CSS Values and Units Module Level 5

1. Introduction

This is a diff spec against CSS Values and Units Level 4.

1.1. Module Interactions

This module extends [CSS-VALUES-4] which replaces and extends the data type definitions in [CSS21] sections 1.4.2.1, 4.3, and A.2.

2. Textual Data Types

See CSS Values 4 § 4 Textual Data Types.

3. Value Definition Syntax

See CSS Values 4 § 2 Value Definition Syntax.

3.1. Functional Notation Definitions

See CSS Values 4 § 2.6 Functional Notation Definitions.

3.1.1. Commas and Semicolons in Functions

Functional notation often uses commas to separate parts of its internal grammar. However, some functions (such as mix()) allow values that, themselves, can contain commas.

To accommodate these sorts of grammars unambiguously, commas in functional grammars are implicitly upgradeable to semicolons in Level 5; that is, commas in a functional notation's grammar can instead be represented as <semicolon-token>s. This is all-or-nothing: either every comma in the functional notation must be written as a semicolon, or none of them must be.

When a functional notation is parsed, initially commas (,) and comma-multipliers (#) in the grammar match only <comma-token>s in the value, and any comma-containing productions are not allowed to contain <comma-token>s (the productions must be interpreted as ending before the comma). If a <semicolon-token> is encountered while parsing the functional notation, the contents must be re-interpreted, with commas and comma-multipliers in the grammar matching only <semicolon-token>s in the value, and comma-containing productions being allowed to contain <comma-token>s.

Commas contained in productions defined as comma-containing productions (such as <any-value> or <whole-value>) are not implicitly upgradeable. Even when a functional notation is being re-interpreted with semicolons, these productions end when a <semicolon-token> is encountered.

random-item( <random-caching-options>, [<declaration-value>?]# )

list-style: random-item(--x, disc, circle, square);

list-style: random-item(--x; disc; circle; square);

list-style: random-item(--x; disc, circle; square);

font-family: random-item(--x; Times, serif; Arial, sans-serif; Courier, monospace);

font-family: random-item(--x, serif, sans-serif, monospace);

Functional notations are serialized with commas (rather than semicolons) whenever possible.

The following generic productions are comma-containing productions:

• <any-value>

• <whole-value>

• <declaration-value>

3.2. Resource Locators: the <url> type

See CSS Values 4 § 4.5 Resource Locators: the <url> type.

3.2.1. Request URL Modifiers

<request-url-modifier>s are <url-modifier>s that affect the <url>’s resource request by applying associated URL request modifier steps. See CSS Values 4 § 4.5.4 URL Processing Model.

This specification defines the following <request-url-modifier>s:

<request-url-modifier> = <crossorigin-modifier> | <integrity-modifier> | <referrerpolicy-modifier>
<crossorigin-modifier> = crossorigin(anonymous | use-credentials)
<integrity-modifier> = integrity(<string>)
<referrerpolicy-modifier> = referrerpolicy(no-referrer | no-referrer-when-downgrade | same-origin | origin | strict-origin | origin-when-cross-origin | strict-origin-when-cross-origin | unsafe-url)

<crossorigin-modifier> = crossorigin(anonymous | use-credentials)

The URL request modifier steps for this modifier given request req are:

1. Set request's mode to "cors".

2. If the given value is use-credentials, set request's credentials mode to "include".

<integrity-modifier> = integrity(<string>)

The URL request modifier steps for this modifier given request req are to set request's integrity metadata to the given <string>.

<referrerpolicy-modifier> = referrerpolicy(no-referrer | no-referrer-when-downgrade | same-origin | origin | strict-origin | origin-when-cross-origin | strict-origin-when-cross-origin | unsafe-url)

The URL request modifier steps for this modifier given request req are to set request's referrer policy to the ReferrerPolicy that matches the given value.

4. Interpolation Progress Functional Notations

This section is an exploratory draft, and not yet approved by the CSSWG. [Issue #6245]

The progress(), media-progress(), and container-progress() functional notations represent the proportional distance of a given value (the progress value) from one value (the progress start value) to another value (the progress end value). They allow drawing a progress ratio from math functions, media features, and container features, respectively, following a common syntactic pattern:

progress-function() = progress-function( progress value from start value to end value )

The resulting ratio can then be input into other calculations, such as a math function or a mix notation.

4.1. Calculated Progress Values: the progress() notation

The progress() functional notation returns a <number> value representing the position of one calculation (the progress value) between two other calculations (the progress start value and progress end value). The argument calculations can resolve to any <number>, <dimension>, or <percentage>, but must have a consistent type or else the function is invalid. The result will be a <number>, made consistent with the consistent type of the arguments.

The syntax of progress() is defined as follows:

<progress()> = progress(<calc-sum> from <calc-sum> to <calc-sum>)

where the first, second, and third <calc-sum> values represent the progress value, progress start value, and progress end value, respectively.

The value returned by a valid progress() notation is (progress value - start value) / (end value - start value), as a <number>.

Do we need a percent-progress() notation, or do enough places auto-convert that it’s not necessary?

