CSS Custom Properties for Cascading Variables Module Level 1

Editor’s Draft,

Specification Metadata
This version:
https://drafts.csswg.org/css-variables/
Latest published version:
https://www.w3.org/TR/css-variables-1/
Previous Versions:
Test Suite:
http://test.csswg.org/suites/css-variables-1_dev/nightly-unstable/
Issue Tracking:
CSSWG Issues Repository
Editor:
Tab Atkins Jr. (Google)
Suggest an Edit for this Spec:
GitHub Editor

Abstract

This module introduces cascading variables as a new primitive value type that is accepted by all CSS properties, and custom properties for defining them.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-variables” in the title, like this: “[css-variables] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 15 September 2020 W3C Process Document.

1. Introduction

This section is not normative.

Large documents or applications (and even small ones) can contain quite a bit of CSS. Many of the values in the CSS file will be duplicate data; for example, a site may establish a color scheme and reuse three or four colors throughout the site. Altering this data can be difficult and error-prone, since it’s scattered throughout the CSS file (and possibly across multiple files), and may not be amenable to Find-and-Replace.

This module introduces a family of custom author-defined properties known collectively as custom properties, which allow an author to assign arbitrary values to a property with an author-chosen name, and the var() function, which allow an author to then use those values in other properties elsewhere in the document. This makes it easier to read large files, as seemingly-arbitrary values now have informative names, and makes editing such files much easier and less error-prone, as one only has to change the value once, in the custom property, and the change will propagate to all uses of that variable automatically.

1.1. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

2. Defining Custom Properties: the --* family of properties

--*

In all current engines.

Firefox31+Safari9.1+Chrome49+
Opera36+Edge79+
Edge (Legacy)15+IENone
Firefox for Android31+iOS Safari9.3+Chrome for Android49+Android WebView49+Samsung Internet5.0+Opera Mobile36+

This specification defines an open-ended set of properties called custom properties, which, among other things, are used to define the substitution value of var() functions.

Name: --*
Value: <declaration-value>?
Initial: the guaranteed-invalid value
Applies to: all elements
Inherited: yes
Percentages: n/a
Computed value: specified value with variables substituted, or the guaranteed-invalid value
Canonical order: per grammar
Animation type: discrete

User agents are expected to support this property on all media, including non-visual ones.

A custom property is any property whose name starts with two dashes (U+002D HYPHEN-MINUS), like --foo. The <custom-property-name> production corresponds to this: it’s defined as any <dashed-ident> (a valid identifier that starts with two dashes), except -- itself, which is reserved for future use by CSS. Custom properties are solely for use by authors and users; CSS will never give them a meaning beyond what is presented here.

Tests
Custom properties define variables, referenced with the var() notation, which can be used for many purposes. For example, a page that consistently uses a small set of colors in its design can store the colors in custom properties and use them with variables:
:root {
  --main-color: #06c;
  --accent-color: #006;
}
/* The rest of the CSS file */
#foo h1 {
  color: var(--main-color);
}

The naming provides a mnemonic for the colors, prevents difficult-to-spot typos in the color codes, and if the theme colors are ever changed, focuses the change on one simple spot (the custom property value) rather than requiring many edits across all stylesheets in the webpage.

Unlike other CSS properties, custom property names are not ASCII case-insensitive. Instead, custom property names are only equal to each other if they are identical to each other.

Tests
While both --foo and --FOO are valid, they are distinct properties —using var(--foo) will refer to the first one, while using var(--FOO) will refer to the second.

Perhaps more surprisingly, --foó and --foó are distinct properties. The first is spelled with U+00F3 (LATIN SMALL LETTER O WITH ACUTE) while the second is spelled with an ASCII "o" followed by U+0301 (COMBINING ACUTE ACCENT), and the "identical to" relation uses direct codepoint-by-codepoint comparison to determine if two strings are equal, to avoid the complexities and pitfalls of unicode normalization and locale-specific collation.

Different OSes or input methods can sometimes output visually-identical text with different codepoints, so authors should be aware of this potential hazard.

Custom properties are not reset by the all property. We may define a property in the future that resets all variables.

The CSS-wide keywords can be used in custom properties, with the same meaning as in any another property.

Tests

Note: That is, they’re interpreted at cascaded-value time as normal, and are not preserved as the custom property’s value, and thus are not substituted in by the corresponding variable.

