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
- variable-declaration-29.html (live test) (source)
- variable-declaration-31.html (live test) (source)
- variable-declaration-32.html (live test) (source)
- variable-declaration-33.html (live test) (source)
- variable-declaration-34.html (live test) (source)
- variable-declaration-35.html (live test) (source)
- variable-declaration-36.html (live test) (source)
- variable-declaration-40.html (live test) (source)
- variable-declaration-41.html (live test) (source)
- variable-declaration-42.html (live test) (source)
- variable-empty-name-reserved.html (live test) (source)
: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.
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.
--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
- variable-declaration-43.html (live test) (source)
- variable-declaration-44.html (live test) (source)
- variable-declaration-45.html (live test) (source)
- variable-declaration-46.html (live test) (source)
- variable-declaration-47.html (live test) (source)
- variable-declaration-56.html (live test) (source)
- variable-declaration-57.html (live test) (source)
- variable-declaration-58.html (live test) (source)
- variable-declaration-60.html (live test) (source)
- variable-definition-keywords.html (live test) (source)
- variable-css-wide-keywords.html (live test) (source)
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
- css-vars-custom-property-inheritance.html (live test) (source)
- variable-created-document.html (live test) (source)
- variable-created-element.html (live test) (source)
- variable-cssText.html (live test) (source)
- variable-declaration-06.html (live test) (source)
- variable-definition-cascading.html (live test) (source)
- variable-external-declaration-01.html (live test) (source)
- variable-external-reference-01.html (live test) (source)
- variable-external-supports-01.html (live test) (source)
- variable-first-letter.html (live test) (source)
- variable-first-line.html (live test) (source)
- variable-pseudo-element.html (live test) (source)
- variable-reference-13.html (live test) (source)
- variable-reference-14.html (live test) (source)
- variable-reference-shorthands.html (live test) (source)
- variable-reference-visited.html (live test) (source)
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
- variable-animation-from-to.html (live test) (source)
- variable-animation-over-transition.html (live test) (source)
- variable-animation-substitute-into-keyframe-shorthand.html (live test) (source)
- variable-animation-substitute-into-keyframe-transform.html (live test) (source)
- variable-animation-substitute-into-keyframe.html (live test) (source)
- variable-animation-substitute-within-keyframe-fallback.html (live test) (source)
- variable-animation-substitute-within-keyframe-multiple.html (live test) (source)
- variable-animation-substitute-within-keyframe.html (live test) (source)
- variable-animation-to-only.html (live test) (source)
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.
: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.
: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>
: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
- long-variable-reference-crash.html (live test) (source)
- test_variable_legal_values.html (live test) (source)
- variable-declaration-15.html (live test) (source)
- variable-declaration-24.html (live test) (source)
- variable-declaration-25.html (live test) (source)
- variable-declaration-26.html (live test) (source)
- variable-declaration-59.html (live test) (source)
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
--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
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
stylesheet,
it will not resolve into an absolute URL immediately;
if that variable is later used in a different
stylesheet
like background: var(--my-image);,
it will resolve at that point to
.
2.2. Guaranteed-Invalid Values
The initial value of a custom property is a guaranteed-invalid value.
The guaranteed-invalid value is, well, gauranteed 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.
: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.