Note: The progress() function is essentially syntactic sugar for a particular pattern of calc() notations.

4.2. Media Query Progress Values: the media-progress() notation

Similar to the progress() notation, the media-progress() functional notation returns a <number> value representing current value of the specified media query [MEDIAQUERIES-4] as a progress value between two explicit values of the media query (as the progress start value and progress end value).

The syntax of media-progress() is defined as follows:

<media-progress()> = media-progress(<media-feature> from <calc-sum> to <calc-sum>)

The value returned by a valid media-progress() notation is progress value / (end value - start value, as a <number>.

The specified media query must be a valid “range” type query, and its specified progress start value and progress end value must be valid values for the specified media query, or else the function is invalid.

The two input calculations but must have a consistent type or else the function is invalid. The result will be a <number>, made consistent with the consistent type of the arguments.

4.3. Container Query Progress Values: the container-progress() notation

The container-progress() functional notation is identical to the media-progress() functional notation, except that it accepts container features [CSS-CONTAIN-3] in place of media features.

The syntax of container-progress() is defined as follows:

<container-progress()> = container-progress(<size-feature> [ of <container-name> ]? from <calc-sum> to <calc-sum>)

where the optional <container-name> component specifies the named containers to consider when selecting a container to resolve against.

The two input calculations but must have a consistent type or else the function is invalid. The result will be a <number>, made consistent with the consistent type of the arguments.

If no appropriate containers are found, container-progress() resolves its <size-feature> query against the small viewport size.

5. Mixing and Interpolation Notations: the *-mix() family

This section is an exploratory draft, and not yet approved by the CSSWG. [Issue #6245]

Several mix notations in CSS allow representing the interpolation of two values, the mix start value and the mix end value, at a given point in progress between them (the mix progress value). These functional notations follow the syntactic pattern:

mix-function() = mix-function( <progress>, [=mix start value|start-value=], [=mix end value|end-value=] )

The mix notations in CSS include:

• calc-mix(), for interpolating <length>, <percentage>, <time>, and other dimensions representable in calc() expressions

• color-mix(), for interpolating two <color> values

• cross-fade(), for interpolating <image> values

• palette-mix(), for interpolating two font-palette values

and finally the generic mix() notation, which can represent the interpolation of any property’s values (but only the property’s entire value, not individual components).

Note: The cross-fade() notation also has an alternative syntax that allows for mixing more than two values, but does not allow for the more complex expressions of <progress>.

The mix() notation also has a variant that takes a set of keyframes. It does this by referring to an @keyframes rule, and pulling the corresponding property declaration out of that. It would be nice to allow the other mix notations to take keyframe also, but how would we represent a set of keyframes for a component value (rather than a full property value)?

5.1. Representing Interpolation Progress: the <progress> type

The <progress> value type represents the mix progress value in a mix notation, and ultimately resolves to a percentage. It can, however, draw that percentage value from sources such as media queries and animation timelines, and can also convert it through an easing function before using it for interpolation.

Its syntax is defined as follows:

<progress> = [ <percentage> | <number> | <'animation-timeline'> ]? && by <easing-function>

where:

<percentage-token>

Computes to the equivalent <number>: 0% becomes 0, 100% becomes 1, etc.

Note: This only allows literal percentages, like 15%; calculations like calc(100% / 7) will not work, as they will instead attempt to use the normal rules for resolving a percentage against another type (such as <length>, in width). Use expressions like calc(1 / 7) instead.

<number>

Represents the mix progress value.

Note: This allows the use of the progress(), media-progress(), and container-progress() notations.

<'animation-timeline'>

Represents the mix progress value as the progress of the specified animation timeline. The values none and auto, however, are invalid. [CSS-ANIMATIONS-2] [WEB-ANIMATIONS-2]

<easing-function>

Converts the specified input mix progress value into an output mix progress value using the specified easing function. [CSS-EASING-1]

Note: Progress values below 0 and above 1 are valid; they allow representing interpolation beyond the range defined by the start and end values.

Note: While <progress> itself can be a <percentage>, mapping directly to the equivalent <number>, a function that resolves to a <number>, like progress(), resolves <percentage>s using the normal rules for the context; for example, in width, they would be resolved against a length.

The computed value of a <progress> value specified with <percentage> or <number> is the computed <number> converted through the <easing-function> (if any). The computed value of a <progress> value specified with <'animation-timeline'> is the computed <'animation-timeline'> and <easing-function> (if any).

5.2. Interpolated Numeric and Dimensional Values: the calc-mix() notation

The calc-mix() mix notation represents an interpolated numeric or dimensional value. Like calc(), it is a math function, with the following syntactic form:

<calc-mix()> = calc-mix( <progress>, <calc-sum>, <calc-sum> )

The <calc-sum> arguments can resolve to any <number>, <dimension>, or <percentage>, but must have a consistent type or else the function is invalid. The result’s type will be the consistent type, made consistent with the type of the <progress> value.

The used value of a valid calc-mix() is the result of interpolating these two values to the progress given by <progress>. If the <progress> value can be computed to a <number>, then the computed value is likewise the result of interpolating the two computed values to that <progress> value (in other words, A * (1-progress) + B * progress) it is otherwise the calc-mix() notation itself with its arguments each computed according to their type.