Note: While this module focuses on the use of custom properties with the var() function to create “variables”, they can also be used as actual custom properties, parsed by and acted on by script. It’s expected that the CSS Extensions spec [CSS-EXTENSIONS] will expand on these use-cases and make them easier to do.

Custom properties are ordinary properties, so they can be declared on any element, are resolved with the normal inheritance and cascade rules, can be made conditional with @media and other conditional rules, can be used in HTML’s style attribute, can be read or set using the CSSOM, etc.

Tests

Notably, they can even be transitioned or animated, but since the UA has no way to interpret their contents, they always use the "flips at 50%" behavior that is used for any other pair of values that can’t be intelligently interpolated. However, any custom property used in a @keyframes rule becomes animation-tainted, which affects how it is treated when referred to via the var() function in an animation property.

Tests

Animation-tainted is "infectious": custom properties which reference animation-tainted properties also become animation-tainted.

This style rule:
:root {
  --header-color: #06c;
}

declares a custom property named --header-color on the root element, and assigns to it the value "#06c". This property is then inherited to the elements in the rest of the document. Its value can be referenced with the var() function:

h1 { background-color: var(--header-color); }

The preceding rule is equivalent to writing background-color: #06c;, except that the variable name makes the origin of the color clearer, and if var(--header-color) is used on other elements in the document, all of the uses can be updated at once by changing the --header-color property on the root element.

If a custom property is declared multiple times, the standard cascade rules help resolve it. Variables always draw from the computed value of the associated custom property on the same element:
:root { --color: blue; }
div { --color: green; }
#alert { --color: red; }
* { color: var(--color); }

<p>I inherited blue from the root element!</p>
<div>I got green set directly on me!</div>
<div id='alert'>
  While I got red set directly on me!
  <p>I’m red too, because of inheritance!</p>
</div>
A real-world example of custom property usage is easily separating out strings from where they’re used, to aid in maintenance of internationalization:
:root,
:root:lang(en) {--external-link: "external link";}
:root:lang(de) {--external-link: "externer Link";}

a[href^="http"]::after {content: " (" var(--external-link) ")"}

The variable declarations can even be kept in a separate file, to make maintaining the translations simpler.

2.1. Custom Property Value Syntax

The allowed syntax for custom properties is extremely permissive. The <declaration-value> production matches any sequence of one or more tokens, so long as the sequence does not contain <bad-string-token>, <bad-url-token>, unmatched <)-token>, <]-token>, or <}-token>, or top-level <semicolon-token> tokens or <delim-token> tokens with a value of "!".

Tests

In addition, if the value of a custom property contains a var() reference, the var() reference must be valid according to the specified var() grammar. If not, the custom property is invalid and must be ignored.

Note: This definition, along with the general CSS syntax rules, implies that a custom property value never includes an unmatched quote or bracket, and so cannot have any effect on larger syntax constructs, like the enclosing style rule, when reserialized.

Note: Custom properties can contain a trailing !important, but this is automatically removed from the property’s value by the CSS parser, and makes the custom property "important" in the CSS cascade. In other words, the prohibition on top-level "!" characters does not prevent !important from being used, as the !important is removed before syntax checking happens.

Tests
For example, the following is a valid custom property:
--foo: if(x > 5) this.width = 10;

While this value is obviously useless as a variable, as it would be invalid in any normal property, it might be read and acted on by JavaScript.

The values of custom properties, and the values of var() functions substituted into custom properties, are case-sensitive, and must be preserved in their original author-given casing. (Many CSS values are ASCII case-insensitive, which user agents can take advantage of by "canonicalizing" them into a single casing, but that isn’t allowed for custom properties.)

Tests
Because custom properties can contain anything, there is no general way to know how to interpret what’s inside of them (until they’re substituted into a known property with var()). Rather than have them partially resolve in some cases but not others, they’re left completely unresolved; they’re a bare stream of CSS tokens interspersed with var() functions.

This has some knock-on implications. For example, relative URLs in CSS are resolved against the base URL of the stylesheet the value appears in. However, if a custom property like --my-image: url(foo.jpg); shows up in an "/a/style.css" stylesheet, it will not resolve into an absolute URL immediately; if that variable is later used in a different "/b/style.css" stylesheet like background: var(--my-image);, it will resolve at that point to "/b/foo.jpg".

