CSS Values and Units Module Level 4

1. Introduction

The value definition field of each CSS property can contain keywords, data types (which appear between < and >), and information on how they can be combined. Generic data types (<length> being the most widely used) that can be used by many properties are described in this specification, while more specific data types (e.g., <spacing-limit>) are described in the corresponding modules.

1.1. Module Interactions

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

2. Value Definition Syntax

The value definition syntax described here is used to define the set of valid values for CSS properties (and the valid syntax of many other parts of CSS). A value so described can have one or more components.

2.1. Component Value Types

Component value types are designated in several ways:

1. Keyword values (such as auto, disc, etc.), which appear literally, without quotes (e.g. auto).

2. Basic data types, which appear between < and > (e.g., <length>, <percentage>, etc.). For numeric data types, this type notation can annotate any range restrictions using the bracketed range notation described below.

3. Property value ranges, which represent the same pattern of values as a property bearing the same name. These are written as the property name, surrounded by single quotes, between < and >, e.g., <'border-width'>, <'background-attachment'>, etc.

   These types do not include CSS-wide keywords such as inherit, and also do not include any top-level comma-separated-list multiplier (i.e. if a property named pairing is defined as [ <custom-ident> <integer>? ]#, then <'pairing'> is equivalent to [ <custom-ident> <integer>? ], not [ <custom-ident> <integer>? ]#).

4. Functional notations and their arguments. These are written as the function’s name, followed by an empty parentheses pair, between < and >, e.g. <calc()>, and references the correspondingly-named functional notation.

5. Other non-terminals. These are written as the name of the non-terminal between < and >, as in <spacing-limit>. Notice the distinction between <border-width> and <'border-width'>: the latter represents the grammar of the border-width property, the former requires an explicit expansion elsewhere. The definition of a non-terminal is typically located near its first appearance in the specification.

Some property value definitions also include the slash (/), the comma (,), and/or parentheses as literals. These represent their corresponding tokens. Other non-keyword literal characters that may appear in a component value, such as “+”, must be written enclosed in single quotes.

Commas specified in the grammar are implicitly omissible in some circumstances, when used to separate optional terms in the grammar. Within a top-level list in a property or other CSS value, or a function’s argument list, a comma specified in the grammar must be omitted if:

• all items preceding the comma have been omitted
• all items following the comma have been omitted
• multiple commas would be adjacent (ignoring white space/comments), due to the items between the commas being omitted.

example( first? , second? , third? )

All CSS properties also accept the CSS-wide keyword values as the sole component of their property value. For readability these are not listed explicitly in the property value syntax definitions. For example, the full value definition of border-color under CSS Cascading and Inheritance Level 3 is <color>{1,4} | inherit | initial | unset (even though it is listed as <color>{1,4}).

Note: This implies that, in general, combining these keywords with other component values in the same declaration results in an invalid declaration. For example, background: url(corner.png) no-repeat, inherit; is invalid.

2.2. Component Value Combinators

Component values can be arranged into property values as follows:

• Juxtaposing components means that all of them must occur, in the given order.
• A double ampersand (&&) separates two or more components, all of which must occur, in any order.
• A double bar (||) separates two or more options: one or more of them must occur, in any order.
• A bar (|) separates two or more alternatives: exactly one of them must occur.
• Brackets ([ ]) are for grouping.

Juxtaposition is stronger than the double ampersand, the double ampersand is stronger than the double bar, and the double bar is stronger than the bar. Thus, the following lines are equivalent:

  a b   |   c ||   d &&   e f
[ a b ] | [ c || [ d && [ e f ]]]

For reorderable combinators (||, &&), ordering of the grammar does not matter: components in the same grouping may be interleaved in any order. Thus, the following lines are equivalent:

a || b || c
b || a || c

Note: Combinators are not associative, so grouping is significant. For example, a || b || c and a || [ b || c ] are distinct grammars: the first allows a value like b a c, but the second does not.

2.3. Component Value Multipliers

Every type, keyword, or bracketed group may be followed by one of the following modifiers:

• An asterisk (*) indicates that the preceding type, word, or group occurs zero or more times.
• A plus (+) indicates that the preceding type, word, or group occurs one or more times.
• A question mark (?) indicates that the preceding type, word, or group is optional (occurs zero or one times).
• A single number in curly braces ({A}) indicates that the preceding type, word, or group occurs A times.
• A comma-separated pair of numbers in curly braces ({A,B}) indicates that the preceding type, word, or group occurs at least A and at most B times. The B may be omitted ({A,}) to indicate that there must be at least A repetitions, with no upper bound on the number of repetitions.
• A hash mark (#) indicates that the preceding type, word, or group occurs one or more times, separated by comma tokens (which may optionally be surrounded by white space and/or comments). It may optionally be followed by the curly brace forms, above, to indicate precisely how many times the repetition occurs, like <length>#{1,4}.
• An exclamation point (!) after a group indicates that the group is required and must produce at least one value; even if the grammar of the items within the group would otherwise allow the entire contents to be omitted, at least one component value must not be omitted.

The + and # multipliers may be stacked as +#; similarly, the # and ? multipliers may be stacked as #?. These stacks each represent the later multiplier applied to the result of the earlier multiplier. (These same stacks can be represented using grouping, but in complex grammars this can push the number of brackets beyond readability.)

For repeated component values (indicated by *, +, or #), UAs must support at least 20 repetitions of the component. If a property value contains more than the supported number of repetitions, the declaration must be ignored as if it were invalid.

2.4. Combinator and Multiplier Patterns

There are a small set of common ways to combine multiple independent component values in particular numbers and orders. In particular, it’s common to want to express that, from a set of component value, the author must select zero or more, one or more, or all of them, and in either the order specified in the grammar or in any order.

All of these can be easily expressed using simple patterns of combinators and multipliers:

Note that all of the "any order" possibilities are expressed using combinators, while the "in order" possibilities are all variants on juxtaposition.

2.5. Component Values and White Space

Unless otherwise specified, white space and/or comments may appear before, after, and/or between components combined using the above combinators and multipliers.

Note: In many cases, spaces will in fact be required between components in order to distinguish them from each other. For example, the value 1em2em would be parsed as a single <dimension-token> with the number 1 and the identifier em2em, which is an invalid unit. In this case, a space would be required before the 2 to get this parsed as the two lengths 1em and 2em.

2.6. Functional Notation Definitions

The syntax of a functional notation is defined as a sequence of:

1. The function’s name written as an identifier followed by an open parenthesis (such as example(), or the <function-token> production to indicate a function with an arbitrary name.

2. The function’s arguments, if any, expressed using the value definition syntax.

3. A literal closing parenthesis.

The function’s arguments are considered implicitly grouped, as if surrounded by brackets ([ ... ]).

example( <length> , <length> )

<pseudo-class-selector> = ':' <ident-token> | ':' <function-token> <any-value> ')'

2.7. Property Value Examples

Below are some examples of properties with their corresponding value definition fields

3. Combining Values: Interpolation, Addition, and Accumulation

Some procedures, for example transitions and animations, combine two CSS property values. The following combining operations—on the two computed values V_a and V_B yielding the computed value V_result—are defined:

interpolation

Given two property values V_a and V_B, produces an intermediate value V_result at a distance of p along the interval between V_a and V_B such that p = 0 produces V_a and p = 1 produces V_B.

The range of p is (−∞, ∞) due to the effect of timing functions. As a result, this procedure must also define extrapolation behavior for p outside [0, 1].

addition

Given two property values V_a and V_B, returns the sum of the two properties, V_result. For addition that is not commutative (for example, matrix multiplication) V_a represents the first term of the operation and V_B represents the second.

Note: While addition can often be expressed in terms of the same weighted sum function used to define interpolation, this is not always the case. For example, interpolation of transform matrices involves decomposing and interpolating the matrix components whilst addition relies on matrix multiplication.

accumulation

Given two property values V_a and V_B, returns the result, V_result, of combining the two operands such that V_B is treated as a delta from V_a. For accumulation that is not commutative (for example, accumulation of mismatched transform lists) V_a represents the first term of the operation and V_B represents the second.

Note: For many types of animation such as numbers or lengths, accumulation is defined to be identical to addition.

A common case where the definitions differ is for list-based types where addition may be defined as appending to a list whilst accumulation may be defined as component-based addition. For example, the filter list values blur(2) and blur(3), when added together would produce blur(2) blur(3), but when accumulated would produce blur(5).

These operations are only defined on computed values. (As a result, it is not necessary to define, for example, how to add a <length> value of 15pt with 5em since such values will be resolved to their canonical unit before being passed to any of the above procedures.)

If a value type does not define a specific procedure for addition or is defined as not additive, its addition operation is simply V_result = V_a.

If a value type does not define a specific procedure for accumulation, its accumulation operation is identical to addition.

3.1. Representing Interpolated Values: the mix() notation

Interpolation of two values can be represented by the mix() functional notation, whose syntax is defined as follows:

<mix()> = mix( <percentage> ';' <declaration-value>? ';' <declaration-value>? )

<percentage>

Represents the interpolation progress between the two values.

<declaration-value>

The first <declaration-value> represents the value at the “start” (0%) of the interpolation range, interpreted as a value for the property this function is used in.

The second <declaration-value> represents the value at the “end” (100%) of the interpolation range, interpreted the same.

Either value can be omitted, to refer to an empty value (this is valid in custom properties, such as --foo:;). The separating ; cannot be omitted; for example, a mix() with an empty starting value would be written like mix(50%;;bar).

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

If the two <s in mix() are interpolable (without using mix() itself) as values for the property in which it is specified, the computed value of mix() is the result of interpolating these two values to the progress given by the <percentage>. Otherwise, the computed value of mix() is the mix() functional notation itself with its <percentage> computed and both <declaration-value>s computed as values for this property.

A mix() notation is invalid if it is not the sole value of the property; if either of its <declaration-value>s are invalid when specified as the entire value for the property the function is used in; or if the property using it is not animatable.

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

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

/* 'display' is not animatable */
display: mix(0% ; inline ; block);

Progress values below 0% and above 100% are valid; they represent interpolation beyond the range represented by the start and end values.

3.2. Range Checking

Interpolation can result in a value outside the valid range for a property, even if all of the inputs to interpolation are valid; this especially happens when p is outside the [0, 1] range, but some easing functions can cause this to occur even within that range. If the final result after interpolation, addition, and accumulation is out-of-range for the target context the value is being used in, it does not cause the declaration to be invalid. Instead, the value must be clamped to the range allowed in the target context, exactly the same as math functions (see § 10.12 Range Checking).

Note: Even if interpolation results in an out-of-range value, addition/accumulation might "correct" the result and bring it back into range. Thus, clamping is only applied to the final result of applying all interpolation-related operations.

