CSS Functions and Mixins Module

1. Introduction

This section is not normative.

Note: At this time, this specification only defines custom functions, which operate at the level of CSS values. It is expected that it will define "mixins" later, which are functions that operate at the style rule level.

Custom properties give authors a lot of power to define useful, sometimes complex values in one place, and then re-use them across their stylesheet. They can vary across the document, or based on Media Queries or other conditionals, making them very flexible and responsive.

However, their values are fixed at the point they’re defined, unable to be changed except by fully overriding their previous definition: a --shadow: 2px 2px var(--shadow-color) declaration takes its --shadow-color value from the element it’s declared on, and later changes to --shadow-color on descendant elements don’t alter the value of --shadow for them; they continue to use the shadow color defined where --shadow was defined. This is a common source of confusion for authors making heavy use of composite variables like this.

Custom functions allow authors the same power as custom properties, but parameterized: they have the same flexibility and conditionality as a custom property definition, but take values from other custom properties (or explicitly as arguments) at the point of use. For example, instead of a --shadow custom property, a --shadow() custom function could be defined instead, like:

@function --shadow(--shadow-color <color> : inherit) {
  /* If --shadow-color argument isn't passed,
     or doesn't successfully parse as a <color>,
     try to use the --shadow-color *property*
     from the element instead */

  /* var(--shadow-color) refers to the --shadow-color parameter,
     rather than a custom property,
     but can still use a fallback value as normal */
  result: 2px 2px var(--shadow-color, black);
}

.foo {
  --shadow-color: blue;
  box-shadow: --shadow(); /* produces a blue shadow */
  /* or just */
  box-shadow: --shadow(blue);
}

2. Defining Custom Functions

A custom function can be thought of as an advanced custom property, which instead of being substituted by a single fixed value, computes its substitution value based on function parameters and the value of custom properties at the point it’s invoked. Rather than the var() syntax that custom properties use for substitution, custom functions are invoked by <dashed-function> syntax, allowing additional values to be passed as arguments.

@function --negative(--value) {
  result: calc(-1 * var(--value));
}

html {
  --gap: 1em;
  padding: --negative(var(--gap));
  /* or by passing the value explicitly, like: */
  padding: --negative(1em);
}

<dashed-function>s are arbitrary substitution functions, like var(). Their presence in a property’s value causes it to be assumed valid at parse time, and only evaluated and parsed at computed-value time, after arbitrary substitution has occurred.

2.1. The @function Rule

The @function rule defines a custom function, and consists of a name, a list of parameters, a function body, and optionally a return type described by a syntax definition.

Each function parameter consists of a name (<custom-property-name>); optionally a parameter type, described by a syntax definition; and optionally a default value.

<@function> = @function <function-token> <function-parameter>#? )
  [ returns <css-type> ]?
{
  <declaration-rule-list>
}

<function-parameter> = <custom-property-name> <css-type>? [ : <declaration-value> ]?
<css-type> = <syntax-component> | <type()>
<type()> = type( <syntax> )

2.1.1. The Function Preamble

The <function-token> production must start with two dashes (U+002D HYPHEN-MINUS), similar to <dashed-ident>, or else the definition is invalid.

The name of the resulting custom function is given by the name of the <function-token>, the optional function parameters are given by the <function-parameter> values (defaulting to an empty set), and the optional return type is given by the <css-type> following the returns keyword (defaulting to *).

@function --foo(--a <length>) { /* ... */ }
@function --foo(--a <color>) { /* ... */ }
@function --foo(--a <length>+) { /* ... */ }

@function --foo(--a type(<number> | <percentage>)) { /* ... */ }

The name of a @function rule is a tree-scoped name. If more than one @function exists for a given name, then the rule in the stronger cascade layer wins, and rules defined later win within the same layer.

If the function parameters contain the same <custom-property-name> more than once, then the @function rule is invalid.