2.2. Guaranteed-Invalid Values

The initial value of a custom property is a guaranteed-invalid value. As defined in § 3 Using Cascading Variables: the var() notation, using var() to substitute a custom property with this as its value makes the property referencing it invalid at computed-value time.

This value serializes as the empty string, but actually writing an empty value into a custom property, like --foo: ;, is a valid (empty) value, not the guaranteed-invalid value. If, for whatever reason, one wants to manually reset a variable to the guaranteed-invalid value, using the keyword initial will do this.

2.3. Resolving Dependency Cycles

Custom properties are left almost entirely unevaluated, except that they allow and evaluate the var() function in their value. This can create cyclic dependencies where a custom property uses a var() referring to itself, or two or more custom properties each attempt to refer to each other.

For each element, create a directed dependency graph, containing nodes for each custom property. If the value of a custom property prop contains a var() function referring to the property var (including in the fallback argument of var()), add an edge between prop and the var. Edges are possible from a custom property to itself.

If there is a cycle in the dependency graph, all the custom properties in the cycle are invalid at computed-value time.

Tests

Note: Defined properties that participate in a dependency cycle either end up with invalid variables in their value (becoming invalid at computed-value time), or define their own cyclic handling (like font-size using em values). They do not compute to the guaranteed-invalid value like custom properties do.

This example shows a custom property safely using a variable:
:root {
  --main-color: #c06;
  --accent-background: linear-gradient(to top, var(--main-color), white);
}

The --accent-background property (along with any other properties that use var(--main-color)) will automatically update when the --main-color property is changed.

On the other hand, this example shows an invalid instance of variables depending on each other:
:root {
  --one: calc(var(--two) + 20px);
  --two: calc(var(--one) - 20px);
}

Both --one and --two are now invalid at computed-value time, and compute to the guaranteed-invalid value rather than lengths.

It is important to note that custom properties resolve any var() functions in their values at computed-value time, which occurs before the value is inherited. In general, cyclic dependencies occur only when multiple custom properties on the same element refer to each other; custom properties defined on elements higher in the element tree can never cause a cyclic reference with properties defined on elements lower in the element tree.

Tests
For example, given the following structure, these custom properties are not cyclic, and all define valid variables:
<one><two><three /></two></one>
<style>
one   { --foo: 10px; }
two   { --bar: calc(var(--foo) + 10px); }
three { --foo: calc(var(--bar) + 10px); }
</style>

The <one> element defines a value for --foo. The <two> element inherits this value, and additionally assigns a value to --bar using the foo variable. Finally, the <three> element inherits the --bar value after variable substitution (in other words, it sees the value calc(10px + 10px)), and then redefines --foo in terms of that value. Since the value it inherited for --bar no longer contains a reference to the --foo property defined on <one>, defining --foo using the var(--bar) variable is not cyclic, and actually defines a value that will eventually (when referenced as a variable in a normal property) resolve to 30px.

3. Using Cascading Variables: the var() notation

var()

In all current engines.

Firefox31+Safari9.1+Chrome49+
Opera36+Edge79+
Edge (Legacy)15+IENone
Firefox for Android31+iOS Safari9.3+Chrome for Android49+Android WebView50+Samsung Internet5.0+Opera Mobile36+

The value of a custom property can be substituted into the value of another property with the var() function. The syntax of var() is:

var() = var( <custom-property-name> , <declaration-value>? )
Tests

@supports


In an exception to the usual comma elision rules, which require commas to be omitted when they’re not separating values, a bare comma, with nothing following it, must be treated as valid in var(), indicating an empty fallback value.

Tests

Note: That is, var(--a,) is a valid function, specifying that if the --a custom property is invalid or missing, the var() should be replaced with nothing.

The var() function can be used in place of any part of a value in any property on an element. The var() function can not be used as property names, selectors, or anything else besides property values. (Doing so usually produces invalid syntax, or else a value whose meaning has no connection to the variable.)

Tests
For example, the following code incorrectly attempts to use a variable as a property name:
.foo {
  --side: margin-top;
  var(--side): 20px;
}

This is not equivalent to setting margin-top: 20px;. Instead, the second declaration is simply thrown away as a syntax error for having an invalid property name.

The first argument to the function is the name of the custom property to be substituted. The second argument to the function, if provided, is a fallback value, which is used as the substitution value when the value of the referenced custom property is the guaranteed-invalid value.