:root{ --one : calc ( var ( --two) +20 px ); --two : calc ( var ( --one) -20 px ); }
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
< 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 >
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
- variable-reference-07.html (live test) (source)
- variable-reference-08.html (live test) (source)
- variable-reference-09.html (live test) (source)
- variable-reference-10.html (live test) (source)
- variable-reference-17.html (live test) (source)
- variable-reference-20.html (live test) (source)
- variable-reference-21.html (live test) (source)
- variable-reference-22.html (live test) (source)
- variable-reference-23.html (live test) (source)
- variable-reference-24.html (live test) (source)
- variable-reference-25.html (live test) (source)
- variable-reference-28.html (live test) (source)
- variable-reference-29.html (live test) (source)
- variable-reference-31.html (live test) (source)
- variable-reference-32.html (live test) (source)
- variable-reference-33.html (live test) (source)
- variable-reference-34.html (live test) (source)
- variable-reference-35.html (live test) (source)
- variable-reference.html (live test) (source)
@supports
- variable-supports-01.html (live test) (source)
- variable-supports-02.html (live test) (source)
- variable-supports-03.html (live test) (source)
- variable-supports-04.html (live test) (source)
- variable-supports-05.html (live test) (source)
- variable-supports-06.html (live test) (source)
- variable-supports-07.html (live test) (source)
- variable-supports-08.html (live test) (source)
- variable-supports-09.html (live test) (source)
- variable-supports-10.html (live test) (source)
- variable-supports-11.html (live test) (source)
- variable-supports-12.html (live test) (source)
- variable-supports-13.html (live test) (source)
- variable-supports-14.html (live test) (source)
- variable-supports-15.html (live test) (source)
- variable-supports-16.html (live test) (source)
- variable-supports-17.html (live test) (source)
- variable-supports-18.html (live test) (source)
- variable-supports-19.html (live test) (source)
- variable-supports-20.html (live test) (source)
- variable-supports-21.html (live test) (source)
- variable-supports-22.html (live test) (source)
- variable-supports-23.html (live test) (source)
- variable-supports-24.html (live test) (source)
- variable-supports-25.html (live test) (source)
- variable-supports-26.html (live test) (source)
- variable-supports-27.html (live test) (source)
- variable-supports-28.html (live test) (source)
- variable-supports-29.html (live test) (source)
- variable-supports-30.html (live test) (source)
- variable-supports-31.html (live test) (source)
- variable-supports-32.html (live test) (source)
- variable-supports-33.html (live test) (source)
- variable-supports-34.html (live test) (source)
- variable-supports-35.html (live test) (source)
- variable-supports-36.html (live test) (source)
- variable-supports-37.html (live test) (source)
- variable-supports-38.html (live test) (source)
- variable-supports-39.html (live test) (source)
- variable-supports-40.html (live test) (source)
- variable-supports-41.html (live test) (source)
- variable-supports-42.html (live test) (source)
- variable-supports-43.html (live test) (source)
- variable-supports-44.html (live test) (source)
- variable-supports-45.html (live test) (source)
- variable-supports-46.html (live test) (source)
- variable-supports-47.html (live test) (source)
- variable-supports-48.html (live test) (source)
- variable-supports-49.html (live test) (source)
- variable-supports-50.html (live test) (source)
- variable-supports-51.html (live test) (source)
- variable-supports-52.html (live test) (source)
- variable-supports-53.html (live test) (source)
- variable-supports-54.html (live test) (source)
- variable-supports-55.html (live test) (source)
- variable-supports-56.html (live test) (source)
- variable-supports-57.html (live test) (source)
- variable-supports-58.html (live test) (source)
- variable-supports-59.html (live test) (source)
- variable-supports-60.html (live test) (source)
- variable-supports-61.html (live test) (source)
- variable-supports-62.html (live test) (source)
- variable-supports-63.html (live test) (source)
- variable-supports-64.html (live test) (source)
- variable-supports-65.html (live test) (source)
- variable-supports-66.html (live test) (source)
- variable-supports-67.html (live test) (source)
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
- variable-declaration-08.html (live test) (source)
- variable-declaration-09.html (live test) (source)
- variable-declaration-10.html (live test) (source)
- variable-declaration-11.html (live test) (source)
- variable-declaration-12.html (live test) (source)
- variable-declaration-13.html (live test) (source)
- variable-declaration-22.html (live test) (source)
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.
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 */ }
-
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.
-
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.
-
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.
-
Return result and fallback.
4. Arbitrary Substitution Functions
An arbitrary substitution function is a function that will, when resolved, substitute itself with other content which is unknowable at parse time.