2.1.2. The Function Body

The body of a @function rule accepts conditional group rules, such as @media. Additionally, it accepts the following descriptors:

• The result descriptor, which determines the result of evaluating the function. If no result descriptor exists, the function is valid, but always returns the guaranteed-invalid value.

• Custom properties, providing local variables.

Unknown descriptors are invalid and ignored, but do not make the @function rule itself invalid.

2.2. The result Descriptor

The result descriptor defines the result of evaluating the custom function defined by its @function rule. Using var() functions, it can reference function parameters, local variables, as well as other custom functions via <dashed-function>s.

The result descriptor itself does not have a type, but its resolved value is type-checked during the substitution of a <dashed-function>.

2.3. Arguments & Local Variables

This section is non-normative.

Within a custom function’s function body, the var() function can access local variables (the custom properties defined in the function body), function parameters (the values passed to the function, or set to default values), and custom properties defined at the call site (an element, or another custom function).

In that list, earlier things "win" over later things of the same name—​if you have a local variable named --foo, var(--foo) will be substituted by that local variable, not by an argument or a custom property defined outside. The other values can still be accessed, however: setting the --foo local variable to initial will resolve it to the --foo parameter, while inherit will resolve it to the --foo custom property from the call site.

@function --outer(--outer-arg) {
  --outer-local: 2;
  result: --inner();
}
@function --inner() returns <number> {
  result: calc(var(--outer-arg) + var(--outer-local));
}
div {
  z-index: --outer(1); /* 3 */
}

@function --double-z() returns <number> {
  result: calc(var(--z) * 2);
}
div {
  --z: 3;
  z-index: --double-z(); /* 6 */
}

@function --add-a-b-c(--b, --c) {
  --c: 300;
  result: calc(var(--a) + var(--b) + var(--c));
  /* uses the --a from the call site's custom property,
     the --b from the function parameter,
     and the --c from the local variable */
}
div {
  --a: 1;
  --b: 2;
  --c: 3;
  z-index: --add-a-b-c(20, 30); /* 321 */
}

3. Using Custom Functions

Similar to how the value of a custom property can be substituted into the value of another property with var(), the result of a custom function evaluation can be substituted into the value of a property with a <dashed-function>.

A <dashed-function> is a functional notation whose function name starts with two dashes (U+002D HYPHEN-MINUS). Its syntax is:

<dashed-function> = --*( <declaration-value># )

A <dashed-function> can only be used where var() is allowed.

If a property contains one or more <dashed-function>s, the entire property’s grammar must be assumed to be valid at parse time. At computed-value time, every <dashed-function> must be substituted before finally being checked against the property’s grammar.

Note: Within the body of a custom function, var() functions might resolve differently than on the element the <dashed-function> is used on. See § 3.1 Evaluating Custom Functions.

A <dashed-function> is evaluated in some context: either in a property value on an element (or in a descriptor that is eventually treated like a property on an element, such as in @keyframes), or in a descriptor in the function body of another custom function that is being applied to a "hypothetical" element. Either way, this provides a calling context, which contains the property or descriptor name containing the <dashed-function>, and the element (or "hypothetical" element) that property/descriptor is being applied to.

As calling contexts are nested by <dashed-function> evaluations inside of custom functions, a calling context’s root element is the real element at the root of the calling context stack.

If substitute a dashed function fails, and the substitution is taking place on a property’s value, then the declaration containing the <dashed-function> becomes invalid at computed-value time.

@function --max-plus-x(--list, --x) {
  result: calc(max(var(--list)) + var(--x));
}
div {
  width: --max-plus-x({ 1px, 7px, 2px }, 3px); /* 10px */
}

3.1. Evaluating Custom Functions

Custom functions are evaluated by, essentially, pretending their function body is a style rule being applied to a hypothetical element, resolving styles as normal, and then returning the value of the result descriptor on that hypothetical element. The hypothetical element "inherits" the values of all custom properties as if it were a child of its calling context, with its function parameters overriding "inherited" custom properties of the same name.