5.3. Interpolated Color Values: the color-mix() notation

This specification extends the color-mix() functional notation as a mix notation accepting the following syntaxes:

<color-mix()> =
  color-mix( <progress> && <color-interpolation-method>?, <color>, <color> ) |
  color-mix( <color-interpolation-method>, [<color> && <percentage [0,100]>?]#{2} )

The used value of the first mix notation variant is equivalent to assigning the <progress> value, as a <percentage>, to the <percentage> of the second <color> argument in the second variant. That is, color-mix(progress, color1, color2) is equivalent to color-mix(color1, color2 progress). See CSS Color 5 § 3 Mixing Colors: the color-mix() Function for the normative definition of the second variant.

<progress> allows returning percentages outside 0-100%, but color-mix() doesn’t allows such values, so need to define how that gets processed.

5.4. Interpolated Image Values: the cross-fade() notation

This specification extends the cross-fade() functional notation as a mix notation accepting the following syntaxes:

<cross-fade()> =
  cross-fade( <progress>, [ <image> | <color> ], [ <image> | <color> ] ) |
  cross-fade( <cf-image># )

The used value of the first mix notation variant is equivalent to assigning the <progress> value as the <percentage> of the second <color> argument in the second variant. That is, cross-fade(progress, image1, image2) is equivalent to cross-fade(image1, image2 progress). See CSS Images 4 § 2.6 Combining images: the cross-fade() notation for the normative definition of the second variant.

5.5. Interpolated Transform Values: the transform-mix() notation

The transform-mix() mix notation represents an interpolated <transform-list>, with the following syntactic form:

<transform-mix()> = transform-mix( <progress>, <transform-list>, <transform-list> )

The used value of a valid transform-mix() is the result of interpolating these two values to the progress given by <progress>. If the <progress> value can be computed to a <percentage>, and the <transform-list>s can be interpolated without used-value-time information, then the computed value is likewise the result of interpolating the two computed values to that <progress> value; it is otherwise the transform-mix() notation itself with its arguments each computed according to their type.

transform-mix() is, itself, a <transform-function>.

5.6. Interpolated Property Values: the mix() notation

Interpolation of any two property values can be represented by the mix() mix notation, which supports two alternative syntax patterns:

<mix()> =
  mix( <progress> , <whole-value> , <whole-value> ) |
  mix( <progress> && of <'animation-name'> )

The first syntax alternative, like other mix notations, interpolates between the first <whole-value> (its mix start value) and the second <whole-value> (its mix end value). The second uses the mix progress value to interpolate the corresponding property declarations from a set of keyframes, allowing for more complex interpolation curves.

Note: This functional notation uses semicolons to separate arguments rather than the more typical comma because the values themselves can contain commas.

For the standard mix notation variant, if the two <whole-value>s being interpolated by mix() are interpolable as values for the property in which it is specified, and the interpolated value can be represented without mix(), the computed value of mix() is the result of interpolating these two values to the progress given by <progress>. Otherwise, the computed value of mix() is the mix() functional notation itself with its <progress> value computed and its <whole-value>s (if provided) computed as values for this property.

color: mix(90%, red, blue);
/* via simple interpolation,
   computes to: */
color: rgb(10% 0 90%);

color: mix(90%, currentcolor, black);
/* can’t be fully resolved at computed-value time,
   but still has a defined representation: */
color: color-mix(currentcolor 90%, black 10%);

float: mix(90%, left, right);
/* discretely animatable */
float: right;

transform: mix(90%, translate(calc(1em + 50%)), rotate(30deg));
/* because functions don’t match, it will interpolate
   via matrix(). But translate(%) needs layout
   information to turn into a matrix(), so the
   interpolated value can’t actually be represented.
   Computes to: */
transform: mix(90%, translate(calc(16px + 50%)), rotate(30deg));
transform: mix(90% of ripple);

The mix() notation is a <whole-value>. Additionally, if any of its <whole-value> arguments are not animatable, the notation is invalid.

/* Invalid start value */
color: mix(90%, #invalid, #F00);

/* Function is mixed with other values */
background: url(ocean) mix(10%, blue, yellow);

/* 'animation-*' is not animatable */
animation-delay: mix(0%, 0s, 2s);

6. Miscellaneous Value Substituting Functions

6.1. Representing An Entire Property Value: the <whole-value> type

Several functions defined in this specification can only be used as the "whole value" of a property. For example, background-position: toggle(50px 50px, center); is valid, but background-position: toggle(50px, center) 50px; is not. The <whole-value> production represents these values.

All properties implicitly accept a <whole-value> as their entire value, just as they accept the CSS-wide keywords as their entire value.

When used as a component value of a function, <whole-value> also represents any CSS value normally valid as the whole value of the property in which it is used (including additional <whole-value> functions). However, some functions may restrict what a <whole-value> argument can include.