Specifically, arbitrary substitution functions are allowed in all properties. When present, the property must be assumed to be valid
Arbitrary substitution functions can be used in place of any part of a value in any property on an element. They can not be used as property names, selectors, or anything else besides property values, unless otherwise specified.
Tests
.foo{ --side : margin-top; var ( --side) :20 px ; }
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.
If a property contains one or more arbitrary substitution 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 arbitrary substitution.
Tests
Each arbitrary substitution function must define how to resolve an arbitrary substitution function for itself, returning optional "result" and "fallback" values. The "result" is used to replace the function as long as it does not contain the guaranteed-invalid value; the "fallback" is used otherwise. The "fallback" does not need to be resolved in any way; substitution will handle that if it’s actually used.
Note: See, for example, resolve a var() function.
-
For each arbitrary substitution function func in value:
-
Resolve func. Let result be the returned result, and fallback be the returned fallback.
If no result was returned, set result to the guaranteed-invalid value. If no fallback was returned, set fallback to the guaranteed-invalid value.
-
- If result does not contain the guaranteed-invalid value
-
Replace func in value with result.
- Otherwise, if fallback does not contain the guaranteed-invalid value
-
Replace func in value with fallback.
- Otherwise
-
Replace all of value with the guaranteed-invalid value. Exit this algorithm.
-
-
If there are still arbitrary substitution functions in value (due to substitution), repeat the previous step.
-
Grammar-check value according to its context as normal. If it is not valid at this point, replace value with the guaranteed-invalid value.
Tests
- css-variable-change-style-001.html (live test) (source)
- css-variable-change-style-002.html (live test) (source)
- variable-declaration-01.html (live test) (source)
- variable-declaration-02.html (live test) (source)
- variable-declaration-03.html (live test) (source)
- variable-declaration-04.html (live test) (source)
- variable-declaration-05.html (live test) (source)
- variable-generated-content-dynamic-001.html (live test) (source)
- variable-presentation-attribute.html (live test) (source)
- variable-reference-01.html (live test) (source)
- variable-reference-02.html (live test) (source)
- variable-reference-03.html (live test) (source)
- variable-reference-04.html (live test) (source)
- variable-reference-05.html (live test) (source)
- variable-reference-12.html (live test) (source)
- variable-reference-16.html (live test) (source)
- variable-reference-40.html (live test) (source)
- variable-reference-refresh.html (live test) (source)
- variable-substitution-background-properties.html (live test) (source)
- variable-substitution-basic.html (live test) (source)
- variable-substitution-filters.html (live test) (source)
- variable-substitution-replaced-size.html (live test) (source)
- variable-substitution-shadow-properties.html (live test) (source)
- variable-substitution-variable-declaration.html (live test) (source)
CSSOM
.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) *1 px ); }
This also implies that the post-substitution value might not be directly serializable as-is. Here’s a similar example to the preceding:
.foo{ --gap : 20 ; --not-px-length : var ( --gap) px; }
The serialization of the computed (post-substitution) value of --not-px-length is not 20px, because that would parse back as the single combined dimension; instead, it will serialize with a comment between the two tokens, like px, to enforce that they are separate tokens even when re-parsing.
Tests
- variable-declaration-14.html (live test) (source)
- variable-declaration-53.html (live test) (source)
- variable-declaration-54.html (live test) (source)
- variable-declaration-55.html (live test) (source)
- variable-reference-15.html (live test) (source)
- variable-reference-without-whitespace.html (live test) (source)
In properties, arbitrary substitution functions are substituted at computed-value time, before any other value transformations or introspection can occur. If a property, after substitution, does not match its declared grammar, the declaration is invalid at computed-value time.
Tests
- variable-declaration-16.html (live test) (source)
- variable-declaration-17.html (live test) (source)
- variable-declaration-18.html (live test) (source)
- variable-declaration-19.html (live test) (source)
- variable-declaration-21.html (live test) (source)
- variable-transitions-transition-property-all-before-value.html (live test) (source)
- variable-transitions-value-before-transition-property-all.html (live test) (source)
If a property, after substitution, contains only a CSS-wide keyword (and possibly whitespace/comments), its value is determined as if that keyword were its specified value all along.