4. Textual Data Types

The textual data types include various keywords and identifiers as well as strings (<string>) and URLs (<url>). Aside from the casing of pre-defined keywords or as explicitly defined for a given property, no normalization is performed, not even Unicode normalization: the specified and computed value of a property are exactly the provided Unicode values after parsing (which includes character set conversion and escaping). [UNICODE] [CSS-SYNTAX-3]

CSS identifiers, generically denoted by <ident>, consist of a sequence of characters conforming to the <ident-token> grammar. [CSS-SYNTAX-3] Identifiers cannot be quoted; otherwise they would be interpreted as strings. CSS properties accept two classes of identifiers: pre-defined keywords and author-defined identifiers.

Note: The <ident> production is not meant for property value definitions—<custom-ident> should be used instead. It is provided as a convenience for defining other syntactic constructs.

All textual data types interpolate as discrete and are not additive.

4.1. Pre-defined Keywords

In the value definition fields, keywords with a pre-defined meaning appear literally. Keywords are identifiers and are interpreted ASCII case-insensitively (i.e., [a-z] and [A-Z] are equivalent).

Value: collapse | separate

table { border-collapse: separate }

4.1.1. CSS-wide keywords: initial, inherit and unset

As defined above, all properties accept the CSS-wide keywords, which represent value computations common to all CSS properties. These keywords are normatively defined in the CSS Cascading and Inheritance Module.

Other CSS specifications can define additional CSS-wide keywords.

4.2. Unprefixed Author-defined Identifiers: the <custom-ident> type

Some properties accept arbitrary author-defined identifiers as a component value. This generic data type is denoted by <custom-ident>, and represents any valid CSS identifier that would not be misinterpreted as a pre-defined keyword in that property’s value definition. Such identifiers are fully case-sensitive (meaning they’re compared using the "identical to" operation), even in the ASCII range (e.g. example and EXAMPLE are two different, unrelated user-defined identifiers).

The CSS-wide keywords are not valid <custom-ident>s. The default keyword is reserved and is also not a valid <custom-ident>. Specifications using <custom-ident> must specify clearly what other keywords are excluded from <custom-ident>, if any—for example by saying that any pre-defined keywords in that property’s value definition are excluded. Excluded keywords are excluded in all ASCII case permutations.

When parsing positionally-ambiguous keywords in a property value, a <custom-ident> production can only claim the keyword if no other unfulfilled production can claim it.

Note: When designing grammars with <custom-ident>, the <custom-ident> should always be “positionally unambiguous”, so that it’s impossible to conflict with any keyword values in the property. Such conflicts can alternatively be avoided by using <dashed-ident>.

4.3. Prefixed Author-defined Identifiers: the <dashed-ident> type

Some contexts accept both author-defined identifiers and CSS-defined identifiers. If not handled carefully, this can result in difficulties adding new CSS-defined values; UAs have to study existing usage and gamble that there are sufficiently few author-defined identifiers in use matching the new CSS-defined one, so giving the new value a special CSS-defined meaning won’t break existing pages.

While there are many legacy cases in CSS that mix these two values spaces in exactly this fraught way, the <dashed-ident> type is meant to be an easy way to distinguish author-defined identifiers from CSS-defined identifiers.

The <dashed-ident> production is a <custom-ident>, with all the case-sensitivity that implies, with the additional restriction that it must start with two dashes (U+002D HYPHEN-MINUS).

<dashed-ident>s are reserved solely for use as author-defined names. CSS will never define a <dashed-ident> for its own use.

.foo {
  --fg-color: blue;
}

@color-profile --foo { src: url(https://example.com/foo.icc); }
.foo {
  color: color(--foo 1 0 .5 / .2);
}

4.4. Quoted Strings: the <string> type

Strings are denoted by <string>. When written literally, they consist of a sequence of characters delimited by double quotes or single quotes, corresponding to the <string-token> production in the CSS Syntax Module [CSS-SYNTAX-3].

content: "this is a 'string'.";
content: "this is a \"string\".";
content: 'this is a "string".';
content: 'this is a \'string\'.'

It is possible to break strings over several lines, for aesthetic or other reasons, but in such a case the newline itself has to be escaped with a backslash (\). The newline is subsequently removed from the string. For instance, the following two selectors are exactly the same:

a[title="a not s\
o very long title"] {/*...*/}
a[title="a not so very long title"] {/*...*/}

Since a string cannot directly represent a newline, to include a newline in a string, use the escape "\A". (Hexadecimal A is the line feed character in Unicode (U+000A), but represents the generic notion of "newline" in CSS.)

4.5. Resource Locators: the <url> type

The <url> type, written with the url() and src() functions, represents a URL, which is a pointer to a resource.

The syntax of <url> is:

<url> = <url()> | <src()>

<url()> = url( <string> <url-modifier>* ) | <url-token>
<src()> = src( <string> <url-modifier>* )

body { background: url("http://www.example.com/pinkish.gif") }

For legacy reasons, a url() can be written without quotation marks around the URL itself, in which case it is specially-parsed as a <url-token> [CSS-SYNTAX-3]. Because of this special parsing, url() is only able to specify its URL literally; src() lacks this special parsing rule, and so its URL can be provided by functions, such as var().

background: url("http://www.example.com/pinkish.gif");
background: url(http://www.example.com/pinkish.gif);

background: src("http://www.example.com/pinkish.gif");
--foo: "http://www.example.com/pinkish.gif";
background: src(var(--foo));

--foo: "http://www.example.com/pinkish.gif";
background: url(var(--foo));

Some CSS contexts (such as @import) also allow a <url> to be represented by a bare <string>, without the function wrapper. In such cases the string behaves identically to a url() function containing that string.

@import url("base-theme.css");
@import "base-theme.css";

4.5.1. Relative URLs

In order to create modular style sheets that are not dependent on the absolute location of a resource, authors should use relative URLs. Relative URLs (as defined in [URL]) are resolved to full URLs using a base URL. RFC 3986, section 3, defines the normative algorithm for this process. For CSS style sheets, the base URL is that of the style sheet itself, not that of the styled source document. Style sheets embedded within a document have the base URL associated with their container.

Note: For HTML documents, the base URL is mutable.

When a <url> appears in the computed value of a property, it is resolved to an absolute URL, as described in the preceding paragraph. The computed value of a URL that the UA cannot resolve to an absolute URL is the specified value.

body { background: url("tile.png") }

http://www.example.org/style/basic.css

http://www.example.org/style/tile.png

4.5.1.1. Fragment URLs

To enable element ID references to work in CSS regardless of base URL changes or shadow DOM, <url>s have special behavior when they contain only a fragment.

If a <url>'s value starts with a U+0023 NUMBER SIGN (#) character, then the URL additionally has its local url flag set, and is a tree-scoped reference for the URL’s fragment.

When matching a <url> with the local url flag set, resolve it as a tree-scoped reference with the tree’s IDs as the associated tree-scoped names: specifically, resolve to the first element in tree order among the associated node tree's descendants with the URL’s fragment as its ID. (And, as usual for tree-scoped references, continuing up to the host’s tree if needed.) If no such element is found, the URL fails to resolve.

Possibly reference find a potential indicated element, but that is defined specifically for Documents, not ShadowRoots.

I’m just folding together "can’t find the ID" and "is a Media Fragment or other non-ID fragment", and treating both of them as a failed reference. I think this is reasonable.

Note: This means that such fragments will resolve against the contents of the current document (or whichever node tree the stylesheet lives in, if shadow DOM is involved) regardless of how such relative URLs would resolve elsewhere (ignoring, for example, base elements changing the base URL, or relative URLs in linked stylesheets resolving against the stylesheet’s URL).

<!DOCTYPE html>
<base href="http://example.com/">
...
<a href="#anchor" style="background-image: url(#image)">link</a>

When serializing a url() with the local url flag set, it must serialize as just the fragment.

4.5.2. Empty URLs

If the value of the <url> is the empty string (like url("") or url()), the url must resolve to an invalid resource (similar to what the url about:invalid does).

Its computed value is url("") or src(""), whichever was specified, and it must serialize as such.

Note: This matches the behavior of empty urls for embedded resources elsewhere in the web platform, and avoids excess traffic re-requesting the stylesheet or host document due to editing mistakes leaving the url() value empty, which are almost certain to be invalid resources for whatever the url() shows up in. Linking on the web platform does allow empty urls, so if/when CSS gains some functionality to control hyperlinks, this restriction can be relaxed in those contexts.

4.5.3. URL Modifiers

<url>s support specifying additional <url-modifier>s, which change the meaning or the interpretation of the URL somehow. A <url-modifier> is either an <ident> or a functional notation.

This specification does not define any <url-modifier>s, but other specs may do so.

Note: A <url> that is either unquoted or not wrapped in url() notation cannot accept any <url-modifier>s.

4.5.4. URL Processing Model

5. Numeric Data Types

Numeric data types are used to represent quantities, indexes, positions, and other such values. Although many syntactic variations can exist in expressing the quantity (numeric aspect) in a given numeric value, the specified and computed value do not distinguish these variations: they represent the value’s abstract quantity, not its syntactic representation.

The numeric data types include <integer>, <number>, <percentage>, and various dimensions including <length>, <angle>, <time>, <frequency>, and <resolution>.

Note: While general-purpose dimensions are defined here, some other modules define additional data types (e.g. [css-grid-1] introduces fr units) whose usage is more localized.

The precision and supported range of numeric values in CSS is explicitly undefined, and can vary based on the property or other context a value is used in. However, within the CSS specifications, infinite precision and range is assumed. When a value cannot be explicitly supported due to range/precision limitations, it must be converted to the closest value supported by the implementation, but how the implementation defines "closest" is explicitly undefined as well.

If an <angle> must be converted due to exceeding the implementation-defined range of supported values, it must be clamped to the nearest supported multiple of 360deg.

5.1. Range Restrictions and Range Definition Notation

Properties can restrict numeric values to some range. If the value is outside the allowed range, then unless otherwise specified, the declaration is invalid and must be ignored. Range restrictions can be annotated in the numeric type notation using CSS bracketed range notation—[min,max]—within the angle brackets, after the identifying keyword, indicating a closed range between (and including) min and max. For example, <integer [0,10]> indicates an integer between 0 and 10, inclusive, while <angle [0,180deg]> indicates an angle between 0deg and 180deg (expressed in any unit).

Note: CSS values generally do not allow open ranges; thus only square-bracket notation is used.

CSS theoretically supports infinite precision and infinite ranges for all value types; however in reality implementations have finite capacity. UAs should support reasonably useful ranges and precisions. Range extremes that are ideally unlimited are indicated using ∞ or −∞ as appropriate. For example, <length [0,∞]> indicates a non-negative length.

