate Recommendation The following changes were made to this specification since the 3 October 2013 Candidate Recommendation: Defined environment encoding of imported style sheets. The environment encoding of an imported style sheet is the encoding of the style sheet that imported it. [CSS3SYN] Referenced [CSS3SYN] for syntax of !important rules. A declaration is important if it has a !importnt annotation, as defined by [CSS3SYN] . Explained reset-only sub-properties and clarified that they also get affected by a CSS-wide keyword value in the shorthand declaration. In other cases, a property might be a reset-only sub-property of the shorthand: Like other sub-properties, it is reset to its initial value by the shorthand when unspecified, but the shorthand might not include syntax to set the sub-property to any of its other values. For example, the border shorthand resets border-image to its initial value of none, but has no syntax to set it to anything else. [CSS3BG] If a shorthand is specified as one of the CSS-wide keywords [CSS3VAL], it sets all of its sub-properties to that keyword, including any that are reset-only sub-properties . (Note that these keywords cannot be combined with other values in a single declaration, not even in a shorthand.) A Disposition of Comments is available. Acknowledgments David Baron, Simon Sapin, and Boris Zbarsky contributed to this specification.ConformanceDocuent conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:


This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.Conformance classes

Conformance to this specification is defined for three conformance classes:style sheet A CSS style sheet. renderer A UA that interprets the semantics of a style sheet and renders documents that use them. authoring tool A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.CR exit criteria