6.2. Selecting the First Supported Value: the first-valid() notation

CSS supports progressive enhancement with its forward-compatible parsing: authors can declare the same property multiple times in a style rule, using different values each time, and a CSS UA will automatically use the last one that it understands and throw out the rest. This principle, together with the @supports rule, allows authors to write stylesheets that work well in old and new UAs simultaneously.

However, using var() (or similar substitution functions that resolve after parsing) thwarts this functionality; CSS UAs must assume any such property is valid at parse-time.

The first-valid() functional notation inlines the fallback behavior intrinsic to parsing declarations. Unlike most notations, it can accept any valid or invalid syntax in its arguments, and represents the first value among its arguments that is supported (parsed as valid) by the UA as the whole value of the property it’s used in.

<first-valid()> = first-valid( <declaration-value># )

If none of the arguments represent a valid value for the property, the property is invalid at computed-value time.

first-valid() is a <whole-value>.

Should this have a different name? We didn’t quite decide on it during the resolution to add this.

Note: Despite effectively taking <whole-value>s as its argument, first-valid() is instead defined to take <declaration-value>s because, by definition, it’s intended to be used in cases where its values might be invalid for the declaration it’s in. <declaration-value> imposes no contextual validity constraints on what it matches, unlike <whole-value>.

6.3. Toggling Between Values: toggle()

The toggle() expression allows descendant elements to cycle over a list of values instead of inheriting the same value.

em { font-style: toggle(italic, normal); }

ul { list-style-type: toggle(disc, circle, square, box); }

The syntax of the toggle() expression is:

<toggle()> = toggle( <whole-value># )

The toggle() notation is a <whole-value>. However, it is not allowed to be nested, nor may it contain attr() or calc() notations; declarations containing such constructs are invalid.

background-position: 10px toggle(50px, 100px);
/* toggle() must be the sole value of the property */

list-style-type: toggle(disc, 50px);
/* 50px isn’t a valid value of 'list-style-type' */

To determine the computed value of toggle(), first evaluate each argument as if it were the sole value of the property in which toggle() is placed to determine the computed value that each represents, called C_n for the n-th argument to toggle(). Then, compare the property’s inherited value with each C_n. For the earliest C_n that matches the inherited value, the computed value of toggle() is C_n+1. If the match was the last argument in the list, or there was no match, the computed value of toggle() is the computed value that the first argument represents.

Note: This means that repeating values in a toggle() short-circuits the list. For example toggle(1em, 2em, 1em, 4em) will be equivalent to toggle(1em, 2em).

Note: That toggle() explicitly looks at the computed value of the parent, so it works even on non-inherited properties. This is similar to the inherit keyword, which works even on non-inherited properties.

Note: That the computed value of a property is an abstract set of values, not a particular serialization [CSS21], so comparison between computed values should always be unambiguous and have the expected result. For example, a Level 2 background-position computed value is just two offsets, each represented as an absolute length or a percentage, so the declarations background-position: top center and background-position: 50% 0% produce identical computed values. If the "Computed Value" line of a property definition seems to define something ambiguous or overly strict, please provide feedback so we can fix it.

If toggle() is used on a shorthand property, it sets each of its longhands to a toggle() value with arguments corresponding to what the longhand would have received had each of the original toggle() arguments been the sole value of the shorthand.

margin: toggle(1px 2px, 4px, 1px 5px 4px);

margin-top:    toggle(1px, 4px, 1px);
margin-right:  toggle(2px, 4px, 5px);
margin-bottom: toggle(1px, 4px, 4px);
margin-left:   toggle(2px, 4px, 5px);

margin-top:    toggle(1px, 4px);
margin-right:  toggle(2px, 4px, 5px);
margin-bottom: toggle(1px, 4px);
margin-left:   toggle(2px, 4px, 5px);

6.4. Attribute References: the attr() function

The attr() function substitutes the value of an attribute on an element into a property, similar to how the var() function substitutes a custom property value into a function.

attr() = attr( <attr-name> <attr-type>? , <declaration-value>?)

<attr-name> = [ <ident-token> '|' ]? <ident-token>

<attr-type> = string | ident | color | number | percentage |
              length | angle | time | frequency | flex | <dimension-unit>

The <dimension-unit> production matches a literal "%" character (that is, a <delim-token> with a value of "%") or an ident whose value is any of the CSS units for <length>, <angle>, <time>, <frequency>, or <flex> values (such as px or ms).

The arguments of attr() are:

<attr-name>

Gives the name of the attribute being referenced, similar to <wq-name> (from [SELECTORS-3]) but without the possibility of a wildcard prefix.

If no namespace is specified (just an identifier is given, like attr(foo)), the null namespace is implied. (This is usually what’s desired, as namespaced attributes are rare. In particular, HTML and SVG do not contain namespaced attributes.) As with attribute selectors, the case-sensitivity of <attr-name> depends on the document language.

If attr() is used in a property applied to an element, it references the attribute of the given name on that element; if applied to a pseudo-element, the attribute is looked up on the pseudo-element’s originating element.

<attr-type>

Specifies what kind of CSS value the attribute’s value will be interpreted into (the attr()’s substitution value) and what, if any, special parsing will be done to the value.