If no range is indicated, either by using the bracketed range notation or in the property description, then [−∞,∞] is assumed.

Values of −∞ or ∞ must be written without units, even if the value type uses units. Values of 0 can be written without units, even if the value type doesn’t allow “unitless zeroes” (such as <time>).

Note: At the time of writing, the bracketed range notation is new; thus in most CSS specifications any range limitations are described only in prose. (For example, “Negative values are not allowed” or “Negative values are invalid” indicate a [0,∞] range.) This does not make them any less binding.

5.2. Integers: the <integer> type

Integer values are denoted by <integer>.

When written literally, an integer is one or more decimal digits 0 through 9 and corresponds to a subset of the <number-token> production in the CSS Syntax Module [CSS-SYNTAX-3]. The first digit of an integer may be immediately preceded by - or + to indicate the integer’s sign.

Unless otherwise specified, in the CSS specifications rounding to the nearest integer requires rounding in the direction of +∞ when the fractional portion is exactly 0.5. (For example, 1.5 rounds to 2, while -1.5 rounds to -1.)

5.2.1. Computation and Combination of <integer>

Unless otherwise specified, the computed value of a specified <integer> is the specified abstract integer.

Interpolation of <integer> is defined as V_result = round((1 - p) × V_a + p × V_b); that is, interpolation happens in the real number space as for <number>s, and the result is converted to an <integer> by rounding to the nearest integer.

Addition of <integer> is defined as V_result = V_a + V_b

5.3. Real Numbers: the <number> type

Number values are denoted by <number>, and represent real numbers, possibly with a fractional component.

When written literally, a number is either an integer, or zero or more decimal digits followed by a dot (.) followed by one or more decimal digits; optionally, it can be concluded by the letter “e” or “E” followed by an integer indicating the base-ten exponent in scientific notation. It corresponds to the <number-token> production in the CSS Syntax Module [CSS-SYNTAX-3]. As with integers, the first character of a number may be immediately preceded by - or + to indicate the number’s sign.

The value <zero> represents a literal number with the value 0. Expressions that merely evaluate to a <number> with the value 0 (for example, calc(0)) do not match <zero>; only literal <number-token>s do.

5.3.1. Computation and Combination of <number>

Unless otherwise specified, the computed value of a specified <number> is the specified abstract number.

Interpolation of <number> is defined as V_result = (1 - p) × V_a + p × V_b

Addition of <number> is defined as V_result = V_a + V_b

5.4. Numbers with Units: dimension values

The general term dimension refers to a number with a unit attached to it; and is denoted by <dimension>.

When written literally, a dimension is a number immediately followed by a unit identifier, which is an identifier. It corresponds to the <dimension-token> production in the CSS Syntax Module [CSS-SYNTAX-3]. Like keywords, unit identifiers are ASCII case-insensitive.

CSS uses <dimension>s to specify distances (<length>), durations (<time>), frequencies (<frequency>), resolutions (<resolution>), and other quantities.

5.4.1. Compatible Units

When serializing computed values [CSSOM], compatible units (those related by a static multiplicative factor, like the 96:1 factor between px and in, or the computed font-size factor between em and px) are converted into a single canonical unit. Each group of compatible units defines which among them is the canonical unit that will be used for serialization.

When serializing resolved values that are used values, all value types (percentages, numbers, keywords, etc.) that represent lengths are considered compatible with lengths. Likewise any future API that returns used values must consider any values that represent distances/durations/frequencies/etc. as compatible with the relevant class of dimensions, and canonicalize accordingly.

5.4.2. Combination of Dimensions

Interpolation of compatible dimensions (for example, two <length> values) is defined as V_result = (1 - p) × V_a + p × V_b

Addition of compatible dimensions is defined as V_result = V_a + V_b

5.5. Percentages: the <percentage> type

Percentage values are denoted by <percentage>, and indicates a value that is some fraction of another reference value.

When written literally, a percentage consists of a number immediately followed by a percent sign %. It corresponds to the <percentage-token> production in the CSS Syntax Module [CSS-SYNTAX-3].

Percentage values are always relative to another quantity, for example a length. Each property that allows percentages also defines the quantity to which the percentage refers. This quantity can be a value of another property for the same element, the value of a property for an ancestor element, a measurement of the formatting context (e.g., the width of a containing block), or something else.

5.5.1. Computation and Combination of <percentage>

Unless otherwise specified (such as in font-size, which computes its <percentage> values to <length>), the computed value of a percentage is the specified percentage.

Interpolation of <percentage> is defined as V_result = (1 - p) × V_a + p × V_b

Addition of <percentage> is defined as V_result = V_a + V_b

5.6. Mixing Percentages and Dimensions

In cases where a <percentage> can represent the same quantity as a dimension in the same component value position, and can therefore be combined with them in a calc() expression, the following convenience notations may be used in the property grammar:

<length-percentage>

Equivalent to [ <length> | <percentage> ], where the <percentage> will resolve to a <length>.

<frequency-percentage>

Equivalent to [ <frequency> | <percentage> ], where the <percentage> will resolve to a <frequency>.

<angle-percentage>

Equivalent to [ <angle> | <percentage> ], where the <percentage> will resolve to an <angle>.

<time-percentage>

Equivalent to [ <time> | <percentage> ], where the <percentage> will resolve to a <time>.

Note: Specifications should never alternate <percentage> in place of a dimension in a grammar unless they are compatible.

Note: More <type-percentage> productions can be added in the future as needed. A <number-percentage> will never be added, as <number> and <percentage> can’t be combined in calc().

5.6.1. Computation and Combination of Percentage and Dimension Mixes

The computed value of a percentage-dimension mix is defined as

• a computed dimension if the percentage component is zero or is defined specifically to compute to a dimension value

• a computed percentage if the dimension component is zero

• a computed calc() expression otherwise

Interpolation of percentage-dimension value combinations (e.g. <length-percentage>, <frequency-percentage>, <angle-percentage>, <time-percentage> or equivalent notations) is defined as

• equivalent to interpolation of <length> if both V_a and V_b are pure <length> values
• equivalent to interpolation of <percentage> if both V_a and V_b are pure <percentage> values
• equivalent to converting both values into a calc() expression representing the sum of the dimension type and a percentage (each possibly zero) and interpolating each component individually (as a <length>/<frequency>/<angle>/<time> and as a <percentage>, respectively)

Addition of <percentage> is defined the same as interpolation except by adding each component rather than interpolating it.

5.7. Ratios: the <ratio> type

Ratio values are denoted by <ratio>, and represent the ratio of two numeric values. It most often represents an aspect ratio, relating a width (first) to a height (second).

When written literally, a ratio has the syntax:

<ratio> = <number [0,∞]> [ / <number [0,∞]> ]?

The second <number> is optional, defaulting to 1. However, <ratio> is always serialized with both components.

The computed value of a <ratio> is the pair of numbers provided.

If either number in the <ratio> is 0 or infinite, it represents a degenerate ratio (and, generally, won’t do anything).

If two <ratio>s need to be compared, divide the first number by the second, and compare the results. For example, 3/2 is less than 2/1, because it resolves to 1.5 while the second resolves to 2. (In other words, “tall” aspect ratios are less than “wide” aspect ratios.)

5.7.1. Combination of <ratio>

The interpolation of a <ratio> is defined by converting each <ratio> to a number by dividing the first value by the second (so a ratio of 3 / 2 would become 1.5), taking the logarithm of that result (so the 1.5 would become approximately 0.176), then interpolating those values. The result during the interpolation is converted back to a <ratio> by inverting the logarithm, then interpreting the result as a <ratio> with the result as the first value and 1 as the second value.

If either <ratio> is degenerate, the values cannot be interpolated.

start  = log(5);   // ≈ 0.69897
end    = log(1.5); // ≈ 0.17609
interp = 0.69897*.5 + 0.17609*.5; // ≈ 0.43753
final  = 10^interp; // ≈ 2.73

Note: Interpolating over the logarithm of the ratio means the results are scale-independent (5 / 1 to 300 / 200 would give the same results as above), that they’re symmetrical over "wide" and "tall" variants (interpolating from 1 / 5 to 2 / 3 would give a ratio approximately equal to 1 / 2.73 at the halfway point), and that they’re symmetrical over whether the width is fixed and the height is based on the ratio or vice versa. These properties are not shared by many other possible interpolation strategies.

Note: Due to the properties of logarithms, any log can be used; the example here uses base-10 log, but if, say, the natural log and e was used, the intermediate results would be different but the final result would be the same.

Addition of <ratio>s is not possible.

6. Distance Units: the <length> type

Lengths refer to distance measurements and are denoted by <length> in the property definitions. A length is a dimension.

For zero lengths the unit identifier is optional (i.e. can be syntactically represented as the <number> 0). However, if a 0 could be parsed as either a <number> or a <length> in a property (such as line-height), it must parse as a <number>.

Properties may restrict the length value to some range. If the value is outside the allowed range, the declaration is invalid and must be ignored.

While some properties allow negative length values, this may complicate the formatting and there may be implementation-specific limits. If a negative length value is allowed but cannot be supported, it must be converted to the nearest value that can be supported.

In cases where the used length cannot be supported, user agents must approximate it in the actual value.

There are two types of length units: relative and absolute. The specified value of a length (specified length) is represented by its quantity and its unit. The computed value of a length (computed length) is the specified length resolved to an absolute length, and its unit is not distinguished: it can be represented by any absolute length unit (but will be serialized using its canonical unit, px).

While the exact supported precision of numeric values, and how they are rounded to match that precision, is generally intentionally undefined, <length>s in border-width and a few other properties are rounded in a specific fashion to ensure reasonable visual display. (This algorithm is called by individual properties explicitly.)

6.1. Relative Lengths

Relative length units specify a length relative to another length. Style sheets that use relative units can more easily scale from one output environment to another.

The relative units are:

Child elements do not inherit the relative values as specified for their parent; they inherit the computed values.

6.1.1. Font-relative Lengths: the em, rem, ex, rex, cap, rcap, ch, rch, ic, ric, lh, rlh units

The font-relative lengths refer to the font metrics either of the element on which they are used (for the local font-relative lengths) or of the root element (for the root font-relative lengths).

em

Equal to the computed value of the font-size property of the element on which it is used.

