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

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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

✔ MDN --* In all current engines. Firefox 31+ Safari 9.1+ Chrome 49+ Opera 36+ Edge 79+ Edge (Legacy) 15+ IE None Firefox for Android 31+ iOS Safari 9.3+ Chrome for Android 49+ Android WebView 49+ Samsung Internet 5.0+ Opera Mobile 36+

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.

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.

:root { --main-color : #06c ; --accent-color : #006 ; } /* The rest of the CSS file */ #foo h1 { color : var ( --main-color ); } 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: 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 case-sensitive.

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

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.

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.

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.

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

: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> 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 , :root :lang ( en ) { --external-link : "external link" ;} :root :lang ( de ) { --external-link : "externer Link" ;} a [ href^= "http" ] ::after { content : " (" var ( --external-link ) ")" } A real-world example of custom property usage is easily separating out strings from where they’re used, to aid in maintenance of internationalization: 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 "!".

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.

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

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 cyclic at computed-value time , and must compute to the guaranteed-invalid value.

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 computed to the guaranteed-invalid value like custom properties do.

:root { --main-color : #c06 ; --accent-background : linear-gradient ( to top , var ( --main-color ), white ); } This example shows a custom property safely using a variable: The --accent-background property (along with any other properties that use var(--main-color)) will automatically update when the --main-color property is changed.

:root { --one : calc ( var ( --two ) + 20 px ); --two : calc ( var ( --one ) - 20 px ); } On the other hand, this example shows an invalid instance of variables depending on each other: Both --one and --two are now cyclic at computed-value time, and compute to the guaranteed-invalid value rather than 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.

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

✔ MDN var In all current engines. Firefox 31+ Safari 9.1+ Chrome 49+ Opera 36+ Edge 79+ Edge (Legacy) 15+ IE None Firefox for Android 31+ iOS Safari 9.3+ Chrome for Android 49+ Android WebView 50+ Samsung Internet 5.0+ Opera Mobile 36+

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

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.)

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

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.

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

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

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.

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

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 of the property is 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.

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

The use of var() functions in shorthand properties presents some unique difficulties.

For non-custom properties, the value of a shorthand property is separated out into its component longhand properties at parse time, and then the longhands themselves participate in the cascade, with the shorthand more-or-less discarded. If a var() functions is used in a shorthand, however, one can’t tell what values are meant to go where; it may in fact be impossible to separate it out at parse time, as a single var() function may substitute in the value of several longhands at once.

To get around this, implementations must fill in longhands with a special, unobservable-to-authors pending-substitution value that indicates the shorthand contains a variable, and thus the longhand’s value is pending variable substitution. 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.

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

Similarly, while [CSSOM] defines that shorthand properties are serialized by appropriately concatenating the values of their corresponding longhands, shorthands that are specified with explicit var() functions must serialize to the original, var()-containing value. For other shorthands, if any of the longhand subproperties for that shorthand have pending-substitution values then the serialized value of the shorthand must be 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.

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.

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.

5. Changes

5.1. Changes Since the 03 December 2015 CR

Now that [css-syntax-3] auto-trims whitespace from declaration values, made <declaration-value> optional in the custom property grammar, so that empty variables are still allowed. (Issue 774)

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

Serialization of longhands when shorthand uses a variable was defined.

Link to DOM’s definition of "case-sensitive".

Added example of using variables with :lang() to do simple i18n.

Clarified that usage of var() in a custom property must be valid per the var() grammar.

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.