The possible values and their behavior are defined in § 6.4.1 attr() Types.

Defaults to string if omitted.

<declaration-value>

Specifies a fallback value for the attr(), which will be substituted instead of the attribute’s value if the attribute is missing or fails to parse as the specified type.

If the <attr-type> argument is string, defaults to the empty string if omitted; otherwise, defaults to the guaranteed-invalid value if omitted.

If a property contains one or more attr() 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 attr() functions have been substituted.

6.4.1. attr() Types

The behavior of the attr() function depends partially on the value of the <attr-type> argument:

string

The substitution value is a CSS string, whose value is the literal value of the attribute. (No CSS parsing or "cleanup" of the value is performed.)

No value triggers fallback.

ident

The substitution value is a CSS <custom-ident>, whose value is the literal value of the attribute, with leading and trailing ASCII whitespace stripped. (No CSS parsing of the value is performed.)

If the attribute value, after trimming, is the empty string, there is instead no substitution value.

If the <custom-ident>’s value is a CSS-wide keyword or default, there is instead no substitution value.

color

Parse a component value from the attribute’s value. If the result is a <hex-color> or a named color ident, the substitution value is that result as a <color>.

Otherwise there is no substitution value.

number

Parse a component value from the attribute’s value. If the result is a <number-token>, the result is the substitution value.

Otherwise, there is no substitution value.

percentage

Parse a component value from the attribute’s value. If the result is a <percentage-token>, the result is the substitution value.

Otherwise, there is no substitution value.

length

angle

time

frequency

flex

Parse a component value from the attribute’s value. If the result is a <dimension-token> whose unit matches the given type, the result is the substitution value.

Otherwise, there is no substitution value.

<dimension-unit>

Parse a component value from the attribute’s value. If the result is a <number-token>, the substitution value is a dimension with the result’s value, and the given unit.

Otherwise, there is no substitution value.

Do we want to allow math functions as attr values for all the numeric types? And color functions for "color"? I think we do, but I’d have to check the contents to make sure they don’t contain further reference functions; foo="rgb(var(--red), 0, 0)" needs to be illegal for attr(foo color).

<stock>
  <wood length="12"/>
  <wood length="5"/>
  <metal length="19"/>
  <wood length="4"/>
</stock>

stock::before {
  display: block;
  content: "To scale, the lengths of materials in stock are:";
}
stock > * {
  display: block;
  width: attr(length em, 0px);
  height: 1em;
  border: solid thin;
  margin: 0.5em;
}
wood {
  background: orange url(wood.png);
}
metal {
  background: silver url(metal.png);
}

6.4.2. attr() Substitution

attr() and var() substitute at the same time, so I should probably rewrite substitute a var() to be more generally about "substitute a reference" and just use that for both of these functions.

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

To substitute an attr():

1. If the attr() function has a substitution value, replace the attr() function by the substitution value.

2. Otherwise, if the attr() function has a fallback value as its last argument, replace the attr() function by the fallback value. If there are any var() or attr() references in the fallback, substitute them as well.

3. Otherwise, the property containing the attr() function is invalid at computed-value time.

6.4.3. Security

An attr() function can reference attributes that were never intended by the page to be used for styling, and might contain sensitive information (for example, a security token used by scripts on the page).

In general, this is fine. It is difficult to use attr() to extract information from a page and send it to a hostile party, in most circumstances. The exception to this is URLs. If a URL can be constructed with the value of an arbitrary attribute, purely from CSS, it can easily send any information stored in attributes to a hostile party, if 3rd-party CSS is allowed at all.

For this reason, attr() does not have a url type. Additionally, attr() is not allowed to be used in any <url> value, whether directly or indirectly. Doing so makes the property it’s used in invalid.

Note: Implementing this restriction requires tracking a dirty bit on values constructed from attr() values, since they can be fully resolved into a string via registered custom properties, so you can’t rely on just examining the value expression. Note that non-string types can even trigger this, via functions like string() that can stringify other types of values: --foo: attr(foo number); background-image: src(string(var(--foo))) needs to be invalid as well.

7. Generating Random Values

It is often useful to incorporate some degree of "randomness" to a design, either to make repeated elements on a page feel less static and identical, or just to add a bit of "flair" to a page without being distracting.

The random() and random-item() functions (the random functions) allow authors to incorporate randomness into their page, while keeping this randomness predictable from a design perspective, letting authors decide whether a random value should be reused in several places or be unique between instances.

The exact random-number generation method is UA-defined. It should be the case that two distinct random values have no easily-detectable correlation, but this specification intentionally does not specify what that means in terms of cryptographic strength. Authors must not rely on random functions for any purposes that depend on quality cryptography.

7.1. Generating a Random Numeric Value: the random() function

The random() function is a math function that represents a random value between a minimum and maximum value, drawn from a uniform distribution, optionally limiting the possible values to a step between those limits:

<random()> = random( <random-caching-options>? , <calc-sum>, <calc-sum>, [by <calc-sum>]? )

<random-caching-options> = <dashed-ident> || per-element

Its arguments are:

<random-caching-options>

The optional <random-caching-options> provides some control over whether a given random() function resolves similarly or differently to other random()s on the page. See § 7.3 Generating/Caching Random Values: the <random-caching-options> value for details.