The rule:

h1 { line-height: 1.2em }

means that the line height of h1 elements will be 20% greater than the font size of h1 element. On the other hand:

h1 { font-size: 1.2em }

means that the font size of h1 elements will be 20% greater than the computed font size inherited by h1 elements.

rem

Equal to the computed value of the em unit on the root element.

ex

Equal to the used x-height of the first available font [CSS3-FONTS]. The x-height is so called because it is often equal to the height of the lowercase "x". However, an ex is defined even for fonts that do not contain an "x". The x-height of a font can be found in different ways. Some fonts contain reliable metrics for the x-height. If reliable font metrics are not available, UAs may determine the x-height from the height of a lowercase glyph. One possible heuristic is to look at how far the glyph for the lowercase "o" extends below the baseline, and subtract that value from the top of its bounding box. In the cases where it is impossible or impractical to determine the x-height, a value of 0.5em must be assumed.

rex

Equal to the value of the ex unit on the root element.

cap

Equal to the used cap-height of the first available font [CSS3-FONTS]. The cap-height is so called because it is approximately equal to the height of a capital Latin letter. However, a cap is defined even for fonts that do not contain Latin letters. The cap-height of a font can be found in different ways. Some fonts contain reliable metrics for the cap-height. If reliable font metrics are not available, UAs may determine the cap-height from the height of an uppercase glyph. One possible heuristic is to look at how far the glyph for the uppercase “O” extends below the baseline, and subtract that value from the top of its bounding box. In the cases where it is impossible or impractical to determine the cap-height, the font’s ascent must be used.

rcap

Equal to the value of the cap unit on the root element.

ch

Represents the typical advance measure of European alphanumeric characters, and measured as the used advance measure of the “0” (ZERO, U+0030) glyph in the font used to render it. (The advance measure of a glyph is its advance width or height, whichever is in the inline axis of the element.)

Note: This measurement is an approximation (and in monospace fonts, an exact measure) of a single narrow glyph’s advance measure, thus allowing measurements based on an expected glyph count.

Note: The advance measure of a glyph depends on writing-mode and text-orientation as well as font settings, text-transform, and any other properties that affect glyph selection or orientation.

In the cases where it is impossible or impractical to determine the measure of the “0” glyph, it must be assumed to be 0.5em wide by 1em tall. Thus, the ch unit falls back to 0.5em in the general case, and to 1em when it would be typeset upright (i.e. writing-mode is vertical-rl or vertical-lr and text-orientation is upright).

rch

Equal to the value of the ch unit on the root element.

ic

Represents the typical advance measure of CJK letters, and measured as the used advance measure of the “水” (CJK water ideograph, U+6C34) glyph found in the font used to render it.

Note: This measurement is a typically an exact measure (in the few fonts with proportional fullwidth glyphs, an approximation) of a single fullwidth glyph’s advance measure, thus allowing measurements based on an expected glyph count.

In the cases where it is impossible or impractical to determine the ideographic advance measure, it must be assumed to be 1em.

ric

Equal to the value of the ic unit on the root element.

lh

Equal to the computed value of the line-height property of the element on which it is used, converting normal to an absolute length by using only the metrics of the first available font.

rlh

Equal to the value of the lh unit on the root element.

Note: Setting the height of an element using either the lh or the rlh units does not enable authors to control the actual number of lines in that element. These units only enable length calculations based on the theoretical size of an ideal empty line; the size of actual lines boxes may differ based on their content. In cases where an author wants to limit the number of actual lines in an element, the max-lines property can be used instead.

When used in the value of the font-size property on the element they refer to, the local font-relative lengths resolve against the computed metrics of the parent element—or against the computed metrics corresponding to the initial values of the font and line-height properties, if the element has no parent. Likewise, when lh or rlh units are used in the value of the line-height property on the element they refer to, they resolve against the computed line-height and font metrics of the parent element—or the computed metrics corresponding to the initial values of the font and line-height properties, if the element has no parent. (The other font-relative lengths continue to resolve against the element’s own metrics when used in line-height.)

When used outside the context of an element (such as in media queries), the font-relative lengths units refer to the metrics corresponding to the initial values of the font and line-height properties. Similarly, when specified in a document with no root element, the root font-relative lengths are resolved assuming the initial values of the font and line-height properties.

Note: Font-relative units such as ch and ic can trigger font downloads, if a required font is not yet loaded.

The font-relative lengths are calculated in the absence of shaping.

Some user-agents allow users to apply additional restrictions to font sizes in a document, such as setting minimum font sizes to ensure readability. Such restrictions must be applied to the used value of the affected properties only; they must not affect the resolution of font-relative lengths used in properties. However, in other contexts (such as in media queries), to the extent that they would impact the used font metrics, such restrictions do affect the resolution of font-relative lengths.

Note: In general, respecting a user’s preferences, like minimum font sizes, is desirable; it’s useful for a media query like (min-width: 40em) to use the actual font size the document will be displayed in. However, having these preferences affect font-relative lengths in properties on an element was found to not be Web-compatible; too many pages expect these units to be exact multiples of the specified font-size, rather than the actual font-size after applying user preferences.

Some user-agents apply restrictions to the line-height values on form controls. These must have no effect on the lh and rlh units. The effect on their descendants, however, is undefined.

6.1.2. Viewport-percentage Lengths: the *vw, *vh, *vi, *vb, *vmin, *vmax units

The viewport-percentage lengths are relative to the size of the initial containing block—which is itself based on the size of either the viewport (for continuous media) or the page area (for paged media). When the height or width of the initial containing block is changed, they are scaled accordingly.

6.1.2.1. The Large, Small, and Dynamic Viewport Sizes

There are four variants of the viewport-percentage length units, corresponding to four (possibly identical) notions of the viewport size.

UA-default viewport

The UA-default viewport-percentage units (v*) are defined with respect to a UA-defined UA-default viewport size, which for any given document should be equivalent to the large viewport size, small viewport size, or some intermediary size.

Should the UA-default viewport size be required to correspond to the size of the initial containing block?

Note: Implementations that choose a size other than the large viewport size or small viewport size are encouraged to explain their choice to the CSSWG for consideration in future specification updates.

large viewport

The large viewport-percentage units (lv*) are defined with respect to the large viewport size: the viewport sized assuming any UA interfaces that are dynamically expanded and retracted to be retracted. This allows authors to size content such that it is guaranteed to fill the viewport, noting that such content might be hidden behind such interfaces when they are expanded.

The sizes of the large viewport-percentage units are fixed (and therefore stable) unless the viewport itself is resized.

For example, on phones, where screen real-estate is at a premium, browsers will often hide part or all of the title and address bar once the user starts scrolling the page. The large viewport-percentage units are sized relative to this larger everything-retracted space, so content using these units will fill the entire visible page when these UI elements are hidden. However, when these retractable elements are shown, they can obscure content that is sized or positioned using these units.

small viewport

The small viewport-percentage units (sv*) are defined with respect to the small viewport size: the viewport sized assuming any UA interfaces that are dynamically expanded and retracted to be expanded. This allows authors to size content such that it can fit within the viewport even when such interfaces are present, noting that such content might not fill the viewport when such interfaces are retracted.

The sizes of the small viewport-percentage units are fixed (and therefore stable) unless the viewport itself is resized.

An element that is sized as height: 100svh, for example, will fill the screen perfectly, without any of its content being obscured, when all the dynamic UI elements of the UA are shown.

Once those UI elements start being hidden, however, there will be extra space around the element. The small viewport-percentage units units are thus “safer” in general, but might not produce the most attractive layout once the user starts interacting with the page.

dynamic viewport

The dynamic viewport-percentage units (dv*) are defined with respect to the dynamic viewport size: the viewport sized with dynamic consideration of any UA interfaces that are dynamically expanded and retracted. This allows authors to size content such that it can exactly fit within the viewport whether or not such interfaces are present.

The sizes of the dynamic viewport-percentage units are not stable even while the viewport itself is unchanged. Using these units can cause content to resize e.g. while the user scrolls the page. Depending on usage, this can be disturbing to the user and/or costly in terms of performance.

The UA is not required to animate the dynamic viewport-percentage units while expanding and retracting any relevant interfaces, and may instead calculate the units as if the relevant interface was fully expanded or retracted during the UI animation. (It is recommended that UAs assume the fully-retracted size for this duration.)

Whether the expansion/retraction of a particular interface (A) changes the sizes of all of the viewport-percentage lengths (and the initial containing block) simultaneously or (B) contributes to the differences between the large viewport size and small viewport size is largely UA-dependent. However:

• Changes in interface that happen as a result of scrolling or other frequent page interactions that would disturb the user if they resulted in substantial layout changes must be categorized as the latter (B).

• Changes in interface that have a sufficiently steady state that re-laying out the document into the adjusted space would be beneficial to the user must be categorized as the former (A).

• Additionally, UAs may have some dynamically-shown interfaces that intentionally overlay content and do not cause any shifts in layout—and therefore have no effect on any of the viewport-percentage lengths. (Typically on-screen keyboards will fit into this category.)

In all cases, scrollbars are assumed not to exist. Note however that the initial containing block's size is affected by the presence of scrollbars on the viewport.

Level 3 assumes scrollbars never exist because it was hard to implement and only Firefox bothered to do so. This is making authors unhappy. Can we improve here?

6.1.2.2. The Various Viewport-relative Units

The viewport-percentage length units are:

vw

svw

lvw

dvw

Equal to 1% of the width of the UA-default viewport size, small viewport size, large viewport size, and dynamic viewport size, respectively.

In the example below, if the width of the viewport is 200mm, the font size of h1 elements will be 16mm (i.e. (8×200mm)/100).

h1 { font-size: 8vw }

vh

svh

lvh

dvh

Equal to 1% of the height of the UA-default viewport size, small viewport size, large viewport size, and dynamic viewport size, respectively.

vi

svi

lvi

dvi

Equal to 1% of the size of the large viewport size, small viewport size, and dynamic viewport size (respectively) in the box’s inline axis.

vb

svb

lvb

dvb

Equal to 1% of the size of the initial containing block UA-default viewport size, small viewport size, large viewport size, and dynamic viewport size (respectively) in the box’s block axis.

vmin

svmin

lvmin

dvmin

Equal to the smaller of *vw or *vh.

vmax

svmax

lvmax

dvmax

Equal to the larger of *vw or *vh.

Originally the (unprefixed) viewport units were defined relative to the viewport size in general. The dynamism of browser chrome shifting in and out during scrolling was invented later, and following Safari’s lead, most UAs mapped these units to the larger size. Defining it this way is prettier in many cases, but can also block critical content (such as toolbars, headers, and footers) in others. It’s therefore not entirely clear whether this is the best mapping.

In situations where there is no element or it hasn’t yet been styled (such as when evaluating media queries), the *vi and *vb units use the initial value of the writing-mode property to determine which axis they correspond to.