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:independent each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement. interoperable passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or mor additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review. implementation a user agent which: implements the specification. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or "nightly build"). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.IndexTerms defined by this specificationactual value, in §4.6 all, in §3.1 cascade, in §6 cascaded value, in §4.2 computed value, in §4.4 declared value, in §4.1 @import, in §2 !important, in §6.2 important, in §6.2 import conditions, in §2 inherit definition of, in §7.2 value for all, in §7.3.2 inheritance, in §7.2 inherited property, in §7.2 inherited value, in §7.2 initial, in §7.3.1 initial value, in §7.1 longhand, in §3 longhand property, in §3 origin, in §6.1 output of the cascade, in §6 reset-only sub-property, in §3 scoped, in §6 scoping element, in §6 shorthand, in §3 shorthand property, in §3 specified value, in §4.3 sub-property, in §3 unset, in §7.3.3 used value, in §4.5Terms defined by reference[css-backgrounds-3] defines the following terms: background background-color background-image border border-bottom-width border-image border-left-width border-right-width border-top-width [CSS3-BRAK] defines the following terms: break-before orphans page [css-conditional-3] defines the following terms: @media [css-flexbox-1] defines the following terms: flex flex item [css-fonts-3] defines the following terms: font font-family font-size font-size-adjust font-style font-variant font-weight [css-lists-3] defines the following terms: list-style-position [CSS3SYN] defines the following terms: @charset environment encoding [css-text-3] defines the following terms: text-align [CSS3VAL] defines the following terms: <string> <url> ? em ex vh vw | [CSS3-WRITING-MODES] defines the following terms: direction unicode-bidi [CSS21] defines the following terms: display height line-height page-break-after width [mediaqueries-4] defines the following terms: <media-query-list> allReferencesNormative References[CSS-BACKGROUNDS-3] CSS Backgrounds and Borders Module Level 3 URL: https://www.w3.org/TR/css3-background/ [CSS-CONDITIONAL-3] CSS Conditional Rules Module Level 3 URL: https://www.w3.org/TR/cs3-conditional/ [CSS-FONTS-3] John Daggett. CSS Fonts Module Level 3. 3 October 2013. CR. URL: http://www.w3.org/TR/css-fonts-3/ [CSS-LISTS-3] Tab Atkins Jr.. CSS Lists and Counters Module Level 3. 20 March 2014. WD. URL: http://www.w3.org/TR/css-lists-3/ [CSS-TEXT-3] Elika Etemad; Koji Ishii. CSS Text Module Level 3. 10 October 2013. LCWD. URL: http://www.w3.org/TR/css-text-3/ [CSS21] Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: http://www.w3.org/TR/CSS2 [CSS3-ANIMATIONS] Dean Jackson; et al. CSS Animations. 19 February 2013. WD. URL: http://www.w3.org/TR/css3-animations/ [CSS3-BREAK] Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 3. 14 January 2016. CR. URL: http://www.w3.org/TR/css-break-3/ [CSS3-CONDITIONAL] David Baron. CSS Conditional Rules Module Level 3. 4 April 2013. CR. URL: http://www.w3.org/TR/css3-conditional/ [CSS3-TRANSITIONS] Dean Jackson; et al. CSS Transitions. 19 November 2013. WD. URL: http://www.w3.org/TR/css3-ransitions/ [CSS3-WRITING-MODES] Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. 15 December 2015. CR. URL: http://www.w3.org/TR/css-writing-modes-3/ [CSS3SYN] Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. 20 February 2014. CR. URL: http://www.w3.org/TR/css-syntax-3/ [CSS3VAL] Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 11 June 2015. CR. URL: http://www.w3.org/TR/css-values/ [CSSSTYLEATTR] Tantek Çelik; Elika Etemad. CSS Style Attributes. 7 November 2013. REC. URL: http://www.w3.org/TR/css-style-attr [DOM-LEVEL-2-STYLE] Chris Wilson; Philippe Le Hégaret. Document Object Model (DOM) Level 2 Style Specification. 13 November 2000. REC. URL: http://www.w3.org/TR/DOM-Level-2-Style/ [MEDIAQ] Florian Rivoal; et al. Media Queries. 19 June 2012. REC. URL: http://www.w3.org/TR/css3-mediaqueries/ [MEDIAQUERIES-4] Florian Rivoal; Tab Atkins Jr.. Media Queries Level 4. 26 January 2016. WD. URL: http://www.w3.org/TR/mediaqueries-4/ [RFC2119] S. Bradner. Key words for use in RFCs toIndicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119 [SELECT] Tantek Çelik; et al. Selectors Level 3. 29 September 2011. REC. URL: http://www.w3.org/TR/css3-selectors/Informative References[CSS-FLEXBOX-1] Tab Atkins Jr.; Elika Etemad; Rossen Atanassov. CSS Flexible Box Layout Module Level 1. 1 March 2016. CR. URL: http://www.w3.org/TR/css-flexbox-1/ [CSS-PAGE-3] CSS Paged Media Module Level 3 URL: https://www.w3.org/TR/css3-page/ [CSS3BG] Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 9 September 2014. CR. URL: http://www.w3.org/TR/css3-background/ [CSS3PAGE] Melinda Grant; et al. CSS Paged Media Module Level 3. 14 March 2013. WD. URL: http://www.w3.org/TR/css3-page/ [HTML5] Ian Hickson; et al. HTML5. 28 October 2014. REC. URL: http://www.w3.org/TR/html5/ [SVG11] Erik Dahlström; et al. Scalable Vector Graphics (SVG) 1.1 (Second Edition). 16 August 2011. REC. URL: http://www.w3.org/TR/SVG11/Property Index
Name Value Initial Applies to Inh. %ages Media Ani­mat­able Com­puted value all initial | inherit | unset See individual properties See individual properties See individual properties See individual properties See individual properties See individual properties See individual properties#at-ruledef-importReferenced in: 2. Importing Style Sheets: the @import rule (2) (3) (4) (5) (6) (7) (8) (9) 6. Cascading (2)#import-conditionsReferenced in: 2. Importing Style Sheets: the @import rule (2)#shorthand-propertyReferenced in: 3. Shorthand Properties (2) (3) (4) (5) (6) (7) (8) (9) 3.1. Resetting All Properties: the all property 6.2. Important Declarations: the !important annotation 7.3.3. Erasing All Declarations: the unset keyword 8.1. Changes Since the 3 October 2013 Candidate Recommendation#longhandReferenced in: 3. Shorthand Properties (2) (3) (4) (5) (6) (7) (8) (9) (10) 8.1. Changes Since the 3 October 2013 Candidate Recommendation (2) (3)#reset-only-sub-propertyReferenced in: 3. Shorthand Properties 8.1 Changes Since the 3 October 2013 Candidate Recommendation (2) (3)#propdef-allReferenced in: 3.1. Resetting All Properties: the all property (2) (3)#declared-valueReferenced in: 4. Value Processing (2) 4.2. Cascaded Values 5. Filtering (2) 6. Cascading (2) 7.3.3. Erasing All Declarations: the unset keyword#cascaded-valueReferenced in: 4. Value Processing (2) 4.2. Cascaded Values 4.3. Specified Values (2) (3) (4) 6. Cascading 7.3.1. Resetting a Property: the initial keyword 7.3.2. Explicit Inheritance: the inherit keyword 7.3.3. Erasing All Declarations: the unset keyword#specified-valueReferenced in: 1. Introduction 4. Value Processing (2) 4.3. Specified Values (2) (3) 4.4. Computed Values (2) (3) 7. Defaulting 7.1. Initial Values 7.3.1. Resetting a Property: the initial keyword 7.3.2. Explicit Inheritance: the inherit keyword#computed-valueReferenced in: 4. Value Processing (2) 4.4. Computed Values (2) (3) (4) 4.5. Used Values (2) 7.2. Inheritance 7.3.2. Explicit Inheritance: the inherit keyword#used-valueeferenced in: 4. Value Processing (2) (3) 4.5. Used Values (2) (3) (4) 4.6. Actual Values (2)#actual-valueReferenced in: 4. Value Processing (2)#cascadeReferenced in: 1. Introduction (2) 4.1. Declared Values 4.2. Cascaded Values 5. Filtering 7. Defaulting 7.1. Initial Values 7.3.3. Erasing All Declarations: the unset keyword#scopedReferenced in: 6. Cascading (2) (3)#scoping-elementReferenced in: 6. Cascading (2) (3)#output-of-the-cascadeReferenced in: 4.2. Cascaded Values (2)#originReferenced in: 2. Importing Style Sheets: the @import rule (2) 6. Cascading (2) (3) (4) 6.1. Cascading Origins (2)#importantReferenced in: 6. Cascading 6.2. Important Declarations: the !important annotation (2) 8.1. Changes Since the 3 October 2013 Candidate Recommendation#initial-valueReferenced in: 1. Introduction 3. Shorthand Properties 7. Defaulting 7.1. Initial Values 7.2. Inheritance 7.3.1. Resetting a Property: the initial keyword#inheritanceReferenced in: 1. Introduction 4.4. Computed Values (2) 7. Defaulting#inherited-vaueReferenced in: 7.2. Inheritance 7.3.2. Explicit Inheritance: the inherit keyword#inherited-propertyReferenced in: 7. Defaulting 7.1. Initial Values#valdef-all-initialReferenced in: 4.3. Specified Values 7. Defaulting 7.3.1. Resetting a Property: the initial keyword (2) 7.3.3. Erasing All Declarations: the unset keyword#valdef-all-inheritReferenced in: 4.3. Specified Values 7. Defaulting 7.2. Inheritance 7.3.2. Explicit Inheritance: the inherit keyword (2) 7.3.3. Erasing All Declarations: the unset keyword#valdef-all-unsetReferenced in: 7.3.3. Erasing All Declarations: the unset keyword (2)