By default, random() resolves to a single value, shared by all elements using that style, and two random() functions with identical arguments will resolve to the same random value.

Providing a <dashed-ident> does nothing, but can make the argument lists distinct between two or more otherwise-identical random() functions, so they’ll generate distinct values.

The per-element keyword causes the random() function to generate a different value on each element the function is applied to, rather than resolving to a single value per usage in the stylesheet.

<calc-sum>, <calc-sum>

The two required calculations specify the minimum and maximum value the function can resolve to. Both limits are inclusive (the result can be the min or the max).

If the maximum value is less than the minimum value, it behaves as if it’s equal to the minimum value.

For example, random(100px, 300px) will resolve to a random <length> between 100px and 300px: it might be 100px, 300px, or any value between them like 234.5px.

by <calc-sum>

The final optional argument specifies a step value: the values the function can resolve to are further restricted to the form min + (N * step), where N is a non-negative integer chosen uniformly randomly from the possible values that result in an in-range value.

For example, random(100px, 300px, by 50px) can only resolve to 100px, 150px, 200px, 250px, or 300px; it will never return a value like 120px.

While the minimum value is always a possible result, the maximum value isn’t always, if it’s not also a multiple of the step from the minimum. For example, in random(100px, 300px, by 30px), the largest possible value it can resolve to is 280px, 6 steps from the minimum value.

Note that rounding issues might have an effect here: in random(100px, 200px, by 100px / 3) you’ll definitely get three possible values (100px, and approximately 133.33px and 166.67px), but whether 200px is possible depends on rounding precision. To be safe, you can put the maximum value slightly above where you expect the final step to land, like random(100px, 201px, by 100px / 3).

As explained in the definition of round(), CSS has no "natural" precision for values, but the step value can be used to assign one.

For example, random(100px, 500px, by 1px) restricts it to resolving only to whole px values; random(1, 10, by 1) is restricted to resolving only to integers; etc.

Note: The definition of the step does not allow for naively generating a random value in the range and then rounding it to the nearest step value, as that can result in the values not appearing with the same weights. For example, random(100px, 200px, by 50px) has to generate the three possible values each with a 1/3 chance; a naive rounding-based method will instead incorrectly generate 150px twice as often as the boundary values.

All of the calculation arguments can resolve to any <number>, <dimension>, or <percentage>, but must have the same type, or else the function is invalid; the result will have the same type as the arguments.

A random() function can be simplified as soon as its argument calculations can be simplified to numeric values.

Note: This means that random() is usually resolved by computed value time, and thus will inherit as a static numeric value. However, if the argument calculations aren’t resolved until used value time (such as if they include <percentage> values that require layout information to resolve), inheritance will transfer the random() function itself. (This is no different, however, to the behavior of the <percentage>s themselves, which would inherit as <percentage>s and thus might resolve to different values on the child elements.)

At least in theory it should be fine to use random() in non-property contexts, so long as per-element isn’t specified; it’s well-defined what happens with @media (max-width: random(100px, 500px)) {...}, for example. I suspect we want to disallow it, tho?

7.1.1. Argument Ranges

In random(A, B, by C), if A or B is infinite, the result is NaN. If C is infinite, the result is A.

(If C is zero or negative, the result is A, but that falls out of the standard definition.)

Note: As usual for math functions, if any argument calculation is NaN, the result is NaN.

7.2. Picking a Random Item From a List: the random-item() function

The random-item() function resolves to a random item from among its list of items.

<random-item()> = random-item( <random-caching-options> , [ <declaration-value>? ]# )

The required <random-caching-options> is interpreted identically to random(). (See § 7.3 Generating/Caching Random Values: the <random-caching-options> value for details.)

Note: The <random-caching-options> argument is required in random-item(), but optional in random(), both for parsing reasons (it’s impossible to tell whether random-item(--foo, --bar, --baz) has three <declaration-value> arguments or two and a <random-caching-options> argument), and because accidentally associating the random generation of random-item() functions together is much easier to do accidentally, since only the number of arguments is used to distinguish instances.

The remaining arguments are arbitrary sequences of CSS values. The random-item() function resolves to one of these sequences, chosen uniformly at random.

The random-item() function is an arbitrary substitution function, like var().

Define arbitrary substitution function, probably over in Variables, since we have several upcoming functions leaning on this functionality.

Since random-item() is var()-like, we probably want to restrict it to only be usable in properties. (This is likely something we want to apply to all such functions.) Tho random() is a fundamentally different kind of value, we probably want to restrict it as well, for thematic consistency.

7.3. Generating/Caching Random Values: the <random-caching-options> value

In a programming language like JavaScript, there’s a clear temporal ordering to code, so you can tell exactly when something like a call to Math.random() is evaluated. You can also store the results in a variable, making it clear when you’re reusing a single random value in multiple places, versus using a distinct random value in each location.