6.2. Absolute Lengths: the cm, mm, Q, in, pt, pc, px units

The absolute length units are fixed in relation to each other and anchored to some physical measurement. They are mainly useful when the output environment is known. The absolute units consist of the physical units (in, cm, mm, pt, pc, Q) and the visual angle unit (pixel unit) (px):

h1 { margin: 0.5in }      /* inches  */
h2 { line-height: 3cm }   /* centimeters */
h3 { word-spacing: 4mm }  /* millimeters */
h3 { letter-spacing: 1Q } /* quarter-millimeters */
h4 { font-size: 12pt }    /* points */
h4 { font-size: 1pc }     /* picas */
p  { font-size: 12px }    /* px */

Note: Lengths in publishing contexts are sometimes written like 2p3, indicating a length of 2 picas and 3 points. These can be written in CSS as calc(2pc + 3pt) (see § 10.1 Basic Arithmetic: calc()).

All of the absolute length units are compatible, and px is their canonical unit.

For a CSS device, these dimensions are anchored either

1. by relating the physical units to their physical measurements, or
2. by relating the pixel unit to the reference pixel.

For print media at typical viewing distances, the anchor unit should be one of the physical units (inches, centimeters, etc). For screen media (including high-resolution devices), low-resolution devices, and devices with unusual viewing distances, it is recommended instead that the anchor unit be the pixel unit. For such devices it is recommended that the pixel unit refer to the whole number of device pixels that best approximates the reference pixel.

Note: If the anchor unit is the pixel unit, the physical units might not match their physical measurements. Alternatively if the anchor unit is a physical unit, the pixel unit might not map to a whole number of device pixels.

Note: This definition of the pixel unit and the physical units differs from the earlier editions of CSS1 and CSS2. In particular, in previous versions of CSS the pixel unit and the physical units were not related by a fixed ratio: the physical units were always tied to their physical measurements while the pixel unit would vary to most closely match the reference pixel. (This unfortunate change was made because too much existing content relies on the assumption of 96dpi, and breaking that assumption broke the content.)

Note: Units are ASCII case-insensitive and serialize as lower case, for example 1Q serializes as 1q.

The reference pixel is the visual angle of one pixel on a device with a device pixel density of 96dpi and a distance from the reader of an arm’s length. For a nominal arm’s length of 28 inches, the visual angle is therefore about 0.0213 degrees. For reading at arm’s length, 1px thus corresponds to about 0.26 mm (1/96 inch).

The image below illustrates the effect of viewing distance on the size of a reference pixel: a reading distance of 71 cm (28 inches) results in a reference pixel of 0.26 mm, while a reading distance of 3.5 m (12 feet) results in a reference pixel of 1.3 mm.

This second image illustrates the effect of a device’s resolution on the pixel unit: an area of 1px by 1px is covered by a single dot in a low-resolution device (e.g. a typical computer display), while the same area is covered by 16 dots in a higher resolution device (such as a printer).

A device pixel is the smallest unit of area on the device output capable of displaying its full range of colors. For typical color screens, it’s a square or somewhat rectangular region containing a red, green, and blue subpixel. Many non-traditional outputs exist that can blur this definition, such as by displaying some colors at higher resolutions. Such devices still expose some equivalent notion of "device pixel", however.

7. Other Quantities

7.1. Angle Units: the <angle> type and deg, grad, rad, turn units

Angle values are <dimension>s denoted by <angle>. The angle unit identifiers are:

deg

Degrees. There are 360 degrees in a full circle.

grad

Gradians, also known as "gons" or "grades". There are 400 gradians in a full circle.

rad

Radians. There are 2π radians in a full circle.

turn

Turns. There is 1 turn in a full circle.

For example, a right angle is 90deg or 100grad or 0.25turn or approximately 1.57rad.

All <angle> units are compatible, and deg is their canonical unit.

Note: For legacy reasons, some uses of <angle> allow a bare 0 to mean 0deg. This is not true in general, however, and will not occur in future uses of the <angle> type.

7.2. Duration Units: the <time> type and s, ms units

Time values are dimensions denoted by <time>. The time unit identifiers are:

s

Seconds.

ms

Milliseconds. There are 1000 milliseconds in a second.

All <time> units are compatible, and s is their canonical unit.

Properties may restrict the time value to some range. If the value is outside the allowed range, the declaration is invalid and must be ignored.

7.3. Frequency Units: the <frequency> type and Hz, kHz units

Frequency values are dimensions denoted by <frequency>. The frequency unit identifiers are:

Hz

Hertz. It represents the number of occurrences per second.

kHz

KiloHertz. A kiloHertz is 1000 Hertz.

For example, when representing sound pitches, 200Hz (or 200hz) is a bass sound, and 6kHz (or 6khz) is a treble sound.

All <frequency> units are compatible, and hz is their canonical unit.

Note: Units are ASCII case-insensitive and serialize as lower case, for example 1Hz serializes as 1hz.

7.4. Resolution Units: the <resolution> type and dpi, dpcm, dppx units

Resolution units are dimensions denoted by <resolution>. The resolution unit identifiers are:

dpi

Dots per inch.

dpcm

Dots per centimeter.

dppx

x

Dots per px unit.

The <resolution> unit represents the size of a single "dot" in a graphical representation by indicating how many of these dots fit in a CSS in, cm, or px. For uses, see e.g. the resolution media query in [MEDIAQ] or the image-resolution property defined in [CSS3-IMAGES].

All <resolution> units are compatible, and dppx is their canonical unit.

The allowed range of <resolution> values always excludes negative values, in addition to any explicit ranges that might be specified.

Note that due to the 1:96 fixed ratio of CSS in to CSS px, 1dppx is equivalent to 96dpi. This corresponds to the default resolution of images displayed in CSS: see image-resolution.

@media (min-resolution: 2dppx) { ... }

8. Data Types Defined Elsewhere

Some data types are defined in their own modules. This example talks about some of the most common ones used across several specifications.

8.1. Colors: the <color> type

The <color> data type is defined in [CSS-COLOR-4]. UAs must interpret <color> as defined therein.

8.1.1. Combination of <color>

Interpolation of <color> is defined in CSS Color 4 §  12. Color Interpolation. Interpolation is done between premultiplied colors, as defined in CSS Color 4 § 12.3 Interpolating with Alpha.

Addition of <color> is likewise defined as the independent addition of each component as a <number> in premultiplied space.

8.2. Images: the <image> type

The <image> data type is defined in [CSS3-IMAGES]. UAs that support CSS Images Level 3 or its successor must interpret <image> as defined therein. UAs that do not yet support CSS Images Level 3 must interpret <image> as <url>.

8.2.1. Combination of <image>

Note: Interpolation of <image> is defined in CSS Images 3 § 6 Interpolation.

Images are not additive.

8.3. 2D Positioning: the <position> type

The <position> value specifies the position of a object area (e.g. background image) inside a positioning area (e.g. background positioning area). It is interpreted as specified for background-position. [CSS3-BACKGROUND]

<position> = [
  [ left | center | right ] || [ top | center | bottom ]
|
  [ left | center | right | <length-percentage> ]
  [ top | center | bottom | <length-percentage> ]?
|
  [ [ left | right ] <length-percentage> ] &&
  [ [ top | bottom ] <length-percentage> ]
]

Note: The background-position property also accepts a three-value syntax. This has been disallowed generically because it creates parsing ambiguities when combined with other length or percentage components in a property value.

8.3.1. Parsing <position>

When specified in a grammar alongside other keywords, <length>s, or <percentage>s, <position> is greedily parsed; it consumes as many components as possible.

8.3.2. Serializing <position>

When serializing the specified value of a <position>:

If only one component is specified:

• The implied center keyword is added, and a 2-component value is serialized.

If two components are specified:

• Keywords are serialized as keywords.

• <length-percentage>s are serialized as <length-percentage>s.

• Components are serialized horizontal first, then vertical.

If four components are specified:

• Keywords and offsets are both serialized.

• Components are serialized horizontal first, then vertical.

Note: Computed values are always serialized as two offsets (without keywords) because the computed value does not preserve syntactic distinctions.

8.3.3. Combination of <position>

Interpolation of <position> is defined as the independent interpolation of each component (x, y) normalized as an offset from the top left corner as a <length-percentage>.

Addition of <position> is likewise defined as the independent addition each component (x, y) normalized as an offset from the top left corner as a <length-percentage>.

9. Functional Notations

A functional notation is a type of component value that can represent more complex types or invoke special processing. The syntax starts with the name of the function immediately followed by a left parenthesis (i.e. a <function-token>) followed by the argument(s) to the notation followed by a right parenthesis. White space is allowed, but optional, immediately inside the parentheses. Functions can take multiple arguments, which are formatted similarly to a CSS property value. See § 2.6 Functional Notation Definitions.

Note: Some legacy functional notations, such as rgba(), use commas unnecessarily, but generally commas are only used to separate items in a list, or pieces of a grammar that would be ambiguous otherwise. If a comma is used to separate arguments, white space is optional before and after the comma.

background: url(http://www.example.org/image);
color: rgb(100, 200, 50 );
content: counter(list-item) ". ";
width: calc(50% - 2em);

The math functions are defined below. Other functional notations are defined in their own modules; for example the color functions are defined in [CSS-COLOR-4].

10. Mathematical Expressions

The math functions (calc(), clamp(), sin(), and others defined in this chapter) allow numeric CSS values to be written as mathematical expressions.

A math function represents a numeric value, one of:

• <length>,

• <frequency>,

• <angle>,

• <time>,

• <flex>,

• <resolution>,

• <percentage>,

• <number>,

• <integer>

...or the <length-percentage>/etc mixed types, and can be used wherever such a value would be valid.

10.1. Basic Arithmetic: calc()

The calc() function is a math function that allows basic arithmetic to be performed on numerical values, using addition (+), subtraction (-), multiplication (*), division (/), and parentheses.

A calc() function contains a single calculation, which is a sequence of values interspersed with operators, and possibly grouped by parentheses (matching the <calc-sum> grammar), which represents the result of evaluating the expression using standard operator precedence rules (* and / bind tighter than + and -, and operators are otherwise evaluated left-to-right). The calc() function represents the result of its contained calculation.

Components of a calculation can be literal values (such as 5px), other math functions, or other expressions, such as var(), that evaluate to a valid argument type (like <length>).

section {
  float: left;
  margin: 1em; border: solid 1px;
  width: calc(100% / 3 - 2 * 1em - 2 * 1px);
}

.fade {
  background-image: linear-gradient(silver 0%, white 20px,
                                    white calc(100% - 20px), silver 100%);
}

:root {
  font-size: calc(100vw / 35);
}

.aspect-ratio-box {
  --ar: calc(16 / 9);
  --w: calc(100% / 3);
  --h: calc(var(--w) / var(--ar));
  width: var(--w);
  height: var(--h);
}

10.2. Comparison Functions: min(), max(), and clamp()

The comparison functions of min(), max(), and clamp() compare multiple calculations and represent the value of one of them.