Tests

Note: The syntax of the fallback, like that of custom properties, allows commas. For example, var(--foo, red, blue) defines a fallback of red, blue; that is, anything between the first comma and the end of the function is considered a fallback value.

The fallback value allows for some types of defensive coding. For example, an author may create a component intended to be included in a larger application, and use variables to style it so that it’s easy for the author of the larger application to theme the component to match the rest of the app.

Without fallback, the app author must supply a value for every variable that your component uses. With fallback, the component author can supply defaults, so the app author only needs to supply values for the variables they wish to override.

/* In the component’s style: */
.component .header {
  color: var(--header-color, blue);
}
.component .text {
  color: var(--text-color, black);
}

/* In the larger application’s style: */
.component {
  --text-color: #080;
  /* header-color isn’t set,
     and so remains blue,
     the fallback value */
}

If a property contains one or more var() functions, and those functions are syntactically valid, the entire property’s grammar must be assumed to be valid at parse time. It is only syntax-checked at computed-value time, after var() functions have been substituted.

Tests

To substitute a var() in a property’s value:

  1. If the custom property named by the first argument to the var() function is animation-tainted, and the var() function is being used in a property that is not animatable, treat the custom property as having its initial value for the rest of this algorithm.
  2. If the value of the custom property named by the first argument to the var() function is anything but the initial value, replace the var() function by the value of the corresponding custom property.
  3. Otherwise, if the var() function has a fallback value as its second argument, replace the var() function by the fallback value. If there are any var() references in the fallback, substitute them as well.
  4. Otherwise, the property containing the var() function is invalid at computed-value time.

    Note: Other things can also make a property invalid at computed-value time.

Tests

CSSOM


Note that var() substitution takes place at the level of CSS tokens [css-syntax-3], not at a textual level; you can’t build up a single token where part of it is provided by a variable:
.foo {
  --gap: 20;
  margin-top: var(--gap)px;
}

This is not equivalent to setting margin-top: 20px; (a length). Instead, it’s equivalent to margin-top: 20 px; (a number followed by an ident), which is simply an invalid value for the margin-top property. Note, though, that calc() can be used to validly achieve the same thing, like so:

.foo {
  --gap: 20;
  margin-top: calc(var(--gap) * 1px);
}
Tests

var() functions are substituted at computed-value time. If a declaration, once all var() functions are substituted in, does not match its declared grammar, the declaration is invalid at computed-value time.

Tests

If a declaration, once all var() functions are substituted in, contains only a CSS-wide keyword (and possibly whitespace), its value is determined as if that keyword were its specified value all along.

Tests
For example, the following usage is fine from a syntax standpoint, but results in nonsense when the variable is substituted in:
:root { --looks-valid: 20px; }
p { background-color: var(--looks-valid); }

Since 20px is an invalid value for background-color, this instance of the property computes to transparent (the initial value for background-color) instead.

If the property was one that’s inherited by default, such as color, it would compute to the inherited value rather than the initial value.

While a var() function can’t get a CSS-wide keyword from the custom property itself—if you tried to specify that, like --foo: initial;, it would just trigger explicit defaulting for the custom property—it can have a CSS-wide keyword in its fallback:
p { color: var(--does-not-exist, initial); }

In the above code, if the --does-not-exist property didn’t exist or is invalid at computed-value time, the var() will instead substitute in the initial keyword, making the property behave as if it was originally color: initial. This will make it take on the document’s initial color value, rather than defaulting to inheritance, as it would if there were no fallback.

3.1. Invalid Variables

When a custom property’s value is the guaranteed-invalid value, var() functions cannot use it for substitution. Attempting to do so makes the declaration invalid at computed-value time, unless a valid fallback is specified.

A declaration can be invalid at computed-value time if it contains a var() that references a custom property with the guaranteed-invalid value, as explained above, or if it uses a valid custom property, but the property value, after substituting its var() functions, is invalid. When this happens, the computed value is one of the following depending on the property’s type:

The property is a non-registered custom property
The property is a registered custom property with universal syntax

The computed value is the guaranteed-invalid value.

Otherwise

Either the property’s inherited value or its initial value depending on whether the property is inherited or not, respectively, as if the property’s value had been specified as the unset keyword.