CSS, on the other hand, is a declarative language (code is not "executed" in any particular order, nor is there any control over how many times something is "executed"); it makes it very easy to apply identical styles to multiple elements but difficult to specify distinct values for each of them (making it unclear whether a property using random() is meant to resolve to the same value on each element it’s applied to or to distinct values on each); and it has very limited "variable" functionality (making it difficult to intentionally reuse a particular randomly-generated value in several places).

To resolve these issues, the random() and random-item() functions are defined to generate random values under the following caching semantics:

• Each instance of random() or random-item() in a stylesheet specifies a random-caching key. Two instances of either function must resolve to identical values if their random-caching keys are identical; they must resolve to distinct values if they’re different.

  ("Distinct" here means generated by a fresh random operation; this might coincidentally result in the same value as another random operation.)

• For random(), the random-caching key is a tuple of:

  1. The used value of the minimum calculation.

  2. The used value of the maximum calculation.

  3. The used value of the step calculation, if present, or null otherwise.

  4. The <dashed-ident> part of the <random-caching-options>, if present, or null otherwise.

  5. If per-element is specified in the <random-caching-options>, a unique value per element or pseudo-element the function appears in.

• For random-item(), the random-caching key is a tuple of:

  1. The number of arguments to the function.

  2. The <dashed-ident> part of the <random-caching-options>, if present, or null otherwise.

  3. If per-element is specified in the <random-caching-options>, a unique value per element or pseudo-element the function appears in.

The "unique value per element or pseudo-element" must have the same lifetime as a JavaScript reference to the element (or to the originating element + sufficient additional info to uniquely identify the pseudo-element). Elements in separate documents (including across refreshes of the same page, which produces distinct documents with distinct elements) should have distinct unique values. (This is not strictly required, to allow for pseudo-random generation of these values, but uniqueness should be likely enough that authors cannot depend on elements having the same values across documents.)

Additionally, the random generation method should generate distinct values for the same operation when invoked on different documents (including refreshes of the same page).

.random-square {
  width: random(100px, 500px);
  height: random(100px, 500px);
}

.random-rect {
  width: random(100px, 500px);
  height: random(--x, 100px, 500px);
}

.random-rect-2 {
  width: random(100px, 500px);
  height: random(100px, 500px, by 50px);
}

.random-square-2 {
  font-size: 16px;
  width: random(160px, 320px);
  height: random(10em, 20em);
}

.foo { width: random(100px, 500px); }

.foo { width: random(per-element, 100px, 500px); }

.random-squares {
  width: random(per-element, 100px, 500px);
  height: random(per-element, 100px, 500px);
}

.foo {
  --size: random(per-element, 100px, 500px);
  width: var(--size);
  height: var(--size);
}

8. Tree Counting Functions: the sibling-count() and sibling-index() notations

The sibling-count() functional notation represents, as an <integer>, the total number of child elements in the parent of the element on which the notation is used.

The sibling-index() functional notation represents, as an <integer>, the index of the element on which the notation is used among the children of its parent. Like :nth-child(), sibling-index() is 1-indexed.

Note: The counter() function can provide similar abilities as sibling-index(), but returns a <string> rather than an <integer>.

When used on a pseudo-element, these both resolve as if specified on its ultimate originating element.

Note: Like the rest of CSS (other than selectors), sibling-count() and sibling-index() operate on the flat tree.

Note: These functions may, in the future, be extended to accept an of <selector> argument, similar to :nth-child(), to filter on a subset of the children.

9. Calculating With Intrinsic Sizes: the calc-size() function

When transitioning between two definite sizes, or slightly adjusting an existing definite size, calc() works great: halfway between 100% and 20px is calc(50% + 10px), 20% with a margin of 15px on either side is calc(20% + 15px * 2), etc.

But these operations are no longer possible if the size you want to adjust or transition to/from is an intrinsic size, for both practical and backward-compatibility reasons. The calc-size() function allows math to be performed on intrinsic sizes in a safe, well-defined way.

<calc-size()> = calc-size( <calc-size-basis>, <calc-sum> )

<calc-size-basis> = [ <intrinsic-size-keyword> | <calc-size()> | any | <calc-sum> ]

The <intrinsic-size-keyword> production matches any intrinsic size keywords allowed in the context. For example, in width, it matches auto, min-content, stretch, etc.

The first argument given is the calc-size basis, and the second is the calc-size calculation. For either argument, if a <calc-sum> is given, its type must match <length-percentage>, and it must resolve to a <length>.

Within the calc-size calculation, if the calc-size basis is not any, the keyword size is allowed. This keyword is a <length>, and resolves at used value time.

calc-size() represents an intrinsic size. It is specifically not a <length>; any place that wants to accept a calc-size() must explicitly include it in its grammar.

9.1. Simplifying calc-size()

Similar to math functions, at both specified value and computed value times the calc-size calculation (and the calc-size basis, if it’s a <calc-sum>) are simplified to the extent possible, as defined in CSS Values 4 § 10.10.1 Simplification.

9.2. Resolving calc-size()

A calc-size() is treated, in all respects, as if it were its calc-size basis (with any acting as an unspecified definite size).