The min() or max() functions contain one or more comma-separated calculations, and represent the smallest (most negative) or largest (most positive) of them, respectively.

The clamp() function takes three calculations—a minimum value, a central value, and a maximum value—and represents its central calculation, clamped according to its min and max calculations, favoring the min calculation if it conflicts with the max. (That is, given clamp(MIN, VAL, MAX), it represents exactly the same value as max(MIN, min(VAL, MAX))).

For all three functions, the argument calculations 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.

.type {
  /* Set font-size to 10x the average of vw and vh,
     but don’t let it go below 12px. */
  font-size: max(10 * (1vw + 1vh) / 2, 12px);
}

.type {
  /* Force the font-size to stay between 12px and 100px */
  font-size: clamp(12px, 10 * (1vw + 1vh) / 2, 100px);
}

10.3. Stepped Value Functions: round(), mod(), and rem()

The stepped-value functions, round(), mod(), and rem(), all transform a given value according to another "step value", in different ways.

The round(<rounding-strategy>?, A, B) function contains an optional rounding strategy, and two calculations A and B, and returns the value of A, rounded according to the rounding strategy, to the nearest integer multiple of B either above or below A. The argument calculations 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.

If A is exactly equal to an integer multiple of B, round() resolves to A exactly (preserving whether A is 0⁻ or 0⁺, if relevant). Otherwise, there are two integer multiples of B that are potentially "closest" to A, lower B which is closer to −∞ and upper B which is closer to +∞. The following <rounding-strategy>s dictate how to choose between them:

nearest

Choose whichever of lower B and upper B that has the smallest absolute difference from A. If both have an equal difference (A is exactly between the two values), choose upper B.

up

Choose upper B.

down

Choose lower B.

to-zero

Choose whichever of lower B and upper B that has the smallest absolute difference from 0.

If lower B would be zero, it is specifically equal to 0⁺; if upper B would be zero, it is specifically equal to 0⁻.

If <rounding-strategy> is omitted, it defaults to nearest. (Aka rounding to the nearest integer.)

CSSOM needs to specify how it rounds, and it’s probably good for CSS functions to round the same way by default. What behavior should be used? [Issue #5689]

Note: JavaScript and other programming languages sometimes separate out the rounding strategies into separate rounding functions. JS’s Math.floor() is equivalent to CSS’s round(down, ...); JS’s Math.ceil() is equivalent to CSS’s round(up, ...); JS’s Math.trunc() is equivalent to CSS’s round(to-zero, ...); and JS’s Math.round() is equivalent to CSS’s round(nearest, ...), or just round(...).

Note: The <rounding-strategy> keywords are the same as the keywords in block-step-size and have the same behavior. (block-step-size just lacks to-zero; since block sizes are always non-negative, to-zero and down would be identical.)

The modulus functions mod(A, B) and rem(A, B) similarly contain two calculations A and B, and return the difference between A and the nearest integer multiple of B either above or below A. The argument calculations 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.

The two functions are very similar, and in fact return identical results if both arguments are positive or both are negative: the value of the function is equal to the value of A shifted by the integer multiple of B that brings the value between zero and B. (Specifically, the range includes zero and excludes B. More specifically, if B is positive the range starts at 0⁺, and if B is negative it starts at 0⁻.)

Their behavior diverges if the A value and the B step are on opposite sides of zero: mod() (short for “modulus”) continues to choose the integer multiple of B that puts the value between zero and B, as above (guaranteeing that the result will either be zero or share the sign of B, not A), while rem() (short for "remainder") chooses the integer multiple of B that puts the value between zero and -B, avoiding changing the sign of the value.

Note: mod() and rem() can also be defined directly in terms of other functions: mod(A, B) is equivalent to calc(A - sign(B)*round(down, A*sign(B), B)) (a hacky way to say "round(down) when B is positive, round(up) when B is negative), while rem(A, B) is equivalent to calc(A - round(to-zero, A, B)). (These expressions don’t always handle 0⁺ and 0⁻ correctly, though, because 0⁻ semantics aren’t commutative for addition.)

10.3.1. Argument Ranges

In round(A, B), if B is 0, the result is NaN. If A and B are both infinite, the result is NaN.

If A is infinite but B is finite, the result is the same infinity.

If A is finite but B is infinite, the result depends on the <rounding-strategy> and the sign of A:

nearest

to-zero

If A is positive or 0⁺, return 0⁺. Otherwise, return 0⁻.

up

If A is positive (not zero), return +∞. If A is 0⁺, return 0⁺. Otherwise, return 0⁻.

down

If A is negative (not zero), return −∞. If A is 0⁻, return 0⁻. Otherwise, return 0⁺.

In mod(A, B) or rem(A, B), if B is 0, the result is NaN. If A is infinite, the result is NaN.

In mod(A, B) only, if B is infinite and A has opposite sign to B (including an oppositely-signed zero), the result is NaN.

Note: All other "infinite B" cases are valid, and just return A immediately.

10.4. Trigonometric Functions: sin(), cos(), tan(), asin(), acos(), atan(), and atan2()

The trigonometric functions—sin(), cos(), tan(), asin(), acos(), atan(), and atan2()—compute the various basic trigonometric relationships.

The sin(A), cos(A), and tan(A) functions all contain a single calculation which must resolve to either a <number> or an <angle>, and compute their corresponding function by interpreting the result of their argument as radians. (That is, sin(45deg), sin(.125turn), and sin(3.14159 / 4) all represent the same value, approximately .707.) They all represent a <number>; sin() and cos() will always return a number between −1 and 1, while tan() can return any number between −∞ and +∞. (See § 10.9 Type Checking for details on how math functions handle ∞.)

The asin(A), acos(A), and atan(A) functions are the "arc" or "inverse" trigonometric functions, representing the inverse function to their corresponding "normal" trig functions. All of them contain a single calculation which must resolve to a <number>, and compute their corresponding function, interpreting their result as a number of radians, representing an <angle>. The angle returned by asin() must be normalized to the range [-90deg, 90deg]; the angle returned by acos() to the range [0deg, 180deg]; and the angle returned by atan() to the range [-90deg, 90deg].

The atan2(A, B) function contains two comma-separated calculations, A and B. A and B can resolve to any <number>, <dimension>, or <percentage>, but must have the same type, or else the function is invalid. The function returns the <angle> between the positive X-axis and the point (B,A). The returned angle must be normalized to the interval (-180deg, 180deg] (that is, greater than -180deg, and less than or equal to 180deg).

Note: atan2(Y, X) is generally equivalent to atan(Y / X), but it gives a better answer when the point in question may include negative components. atan2(1, -1), corresponding to the point (-1, 1), returns 135deg, distinct from atan2(-1, 1), corresponding to the point (1, -1), which returns -45deg. In contrast, atan(1 / -1) and atan(-1 / 1) both return-45deg, because the internal calculation resolves to -1 for both.

10.4.1. Argument Ranges

In sin(A), cos(A), or tan(A), if A is infinite, the result is NaN. (See § 10.9 Type Checking for details on how math functions handle NaN.)

In sin(A) or tan(A), if A is 0⁻, the result is 0⁻.

In tan(A), if A is one of the asymptote values (such as 90deg, 270deg, etc), the result is explicitly undefined. If an implementation is capable of exactly representing these inputs, it should return +∞ for the asymptotes at 90deg + N*360deg, and −∞ for the asymptotes at -90deg + N*360deg, but implementations are not required to be able to exactly represent these inputs (and if they can’t, will return whatever the correct numeric answer is for the closest approximation to the input they are capable of representing). Authors must not rely on tan() returning any particular value for these inputs.

In asin(A) or acos(A), if A is less than -1 or greater than 1, the result is NaN.

In acos(A), if A is exactly 1, the result is 0.

In asin(A) or atan(A), if A is 0⁻, the result is 0⁻.

In atan(A), if A is +∞, the result is 90deg; if A is −∞, the result is -90deg.

In atan2(Y, X), the following table gives the results for all unusual argument combinations:

Note: All of these behaviors are intended to match the "standard" definitions of these functions as implemented by most programming languages, in particular as implemented in JS.

10.5. Exponential Functions: pow(), sqrt(), hypot(), log(), exp()

The exponential functions—pow(), sqrt(), hypot(), log(), and exp()—compute various exponential functions with their arguments.

The pow(A, B) function contains two comma-separated calculations A and B, both of which must resolve to <number>s, and returns the result of raising A to the power of B, returning the value as a <number>.

The sqrt(A) function contains a single calculation which must resolve to a <number>, and returns the square root of the value as a <number>. (sqrt(X) and pow(X, .5) are basically equivalent, differing only in some error-handling; sqrt() is a common enough function that it is provided as a convenience.)

The hypot(A, …) function contains one or more comma-separated calculations, and returns the length of an N-dimensional vector with components equal to each of the calculations. (That is, the square root of the sum of the squares of its arguments.) The argument calculations 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.

The log(A, B?) function contains one or two calculations (representing the value to be logarithmed, and the base of the logarithm, defaulting to e), which must resolve to <number>s, and returns the logarithm base B of the value A, as a <number>.

The exp(A) function contains one calculation which must resolve to a <number>, and returns the same value as pow(e, A) as a <number>.

:root {
  --h6: calc(1rem * pow(1.5, -1));
  --h5: calc(1rem * pow(1.5, 0));
  --h4: calc(1rem * pow(1.5, 1));
  --h3: calc(1rem * pow(1.5, 2));
  --h2: calc(1rem * pow(1.5, 3));
  --h1: calc(1rem * pow(1.5, 4));
}

10.5.1. Argument Ranges

In pow(A, B), if A is negative and finite, and B is finite, B must be an integer, or else the result is NaN.

If A or B are infinite or 0, the following tables give the results:

In sqrt(A), if A is +∞, the result is +∞. If A is 0⁻, the result is 0⁻. If A is less than 0, the result is NaN.

In hypot(A, …), if any of the inputs are infinite, the result is +∞.

In log(A, B), if B is 1 or negative, B values between 0 and 1, or greater than 1, are valid. the result is NaN. If A is negative, the result is NaN. If A is 0⁺ or 0⁻, the result is −∞. If A is 1, the result is 0⁺. If A is +∞, the result is +∞.

In exp(A), if A is +∞, the result is +∞. If A is −∞, the result is 0⁺.

(See § 10.9 Type Checking for details on how math functions handle NaN and infinities.)

10.6. Sign-Related Functions: abs(), sign()

The sign-related functions—abs() and sign()—compute various functions related to the sign of their argument.

The abs(A) function contains one calculation A, and returns the absolute value of A, as the same type as the input: if A’s numeric value is positive or 0⁺, just A again; otherwise -1 * A.