Tests
For example, in the following code:
:root { --not-a-color: 20px; }
p { background-color: red; }
p { background-color: var(--not-a-color); }

the <p> elements will have transparent backgrounds (the initial value for background-color), rather than red backgrounds. The same would happen if the custom property itself was unset, or contained an invalid var() function.

Note the difference between this and what happens if the author had just written background-color: 20px directly in their stylesheet - that would be a normal syntax error, which would cause the rule to be discarded, so the background-color: red rule would be used instead.

Note: The invalid at computed-value time concept exists because variables can’t "fail early" like other syntax errors can, so by the time the user agent realizes a property value is invalid, it’s already thrown away the other cascaded values.

3.2. Variables in Shorthand Properties

var() functions produce some complications when parsing shorthand properties into their component longhands, and when serializing shorthand properties from their component longhands.

If a shorthand property contains a var() function in its value, the longhand properties it’s associated with must instead be filled in with a special, unobservable-to-authors pending-substitution value that indicates the shorthand contains a variable, and thus the longhand’s value can’t be determined until variables are substituted.

This value must then be cascaded as normal, and at computed-value time, after var() functions are finally substituted in, the shorthand must be parsed and the longhands must be given their appropriate values at that point.

Tests

Note: When a shorthand is written without a var(), it is parsed and separated out into its component longhand properties at parse time; the longhands then participate in the cascade, with the shorthand property more or less discarded. When the shorthand contains a var(), however, this can’t be done, as the var() could be substituted with anything.

Pending-substitution values must be serialized as the empty string, if an API allows them to be observed.

Tests

Shorthand properties are serialized by gathering the values of their component longhand properties, and synthesizing a value that will parse into the same set of values.

If all of the component longhand properties for a given shorthand are pending-substitution values from the same original shorthand value, the shorthand property must serialize to that original (var()-containing) value.

Otherwise, if any of the component longhand properties for a given shorthand are pending-substitution values, or contain var() functions of their own that have not yet been substituted, the shorthand property must serialize to the empty string.

3.3. Safely Handling Overly-Long Variables

Naively implemented, var() functions can be used in a variation of the "billion laughs attack":

.foo {
  --prop1: lol;
  --prop2: var(--prop1) var(--prop1);
  --prop3: var(--prop2) var(--prop2);
  --prop4: var(--prop3) var(--prop3);
  /* etc */
}

In this short example, --prop4’s computed value is lol lol lol lol lol lol lol lol, containing 8 copies of the original lol. Every additional level added to this doubles the number of identifiers; extending it to a mere 30 levels, the work of a few minutes by hand, would make --prop30 contain nearly a billion instances of the identifier.

To avoid this sort of attack, UAs must impose a UA-defined limit on the allowed length of the token stream that a var() function expands into. If a var() would expand into a longer token stream than this limit, it instead makes the property it’s expanding into invalid at computed-value time.

Tests

This specification does not define what size limit should be imposed. However, since there are valid use-cases for custom properties that contain a kilobyte or more of text, it’s recommended that the limit be set relatively high.

Note: The general principle that UAs are allowed to violate standards due to resource constraints is still generally true here; a UA might, separately, have limits on how long of a custom property they can support, or how large of an identifier they can support. This section calls out this attack specifically because of its long history, and the fact that it can be done without any of the pieces seeming to be too large on first inspection.

4. APIs

All custom property declarations have the case-sensitive flag set.

Tests

Note: Custom properties do not appear on a CSSStyleDeclaration object in camel-cased form, because their names may have both upper and lower case letters which indicate distinct custom properties. The sort of text transformation that automatic camel-casing performs is incompatible with this. They can still be accessed by their proper name via getPropertyValue()/etc.

4.1. Serializing Custom Properties

Custom property names must be serialized with the casing as provided by the author.

Note: For non-custom properties, property names are restricted to the ASCII range and are ASCII case-insensitive, so implementations typically serialize the name lowercased.

Specified values of custom properties must be serialized exactly as specified by the author. Simplifications that might occur in other properties, such as dropping comments, normalizing whitespace, reserializing numeric tokens from their value, etc., must not occur.

Computed values of custom properties must similarly be serialized exactly as specified by the author, save for the replacement of any var() functions.

Tests
For example, given the following properties:
--y: /* baz */;
--x: /* foo */ var(--y) /* bar */;

the serialization of the specified value of --x must be "/* foo */ var(--y) /* bar */", while the serialization of the computed value of --x must be "/* foo */ /* baz */ /* bar */".