Tests
:root{ --looks-valid : 20 px ; } p{ background-color : var ( --looks-valid); }
Since 20px is an invalid value for background-color, the property becomes invalid at computed-value time, and instead resolves to transparent (the initial value for background-color).
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.
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.
4.1. Invalid Substitution
When substitution results in a property’s value containing the guaranteed-invalid value, this makes the declaration invalid at computed-value time. 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.
:root{ --not-a-color : 20 px ; } 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 arbitrary substitution functions 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.
4.2. Substitution in Shorthand Properties
Arbitrary substitution 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 an arbitrary substitution 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 an arbitrary substitution function, and thus the longhand’s value can’t be determined until after substituted.
This value must then be cascaded as normal, and at computed-value time, after substitution, the shorthand must be parsed and the longhands must be given their appropriate values at that point.
Tests
- variable-reference-36.html (live test) (source)
- variable-reference-37.html (live test) (source)
- variable-reference-38.html (live test) (source)
- variable-substitution-shorthands.html (live test) (source)
- vars-background-shorthand-001.html (live test) (source)
- vars-font-shorthand-001.html (live test) (source)
Note: When a shorthand is written without an arbitrary substitution function, 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 (arbitrary substitution function-containing) value.
Otherwise, if any of the component longhand properties for a given shorthand are pending-substitution values, or contain arbitrary substitution functions of their own that have not yet been substituted, the shorthand property must serialize to the empty string.
4.3. Safely Handling Overly-Long Substitution
Naively implemented, some arbitrary substitution functions (such as var()) 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 an arbitrary substitution function expands into. If an arbitrary substitution function would expand into a longer token stream than this limit, it instead is replaced with the guaranteed-invalid value.
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.
5. 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 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.
5.1. 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
--y : /* baz */ ; --x : /* foo */ var ( --y) /* bar */ ;
the serialization of the specified value of --x must be
,
while the serialization of the computed value of --x must be
.
(Note that the leading whitespace on the value is automatically trimmed by the CSS parser; it’s not preserved here.)
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.
6. Changes
6.1. Changes Since the 16 June 2022 CR Snapshot
-
Clarified that the comment-insertion can happen with 0+ comments between the original tokens, not just exactly 1.
-
Clarified the transition behavior of custom properties, in a note
6.2. Changes Since the 11 November 2021 CR Draft
-
Clarified that custom properties apply all pseudo-elements (including those with restricted property lists)
-
Added example to illustrate issues with combining characters, ligatures, etc
-
Strengthened wording around similar-appearing variable names that use distinct codepoint sequences
-
Clarified an example by using more visually distinct languages as examples (English and Greek)
-
Split Security and Privacy into separate sections
6.3. 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)
-
Similarly, made empty fallbacks valid in var().
-
The -- property is reserved for future use by CSS.
-
Added concept of "animation-tainted", to prevent non-animatable properties from using a variable to smuggle in some animatability.
-
Defined the guaranteed-invalid value to make the initial value of custom properties and the result of cycles or substitution failure more straightforward, and allow failure to propagate thru substitutions until finally intercepted by a fallback.
-
Defined that cycles trigger invalid at computed-value time behavior.
-
Allowed variables to resolve to a CSS-wide keyword (only possible by providing it as a fallback).
-
Clarified that registered custom properties act like non-custom properties when they’re invalid at computed-value time.
-
Made longhands with var()s also trigger their shorthands to be unserializable, like longhands with pending-substitution values already did.
-
Required UAs to defend against exponential substitution attacks.
-
Defined how to serialize the values of custom properties (previously, only the property name’s serialization was specified).
6.4. 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.
7. 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.
8. 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.
9. Security Considerations
§ 4.3 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.