The sign(A) function contains one calculation A, and returns -1 if A’s numeric value is negative, +1 if A’s numeric value is positive, 0⁺ if A’s numeric value is 0⁺, and 0⁻ if A’s numeric value is 0⁻.

Note: Both of these functions operate on the fully simplified/resolved form of their arguments, which may give unintuitive results at first glance. In particular, an expression like 10% might be positive or negative once it’s resolved, depending on what value it’s resolved against. For example, in background-position positive percentages resolve to a negative length, and vice versa, if the background image is larger than the background area. Thus sign(10%) might return 1 or -1, depending on how the percentage is resolved! (Or even 0, if it’s resolved against a zero length.)

10.7. Numeric Constants: e, pi

While the trigonometric and exponential functions handle many complex numeric operations, some reasonable calculations must be put together more manually, and many times these include well-known constants, such as e and π.

Rather than require authors to manually type out several digits of these constants, a few of them are provided directly:

e is the base of the natural logarithm, approximately equal to 2.7182818284590452354.

pi is the ratio of a circle’s circumference to its diameter, approximately equal to 3.1415926535897932.

Note: These keywords are only usable within a calculation, such as calc(pow(e, pi) - pi), or min(pi, 5, e). If used outside of a calculation, they’re treated like any other keyword: animation-name: pi; refers to an animation named "pi"; line-height: e; is invalid (not similar to line-height: 2.7, but line-height: calc(e); is).

10.7.1. Degenerate Numeric Constants: infinity, -infinity, NaN

When a calculation or a subtree of a calculation becomes infinite or NaN, representing it with a numeric value is no longer possible. To aid in serialization of these degenerate values, the additional math constants infinity (with the value +∞), -infinity (with the value −∞), and NaN (with the value NaN) are defined.

As usual for CSS keywords, these are ASCII case-insensitive. Thus, calc(InFiNiTy) is perfectly valid. However, NaN must be serialized with this canonical casing.

Note: While not technically numbers, these keywords act as numeric values, similar to e and pi. Thus to get an infinite length, for example, requires an expression like calc(infinity * 1px).

Note: These constants are defined mostly to make serialization of infinite/NaN values simpler and more obvious, but can be used to indicate a "largest possible value", since an infinite value gets clamped to the allowed range. It’s rare for this to be reasonable, but when it is, using infinity is clearer in its intent than just putting an enormous number in one’s stylesheet.

10.8. Syntax

The syntax of a math function is:

<calc()>  = calc( <calc-sum> )
<min()>   = min( <calc-sum># )
<max()>   = max( <calc-sum># )
<clamp()> = clamp( <calc-sum>#{3} )
<round()> = round( <rounding-strategy>?, <calc-sum>, <calc-sum> )
<mod()>   = mod( <calc-sum>, <calc-sum> )
<rem()>   = rem( <calc-sum>, <calc-sum> )
<sin()>   = sin( <calc-sum> )
<cos()>   = cos( <calc-sum> )
<tan()>   = tan( <calc-sum> )
<asin()>  = asin( <calc-sum> )
<acos()>  = acos( <calc-sum> )
<atan()>  = atan( <calc-sum> )
<atan2()> = atan2( <calc-sum>, <calc-sum> )
<pow()>   = pow( <calc-sum>, <calc-sum> )
<sqrt()>  = sqrt( <calc-sum> )
<hypot()> = hypot( <calc-sum># )
<log()>   = log( <calc-sum>, <calc-sum>? )
<exp()>   = exp( <calc-sum> )
<abs()>   = abs( <calc-sum> )
<sign()>  = sign( <calc-sum> )
<calc-sum> = <calc-product> [ [ '+' | '-' ] <calc-product> ]*
<calc-product> = <calc-value> [ [ '*' | '/' ] <calc-value> ]*
<calc-value> = <number> | <dimension> | <percentage> |
               <calc-constant> | ( <calc-sum> )
<calc-constant> = e | pi | infinity | -infinity | NaN
<rounding-strategy> = nearest | up | down | to-zero

In addition, whitespace is required on both sides of the + and - operators. (The * and / operators can be used without white space around them.)

Several of the math functions above have additional constraints on what their <calc-sum> arguments can contain. Check the definitions of the individual functions for details.

UAs must support calculations of at least 32 <calc-value> terms and at least 32 levels of nesting (parentheses and/or functions). For functions that support an arbitrary number of arguments (such as min()), it must also support at least 32 arguments. If a calculation contains more than the supported number of terms, arguments, or nesting it must be treated as if it were invalid.

10.9. Type Checking

A math function can be many possible types, such as <length>, <number>, etc., depending on the calculations it contains, as defined below. It can be used anywhere a value of that type is allowed.

Additionally, math functions that resolve to <number> can be used in any place that only accepts <integer>; the value is rounded to the nearest integer as it resolves.

Operators form sub-expressions, which gain types based on their arguments.

Note: In previous versions of this specification, multiplication and division were limited in what arguments they could take, to avoid producing more complex intermediate results (such as 1px * 1em, which is <length>²) and to make division-by-zero detectable at parse time. This version now relaxes those restrictions.

Math functions themselves have types, according to their contained calculations:

calc()

abs()

The type of its contained calculation.

min()

max()

clamp()

hypot()

round()

mod()

rem()

The result of adding the types of its comma-separated calculations.

sign()

sin()

cos()

tan()

asin()

acos()

atan()

atan2()

pow()

sqrt()

log()

exp()

«[ ]» (empty map).

For each of the above, if the type is failure, the math function is invalid.

A math function resolves to <number>, <length>, <angle>, <time>, <frequency>, <resolution>, <flex>, or <percentage> according to which of those productions its type matches. (These categories are mutually exclusive.) If it can’t match any of these, the math function is invalid.

Division by zero is possible, which introduces certain complications. Math functions follow IEEE-754 semantics for these operations:

• Dividing a positive value by zero produces +∞.

• Dividing a negative value by zero produces −∞.

• Adding or subtracting ±∞ to anything produces the appropriate infinity, unless a following rule would define it as producing NaN.

• Multiplying any value by ±∞ produces the appropriate infinity, unless a following rule would define it as producing NaN.

• Dividing any value by ±∞ produces zero, unless a following rule would define it as producing NaN.

• Dividing zero by zero, dividing ±∞ by ±∞, multiplying 0 by ±∞, adding +∞ to −∞ (or the equivalent subtractions) produces NaN.

• Any operation with at least one NaN argument produces NaN.

Additionally, IEEE-754 introduces the concept of a "signed zero", which is specifically positive or negative, and which must be tracked within a calculation and between nested calculations:

• Negative zero (0⁻) can be produced by a multiplication or division that produces zero with exactly one negative argument (such as -5 * 0 or 1 / (-infinity)), or by certain argument combinations in the other math functions.

  Note: Note that negative zeros don’t escape a math function; as detailed below, they’re "censored" away into an "unsigned" zero.

• 0⁻ + 0⁻ or 0⁻ - 0 produces 0⁻. All other additions or subtractions that would produce a zero produce 0⁺.

• Multiplying or dividing 0⁻ with a positive number (including 0⁺) produces a negative result (either 0⁻ or −∞), while multiplying or dividing 0⁻ with a negative number produces a positive result.

  (In other words, multiplying or dividing with 0⁻ follows standard sign rules.)

• When comparing 0⁺ and 0⁻, 0⁻ is less than 0⁺. For example, min(0⁺, 0⁻) must produce 0⁻, max(0⁺, 0⁻) must produce 0⁺, and clamp(0⁺, 0⁻, 1) must produce 0⁺.

• Unless otherwise specified, any other operation that produces a zero produces positive zero (0⁺).

If a top-level calculation (a math function not nested inside of another math function) would produce a value whose numeric part is NaN, it instead act as though the numeric part is 0. If a top-level calculation would produce a value whose numeric part is 0⁻ or 0⁺, it instead acts as though the numeric part is the standard "unsigned" zero.

Note: Algebraic simplifications do not affect the validity of a math function or its resolved type. For example, calc(5px - 5px + 10s) and calc(0 * 5px + 10s) are both invalid due to the attempt to add a length and a time.

Note: Note that <percentage>s relative to <number>s, such as in opacity, are not combinable with those numbers—opacity: calc(.25 + 25%) is invalid. Allowing this causes significant problems with "unit algebra" (allowing multiplication/division of <dimension>s), and in every case so far, doesn’t provide any new functionality. (For example, opacity: 25% is identical to opacity: .25; it’s just a trivial syntax transform.) You can still perform other operations with them, such as opacity: calc(100% / 3);, which is valid.

Note: Because <number-token>s are always interpreted as <number>s or <integer>s, "unitless 0" <length>s aren’t supported in math functions. That is, width: calc(0 + 5px); is invalid, because it’s trying to add a <number> to a <length>, even though both width: 0; and width: 5px; are valid.

Note: Altho there are a few properties in which a bare <number> becomes a <length> at used-value time (specifically, line-height and tab-size), <number>s never become "length-like" in calc(). They always stay as <number>s.

Note: In Quirks Mode [QUIRKS], some properties that would normally only accept <length>s are defined to also accept <number>s, interpreting them as px lengths. Like unitless zeroes, this has no effect on the parsing or behavior of math functions, tho a math function that resolves to a <number> value might become valid in Quirks Mode (and have its result interpreted as a px length).

10.10. Internal Representation

The internal representation of a math function is a calculation tree: a tree where the branch nodes are operator nodes corresponding either to math functions (such as Min, Cos, Sqrt, etc) or to operators in a calculation (Sum, Product, Negate, and Invert, the calc-operator nodes), and the leaf nodes are either numeric values (such as numbers, dimensions, and percentages) or non-math functions that resolve to a numeric type.

Math functions are turned into calculation trees depending on the function:

calc()

The internal representation of a calc() function is the result of parsing a calculation from its argument.

any other math function

The internal representation is an operator node with the same name as the function, whose children are the result of parsing a calculation from each of the function’s arguments, in the order they appear.

10.10.1. Simplification

Internal representations of math functions are eagerly simplified to the extent possible, using standard algebraic simplifications (distributing multiplication over sums, combining similar units, etc.).

10.11. Computed Value

The computed value of a math function is its calculation tree simplified, using all the information available at computed value time. (Such as the em to px ratio, how to resolve percentages in some properties, etc.)

Where percentages are not resolved at computed-value time, they are not resolved in math functions, e.g. calc(100% - 100% + 1px) resolves to calc(0% + 1px), not to 1px. If there are special rules for computing percentages in a value (e.g. the height property), they apply whenever a math function contains percentages.

The calculation tree is again simplified at used value time; with used value time information, a math function always simplifies down to a single numeric value.

Given the complexities of width and height calculations on table cells and table elements, math expressions mixing both percentages and non-zero lengths for widths and heights on table columns, table column groups, table rows, table row groups, and table cells in both auto and fixed layout tables MUST be treated as if auto had been specified.