(Note that the leading whitespace on the value is automatically trimmed by the CSS parser; it’s not preserved here.)

This requirement exists because authors sometimes store non-CSS information in custom properties, and "normalizing" this information can change it in ways that break author code.

For example, storing a UUID in a custom property, like --uuid: 12345678-12e3-8d9b-a456-426614174000, requires the UUID to be echoed back out as written when it’s accessed by script.

This value is technically parsed by CSS as a series of adjacent numbers and dimensions. In particular, the segment "-12e3" parses as a number, equal to -12000. Reserializing it in that form, as required by CSSOM in other contexts, would fatally break the author’s use of the value.

5. Changes

5.1. Changes Since the 03 December 2015 CR

5.2. Changes since the May 6 2014 Last Call Working Draft

6. Acknowledgments

Many thanks to several people in the CSS Working Group for keeping the dream of variables alive over the years, particularly Daniel Glazman and David Hyatt. Thanks to multiple people on the mailing list for helping contribute to the development of this incarnation of variables, particularly Brian Kardell, David Baron, François Remy, Roland Steiner, and Shane Stephens.

7. Privacy and Security Considerations

This specification defines a purely author-level mechanism for passing styling information around within a page they control. As such, there are no new privacy considerations.

§ 3.3 Safely Handling Overly-Long Variables calls out a long-standing Denial-of-Service attack that can be mounted against "macro-expansion"-like mechanisms, such as the var() function, and mandates a defense against that attack.

Conformance

Document 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.

Tests

Tests relating to the content of this specification may be documented in “Tests” blocks like this one. Any such block is non-normative.


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.

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 component 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.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

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.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-ANIMATIONS-1]
Dean Jackson; et al. CSS Animations Level 1. 11 October 2018. WD. URL: https://www.w3.org/TR/css-animations-1/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. 15 October 2021. WD. URL: https://www.w3.org/TR/css-cascade-5/
[CSS-CONDITIONAL-3]
David Baron; Elika Etemad; Chris Lilley. CSS Conditional Rules Module Level 3. 8 December 2020. CR. URL: https://www.w3.org/TR/css-conditional-3/
[CSS-PROPERTIES-VALUES-API-1]
Tab Atkins Jr.; et al. CSS Properties and Values API Level 1. 13 October 2020. WD. URL: https://www.w3.org/TR/css-properties-values-api-1/
[CSS-SYNTAX-3]
Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. 16 July 2019. CR. URL: https://www.w3.org/TR/css-syntax-3/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 6 June 2019. CR. URL: https://www.w3.org/TR/css-values-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 16 October 2021. WD. URL: https://www.w3.org/TR/css-values-4/
[CSS2]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS21/
[CSSOM-1]
Daniel Glazman; Emilio Cobos Álvarez. CSS Object Model (CSSOM). 26 August 2021. WD. URL: https://www.w3.org/TR/cssom-1/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[SELECTORS-4]
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. 21 November 2018. WD. URL: https://www.w3.org/TR/selectors-4/
[WEB-ANIMATIONS-1]
Brian Birtles; et al. Web Animations. 18 May 2021. WD. URL: https://www.w3.org/TR/web-animations-1/

Informative References

[CSS-BACKGROUNDS-3]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 26 July 2021. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
[CSS-BOX-4]
Elika Etemad. CSS Box Model Module Level 4. 21 April 2020. WD. URL: https://www.w3.org/TR/css-box-4/
[CSS-CASCADE-6]
CSS Cascading and Inheritance Level 6 URL: https://drafts.csswg.org/css-cascade-6/
[CSS-COLOR-4]
Tab Atkins Jr.; Chris Lilley. CSS Color Module Level 4. 1 June 2021. WD. URL: https://www.w3.org/TR/css-color-4/
[CSS-EXTENSIONS]
Tab Atkins Jr.. CSS Extensions. ED. URL: https://drafts.csswg.org/css-extensions/
[CSS-FONTS-5]
Myles Maxfield; Chris Lilley. CSS Fonts Module Level 5. 29 July 2021. WD. URL: https://www.w3.org/TR/css-fonts-5/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
--* <declaration-value>? the guaranteed-invalid value all elements yes n/a discrete per grammar specified value with variables substituted, or the guaranteed-invalid value