3.2. Cycles

The result descriptor and local variables within a custom function may reference other custom functions or custom properties, and may therefore create cycles.

For each element, add a node for every specified custom function to the graph described in CSS Variables 1 § 2.3 Resolving Dependency Cycles; add a node for each local variable defined within each of those functions; then, for each custom function func, add edges as follows:

• From func to any custom function referenced by a <dashed-function> within func’s body.

• From func to any custom property or local variable referenced by a var() within func’s body.

• To func from any custom property or local variable that references func using a <dashed-function>.

A <dashed-function> referencing a custom function which is part of a cycle makes the containing declaration invalid at computed-value time.

Note: Cycles are disallowed even through branches that are not taken during execution.

@function --foo(--x) {
  @media (unknown-feature) {
    result: --foo(42);
  }
  result: 1;
}

@function --bar() {
  --x: --bar();
  result: 1;
}

@function --baz(--x) {
  --y: 10px;
  result: calc(var(--x) + var(--y));
}

div {
  --x: --baz(1px);
  --y: --baz(2px);
  width: var(--x);  /* 11px */
  height: var(--y); /* 12px */
}

4. Execution Model of Custom Functions

Like the rest of CSS, custom functions adhere to a declarative model.

The local variable descriptors and result descriptor can appear in any order, and may be provided multiple times. If this happens, then declarations appearing later win over earlier ones.

@function --mypi() {
  result: 3;
  result: 3.14;
}

@function --circle-area(--r) {
  result: calc(pi * var(--r2));
  --r2: var(--r) * var(--r);
}

4.1. Conditional Rules

A conditional group rule that appears within a @function becomes a nested group rule, with the additional restriction that only descriptors allowed within @function are allowed within the nested group rule.

Conditional group rules within @function are processed as normal, acting as if the contents of the rule were present at the conditional group rule’s location when the condition is true, or acting as if nothing exists at that location otherwise.

@function --suitable-font-size() {
  result: 16px;
  @media (width > 1000px) {
    result: 20px;
  }
}

@function --suitable-font-size() {
  @media (width > 1000px) {
    result: 20px;
  }
  result: 16px;
}

@function --suitable-font-size() {
  --size: 16px;
  @media (width > 1000px) {
    --size: 20px;
  }
  result: var(--size);
}

5. CSSOM

The CSSFunctionRule interface represents a @function rule.

[Exposed=Window]
interface CSSFunctionRule : CSSGroupingRule { };

While declarations may be specified directly within a @function rule, they are not represented as such in the CSSOM. Instead, consecutive segments of declarations appear as if wrapped in CSSFunctionDeclarations rules.

Note: This also applies to the "leading" declarations in the @function rule, i.e those that do not follow another nested rule.

@function --bar() {
  --x: 42;
  result: var(--y);
  @media (width > 1000px) {
    /* ... */
  }
  --y: var(--x);
}

@function --bar() {
  /* CSSFunctionDeclarations { */
    --x: 42;
    result: var(--y);
  /* } */
  @media (width > 1000px) {
    /* ... */
  }
  /* CSSFunctionDeclarations { */
    --y: var(--x);
  /* } */
}

5.1. The CSSFunctionDeclarations Interface

The CSSFunctionDeclarations interface represents a run of consecutive declarations within a @function rule.

[Exposed=Window]
interface CSSFunctionDescriptors : CSSStyleDeclaration {
  attribute [LegacyNullToEmptyString] CSSOMString result;
};

[Exposed=Window]
interface CSSFunctionDeclarations : CSSRule {
  [SameObject, PutForwards=cssText] readonly attribute CSSFunctionDescriptors style;
};

The CSSFunctionDeclarations rule, like CSSNestedDeclarations, serializes as if its declaration block had been serialized directly.

6. Privacy Considerations

The constructs defined by this specification are defined and used entirely within CSS; they expose no new information.

7. Security Considerations

No issues have been opened against this specification.