10.12. Range Checking

Parse-time range-checking of values is not performed within math functions, and therefore out-of-range values do not cause the declaration to become invalid. However, the value resulting from a top-level calculation must be clamped to the range allowed in the target context. Clamping is performed on computed values to the extent possible, and also on used values if computation was unable to sufficiently simplify the expression to allow range-checking. (Clamping is not performed on specified values.)

Note: This requires all contexts accepting calc() to define their allowable values as a closed (not open) interval.

Note: By definition, ±∞ are outside the allowed range for any property, and will clamp to the minimum/maximum value allowed. Even properties that can explicitly represent infinity as a keyword value, such as animation-iteration-count, will end up clamping ±∞, as math functions can’t resolve to keyword values; the numeric part of the property’s syntax still has a minimum/maximum value.

Additionally, if a math function that resolves to <number> is used somewhere that only accepts <integer>, the computed value and used value are rounded to the nearest integer, in the same manner as clamping, above.

width: calc(5px - 10px);
width: calc(-5px);
width: 0px;

10.13. Serialization

This section is still under discussion.

See [CSSOM] for further information on serialization.

10.14. Combination of Math Functions

Interpolation of math functions, with each other or with numeric values and other numeric-valued functions, is defined as V_result = calc((1 - p) * V_a + p * V_b). (Simplification of the value might then reduce the expression to a smaller, simpler form.)

Addition of math functions, with each other or with numeric values and other numeric-valued functions, is defined as V_result = calc(V_a + V_b). (Simplification of the value might then reduce the expression to a smaller, simpler form.)

Appendix A: Coordinating List-Valued Properties

Some list-valued properties have coordinated effects: each item in their value list applies to a distinct effect, and corresponding entries in each property’s list all refer to the same effect. Often the coordinating values can also be specified together as a single entry in a list-valued shorthand property.

A typical example is the list-valued background-* properties, which can specify multiple background image layers. For each property controlling how the image is sized, tiled, placed, etc., the Nth item in its list describes some effect that applies to the Nth background image.

A coordinating list property group creates a coordinated value list, which has, for each entry, a value from each property in the group; these are used together to define a single effect, such as a background image layer or an animation. The coordinated value list is assembled as follows:

• The length of the coordinated value list is determined by the number of items specified in one particular coordinating list property, the coordinating list base property. (In the case of backgrounds, this is the background-image property.)

• The Nth value of the coordinated value list is constructed by collecting the Nth use value of each coordinating list property

• If a coordinating list property has too many values specified, excess values at the end of its list are not used.

• If a coordinating list property has too few values specified, its value list is repeated to add more used values.

• The computed values of the coordinating list properties are not affected by such truncation or repetition.

Appendix B: IANA Considerations

Registration for the about:invalid URL scheme

This sections defines and registers the about:invalid URL, in accordance with the registration procedure defined in [RFC6694].

The official record of this registration can be found at http://www.iana.org/assignments/about-uri-tokens/about-uri-tokens.xhtml.

Appendix C: Quirky Lengths

When CSS is being parsed in quirks mode, <quirky-length> is a type of <length> that is only valid in certain properties:

• background-position

• border-spacing

• border-top-width

• border-right-width

• border-bottom-width

• border-left-width

• border-width

• bottom

• clip

• font-size

• height

• left

• letter-spacing

• margin-right

• margin-left

• margin-top

• margin-bottom

• margin

• max-height

• max-width

• min-height

• min-width

• padding-top

• padding-right

• padding-bottom

• padding-left

• padding

• right

• text-indent

• top

• vertical-align

• width

• word-spacing

It is not valid in properties that include or reference these properties, such as the background shorthand, or inside functional notations such as calc(), except that they must be allowed in rect() in the clip property.

Additionally, while <quirky-length> must be valid as a <length> when parsing the affected properties in the @supports rule, it is not valid for those properties when used in the CSS.supports() method.

A <quirky-length> is syntactically identical to a <number-token>, and is interpreted as a px length with the same value.

(In other words, Quirks Mode allows all px lengths in the affected properties to be written without a unit, similar to unitless zero lengths.)

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 Anthony Frehner, Emilio Cobos Álvarez, Koji Ishii, Noam Rosenthal, and Xidorn Quan for their comments and suggestions, which have improved Level 4.

Changes

Recent Changes

(This is a subset of Additions Since Level 3.)

Substantial changes since 19 October 2022 WD:

• Added § 2.6 Functional Notation Definitions to formally define the way that functional notation syntaxes are defined. (Issue 2921)
• Added algorithm for snap as a border width, to reflect the interoperable rules for rendering consistent stroke widths. (Issue 5210)
• Clarified grammar and computed value of mix(). (Issue 8096)
• Undefined the behavior of tan() at the asymptote values. (Issue 8527)
• Specified that negative <resolution> values are out-of-range by definition. (Issue 8532)
• Clarified that fully omitted mix() arguments are valid. (Issue 8556)
• Clarified that range clamping happens specifically to top-level calculations (rather than the unclear term "expressions"). (Issue 8158)
• Added formal definitions for <url()> and <src()> (in addition to <url>).
• Rephrased fragment-only URLs in terms of tree-scoped references. (Issue 3320)

Substantial changes since 16 December 2021 WD:

• Changed resolution of a url() with the local url flag to reference the current node tree (regardless of document base URL modifications). (Issue 3320)
• Switched censoring of NaN that escapes a math function from infinity to zero. (Issue 7067)
• Added Appendix A: Coordinating List-Valued Properties to allow this property pattern to be easily referenced. (Issue 7164)
• Restricted mix() to be the sole value of a declaration. (Issue 6700)
• Updated to match latest Fetch terminology. (Fetch PR 1413, CSS PR 7160)
• Clarified that the font-relative lengths are calculated without text shaping.
• Defined serialization of empty urls to be url(""). (Issue 6447)
• Defined serialization of <position> specified values. (Issue 2274)
• Fixed definition of numbers to allow decimals in combination with scientific notation, as originally intended and as defined in [CSS-SYNTAX-3]. (Issue 7248)
• Corrected various functions to return an empty map for their type instead of «[ "number" → 1 ]». (Issue 7486)
• Clarified effect of special UA restrictions on line-height on lh and rlh. (Issue 3257)
• Defined <function()> notation to refer to functional notations. (Issue 5728)

Substantial changes since 16 October 2021 WD:

• Switched *vi and *vb units to resolve against the computed writing mode of the element itself. (Issue 6873)
• Added § 4.5.4 URL Processing Model to define integration with CORS, etc. (Issue 562)
• Fixed the inverted assignment of viewport-percentage length behaviors to types of interface changes (A vs. B).

  • Changes in interface that happen as a result of scrolling or other frequent page interactions that would disturb the user if they resulted in substantial layout changes must be categorized as the former (A) latter (B) .
  • Changes in interface that have a sufficiently steady state that re-laying out the document into the adjusted space would be beneficial to the user must be categorized as the latter (B) former (A) .

• Defined minimum number of calc() terms, arguments, and nesting as 32. (Issue 3462)
• Defined that mod(-0, infinity) returns NaN. (Issue 4723)
• Deferred toggle() and attr() to Level 5.

Changes since 30 September 2021 WD:

• Added rex, rcap, rch, and ric units.
• Switched toggle() to use semicolons, matching with mix(). (Issue 6701)
• Fixed some wording errors in the definition of calc(). (Issue 6506)
• Imported definition of <quirky-length> from [QUIRKS]. (Issue 6100)

Changes since 7 July 2021 WD:

• Added mix() notation for representing interpolated values.
• Defined generically the computation of <integer>, <number>, <percentage>, and <length>.
• Clarified that only non-zero lengths create a percentage+length mix that switches table cells to auto sizing.

Changes since 11 November 2020 WD:

• Updated interpolation of colors to reference [CSS-COLOR-4] instead of [CSS-COLOR-3].
• Added the svh, svw, svi, svb, svmin, and svmax small viewport-percentage units; lvh, lvw, lvi, lvb, lvmin, and lvmax large viewport-percentage units; and dvh, dvw, dvi, dvb, dvmin, and dvmax dynamic viewport-percentage units. (Issue 4329 and Issue 6113)
• Clamped excessively large <angle> values to multiples of 360deg. (Issue 6105)
• Added back rules on range-checking combined values lost during move from the CSS Transitions specification. (Issue 6097)
• Specified that UA-imposed minimum font sizes apply to the used font-size and not to resolution of font-relative lengths. (Issue 5858)
• Clarified how min() and max() percentages can partially simplify. (Issue 6293)

Additions Since Level 3

Changes since CSS Values and Units Level 3:

• Explicitly undefined numeric precision/range.
• Added rules for interpolation per value type, and their clarified computed values.
• Updated interpolation of colors to reference [CSS-COLOR-4].

Additions since CSS Values and Units Level 3:

• Added the mix() notation for interpolation.
• Defined the <dashed-ident> type.
• Defined the <ratio> type.
• Added src() to the <url> type.
• Added the vi, vb, ic, cap, lh and rlh length units.
• Added the svh, svw, svi, svb, svmin, and svmax small viewport-percentage units and dvh, dvw, dvi, dvb, dvmin, and dvmax dynamic viewport-percentage units.
• Added the x alias to dppx.
• Added min(), max(), and clamp() comparison functions.
• Added round(), mod(), rem(), sin(), cos(), tan(), asin(), acos(), atan(), atan2(), pow(), sqrt(), hypot(), log(), exp(), abs(), sign() math functions.
• Added e, pi, infinity, -infinity, NaN constants for use in calc().
• Added unit algebra to calc(), allowing multiplication and division of dimensions.
• A non-integer in a calc() automatically rounds to the nearest integer when used where an <integer> is required.
• Defined serialization of math functions.
• Added a genericized definition of coordinating list property groups, to make it easier to reference the coordinating behavior of the background properties.

Security Considerations

This specification presents no new security considerations.

This specification defines the url() and src() functions (<url>), which allow CSS to make network requests. Depending on what features they are used in, these can potentially expose whether or not the user has access to resources on a network, and expose information about their contents (such as the rules within a style sheet, the size of an image, the metrics of a font). They can also allow exfiltrating data via URL.

Privacy Considerations

This specification defines units that expose the user’s screen size (the viewport-percentage lengths), default font size, and potentially some information about which fonts are available on the user’s system (the font-relative lengths).

This specification defines the url() and src() functions (<url>), which allow CSS to make network requests. Depending on what features they are used in, these can potentially expose whether or not the user has access to resources on a network, and expose information about their contents (such as the rules within a style sheet, the size of an image, the metrics of a font). They can also allow exfiltrating data via URL.