When actually performing layout calculations, however, the size represented by its calc-size basis is modified to be the value of its calc-size calculation, with the size keyword evaluating to the calc-size basis’s original size.

(If the calc-size basis is any, the calc-size() is a definite length, equal to its calc-size calculation.)

When evaluating the calc-size calculation, if percentages are not definite in the given context, they resolve to 0px. Otherwise, they resolve as normal.

(A percentage in the calc-size basis is treated differently; simplification moves the percentage into the calc-size calculation and replaces it with size references. The calc-size basis then becomes 100%, behaving as whatever 100% would normally do in that context, including possibly making a property behave as auto, etc.)

9.3. Interpolating calc-size()

Two calc-size() functions can be interpolated if (after being canonicalized for interpolation):

Either function returned failure from being canonicalized for interpolation

The values cannot be interpolated.

Both calc-size basises are identical

The result’s calc-size basis is the that basis value.

Either calc-size basis is any

The result’s calc-size basis is the non-any basis.

The result’s calc-size calculation is the interpolation of the two input calc-size calculations.

Note: These interpolation restrictions ensure that a calc-size() doesn’t try to act in two different ways at once; there are some cases where a min-content and max-content would produce different layout behaviors, for example, so the calc-size() has to masquerade as one or the other. This, unfortunately, means you can’t transition between keywords, like going from auto to min-content.

Some calc-size() values can also be interpolated with a <length-percentage> or an <intrinsic-size-keyword>. To determine whether the values can interpolate and what the interpolation behavior is, treat the non-calc-size() value as calc-size(any, value ) if the value is a <calc-sum> or as calc-size( value , size) otherwise, and apply the rules above.

details {
  transition: height 1s;
}
details::details-content {
  display: block;
}
details[open]::details-content {
  height: auto;
}
details:not([open])::details-content {
  height: calc-size(any, 0px);
}

Note: calc-size() is designed such that transitioning to/from calc-size(any, definite length) will always work smoothly, regardless of how the other side of the transition is specified.

Note: This "upgrade a plain value into a calc-size()" behavior puts <length-percentage> values into the calc-size calculation. This allows values with percentages to interpolate with intrinsic size keywords, but does mean that when a percentage isn’t definite, it’ll resolve to zero. If you want to resolve to the actual size the percentage would make the element, explicitly write a calc-size() with the value in its calc-size basis, like calc-size(50%, size).

9.4. Interpolating sizing keywords: the interpolate-size property

Note: If we had a time machine, this property wouldn’t need to exist. It exists because many existing style sheets assume that intrinsic sizing keywords (such as auto, min-content, etc.) cannot animate. Therefore this property exists to allow style sheets to choose to get the expected behavior. Specifying interpolate-size: allow-keywords on the root element chooses the new behavior for the entire page. We suggest doing this whenever compatibility isn’t an issue.

numeric-only

An <intrinsic-size-keyword> cannot be interpolated.

allow-keywords

Two values can be interpolated if one of them is an <intrinsic-size-keyword> and the other is a <length-percentage>. This is done by treating the <intrinsic-size-keyword> keyword as though it is calc-size(keyword, size) and applying the rules in § 9.3 Interpolating calc-size(). In other cases, an <intrinsic-size-keyword> still cannot be interpolated.

The value of interpolate-size that matters is the computed value on the element at the time the animation might start. For CSS transitions, this means the value in the after-change style. An animation is not stopped or started later because interpolate-size changes.

Acknowledgments

Firstly, the editors would like to thank all of the contributors to the previous level of this module.

Secondly, we would like to acknowledge L. David Baron and Mike Bremford for their comments and suggestions, which have improved Level 5.

Changes

Additions Since Level 4

Additions since CSS Values and Units Level 4:

• Added the "comma-upgrading" rules for functional notations.

• Defined several <url-modifier>s for <url> functions.

• Added the *-progress() family of functions, to represent interpolation progress between two values.

• Added the *-mix() family of functions, to represent actually interpolating between two values.

• Added first-valid(), to allow CSS’s forward-compatible parsing behavior (drop invalid things, go with what’s left) to be used with custom properties and other contexts where validity isn’t known until after parsing.

• Added the toggle() and attr() functions.

• Added the random() and random-item() functions.

• Added the sibling-count() and sibling-index() functions.

• Added the calc-size() function, and the related interpolate-size property.

Security Considerations

This specification allows CSS <url> values to have various aspects of their request modified. Although this is new to CSS, every ability is already present in img or link, as well as via JavaScript.

The attr() function allows HTML attribute values to be used in CSS values, potentially exposing sensitive information that was previously not accessible via CSS. See § 6.4.3 Security.

Privacy Considerations

This specification defines units that expose the user’s screen size and default font size, but both are trivially observable from JS, so they do not constitute a new privacy risk. Similarly the media-progress() notation exposes information about the user’s environment and preferences that are already observiable via media queries.

The attr() function allows HTML attribute values to be used in CSS values, potentially exposing sensitive information that was previously not accessible via CSS. See § 6.4.3 Security.