CSS Custom Properties for Cascading Variables Module Level 1

Editor’s Draft,

More details about this document
This version:
https://drafts.csswg.org/css-variables/
Latest published version:
https://www.w3.org/TR/css-variables-1/
Previous Versions:
Implementation Report:
https://wpt.fyi/results/css/css-variables
Feedback:
CSSWG Issues Repository
Editor:
Tab Atkins Jr. (Google)
Suggest an Edit for this Spec:
GitHub Editor
Test Suite:
https://wpt.fyi/results/css/css-variables/

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 [email protected].

This document is governed by the 03 November 2023 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

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 and all pseudo-elements (including those with restricted property lists)
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.

Operating systems, keyboards, or input methods sometimes encode visually-identical text using different codepoint sequences. Authors are advised to choose variable names that avoid potential confusion or to use escapes and other means to ensure that similar appearing sequences are identical. See Section 2.3 in [CHARMOD-NORM] for examples.

Developers maintaining the following CSS might be confused why the test patch is red:
--fijord: red;
--fijord: green;
--fijord: blue;

.test {
  background-color: var(--fijord);
}

The reason is that the first custom property uses the character sequence LATIN SMALL LETTER F + LATIN SMALL LETTER I + LATIN SMALL LETTER J; the second, identical-looking one uses the character sequence LATIN SMALL LETTER F + LATIN SMALL LIGATURE IJ while the third uses the character sequence LATIN SMALL LIGATURE FI + LATIN SMALL LETTER J.

So the CSS contains three distinct custom properties, two of which are unused.

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 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

Note: Like any other property that animates discretely, custom properties can’t be transitioned. Registered custom properties can, however, if given a syntax that has non-discrete animation behavior.

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(el) {--external-link: "εξωτερικός σύνδεσμος";}

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.

The guaranteed-invalid value is, well, guaranteed to be invalid. If it ever appears in a property value, then at computed value time that property becomes invalid at computed-value time.

Non-property contexts will define their own behavior for the guaranteed-invalid value, but it will always be "invalid" in some sense.

The guaranteed-invalid 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 custom property to the guaranteed-invalid value, using the keyword initial will do this.

Note: Other than invoking the initial value of a non-registered custom property, the only way to create the guaranteed-invalid value is by having an invalid arbitrary substitution function.

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

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>? )

The var() function is an arbitrary substitution function.

Tests

@supports


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

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.

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 */
}
To resolve a var() function:
  1. Let result be the value of the custom property named by the function’s first argument, on the element the function’s property is being applied to.

  2. Let fallback be the value of the function’s second argument, defaulting to the guaranteed-invalid value if it doesn’t have a second argument.

  3. If the custom property named by the var()’s first argument is animation-tainted, and the var() is being used in a property that is not animatable, set result to the guaranteed-invalid value.

  4. Return result and fallback.

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 lowercase 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 as the exact code point sequence provided by the author, including not altering the case.

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 16 June 2022 CR Snapshot

5.2. Changes Since the 11 November 2021 CR Draft

5.3. Changes Since the 03 December 2015 CR

5.4. 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 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.

8. Security Considerations

CSS Values 5 §  Safely Handling Overly-Long Substitution 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.

Tests

Tests

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 [email protected] mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-ANIMATIONS-1]
David Baron; et al. CSS Animations Level 1. URL: https://drafts.csswg.org/css-animations/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. URL: https://drafts.csswg.org/css-cascade-5/
[CSS-CONDITIONAL-3]
Chris Lilley; David Baron; Elika Etemad. CSS Conditional Rules Module Level 3. URL: https://drafts.csswg.org/css-conditional-3/
[CSS-PROPERTIES-VALUES-API-1]
Tab Atkins Jr.; Alan Stearns; Greg Whitworth. CSS Properties and Values API Level 1. URL: https://drafts.css-houdini.org/css-properties-values-api-1/
[CSS-SYNTAX-3]
Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. URL: https://drafts.csswg.org/css-syntax/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. URL: https://drafts.csswg.org/css-values-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. URL: https://drafts.csswg.org/css-values-4/
[CSS-VALUES-5]
Tab Atkins Jr.; Elika Etemad; Miriam Suzanne. CSS Values and Units Module Level 5. URL: https://drafts.csswg.org/css-values-5/
[CSS2]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. URL: https://drafts.csswg.org/css2/
[CSSOM-1]
Daniel Glazman; Emilio Cobos Álvarez. CSS Object Model (CSSOM). URL: https://drafts.csswg.org/cssom/
[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. URL: https://drafts.csswg.org/selectors/
[WEB-ANIMATIONS-1]
Brian Birtles; et al. Web Animations. URL: https://drafts.csswg.org/web-animations-1/

Informative References

[CHARMOD-NORM]
Addison Phillips; et al. Character Model for the World Wide Web: String Matching. URL: https://w3c.github.io/charmod-norm/
[CSS-BACKGROUNDS-3]
Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. URL: https://drafts.csswg.org/css-backgrounds/
[CSS-EXTENSIONS]
Tab Atkins Jr.. CSS Extensions. ED. URL: https://drafts.csswg.org/css-extensions/
[CSS-FONTS-4]
Chris Lilley. CSS Fonts Module Level 4. URL: https://drafts.csswg.org/css-fonts-4/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
--* <declaration-value>? the guaranteed-invalid value all elements and all pseudo-elements (including those with restricted property lists) yes n/a discrete per grammar specified value with variables substituted, or the guaranteed-invalid value
MDN

var()

In all current engines.

Firefox31+Safari9.1+Chrome49+
Opera?Edge79+
Edge (Legacy)15+IENone
Firefox for Android?iOS Safari?Chrome for Android?Android WebView50+Samsung Internet?Opera Mobile?
MDN

--*

In all current engines.

Firefox31+Safari9.1+Chrome49+
Opera?Edge79+
Edge (Legacy)15+IENone
Firefox for Android?iOS Safari?Chrome for Android?Android WebView?Samsung Internet?Opera Mobile?