CSS Values and Units Module Level 3

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 syntax described here is used to define the set of valid values for CSS properties. A property value 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.).
3. types that have the same range of values as a property bearing the same name (e.g., <‘border-width’>, <‘background-attachment’>, etc.). In this case, the type name is the property name (complete with quotes) between the brackets. Such a type does not include CSS-wide keywords such as inherit, and also does not include any top-level comma-separated-list multiplier (i.e. if property pairing is defined as [ <custom-ident> <integer>? ]#, then <‘pairing’> is equivalent to [ <custom-ident> <integer>? ], not <custom-ident> <integer> ]#).
4. non-terminals that do not share the same name as a property. In this case, the non-terminal name appears between < and >, as in <spacing-limit>. Notice the distinction between <border-width> and <‘border-width’>: the latter is defined as the value 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 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

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.

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:

A? B? C?

 A? || B? || C?

 [ A? B? C? ]!

A || B || C

A B C 

A && B && C

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. Property value examples

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

3. Textual Data Types

Identifiers, generically denoted by <ident>, consist of a sequence of characters conforming to the <ident-token> grammar. [CSS3SYN] Identifiers cannot be quoted; otherwise they would be interpreted as strings.

3.1. Pre-defined Keywords

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

Value: collapse | separate

table { border-collapse: separate }

3.1.1. 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.

The initial keyword represents the value specified as the property’s initial value. The inherit keyword represents the computed value of the property on the element’s parent. The unset keyword acts as either inherit or initial, depending on whether the property is inherited or not. All of these keywords are normatively defined in the Cascade module. [CSS3CASCADE]

Other CSS specifications can define additional CSS-wide keywords.

3.2. 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, 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.

3.3. Quoted Strings: the <string> type

Strings are denoted by <string> and consist of a sequence of characters delimited by double quotes or single quotes. They correspond to the <string-token> production in the CSS Syntax Module [CSS3SYN].

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.)

3.4. Resource Locators: the <url> type

The url() functional notation, denoted by <url>, represents a URL, which is a pointer to a resource. The typical syntax of a <url> is:

<url> = url( <string> <url-modifier>* )

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

A <url> may alternately be written without quotation marks around the URL itself, in which case it is specially-parsed as a <url-token> [CSS3SYN].

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

Note: This unquoted syntax is cannot accept a <url-modifier> argument and has extra escaping requirements: parentheses, whitespace characters, single quotes (') and double quotes (") appearing in a URL must be escaped with a backslash, e.g. url(open\(parens), url(close\)parens). (In quoted <string> url()s, only newlines and the character used to quote the string need to be escaped.) Depending on the type of URL, it might also be possible to write these characters as URL-escapes (e.g. url(open%28parens) or url(close%29parens)) as described in [URL].

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

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

3.4.1. 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.

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

3.4.1.1. Fragment URLs

To work around some common eccentriticites in browser URL handling, CSS has special behavior for fragment-only urls.

If a url()’s value starts with a U+0023 NUMBER SIGN (#) character, parse it as per normal for URLs, but additionally set the local url flag of the url().

When matching a url() with the local url flag set, ignore everything but the URL’s fragment, and resolve that fragment against the current document that relative URLs are resolved against. This reference must always be treated as same-document (rather than cross-document).

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

3.4.2. 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).

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 editting 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.

3.4.3. URL Modifiers

The url() function supports 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. Numeric Data Types

Properties may restrict numeric values to some range. If the value is outside the allowed range, unless otherwise specified, the declaration is invalid and must be ignored.

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.

4.1. 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 [CSS3SYN]. The first digit of an integer may be immediately preceded by - or + to indicate the integer’s sign.

4.2. 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 and optionally an exponent composed of "e" or "E" and an integer. It corresponds to the <number-token> production in the CSS Syntax Module [CSS3SYN]. As with integers, the first character of a number may be immediately preceded by - or + to indicate the number’s sign.

4.3. 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 [CSS3SYN]. 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.

4.3.1. 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 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 represent distances/durations/frequencies/etc. as compatible with the relevant class of dimensions, and canonicalize accordingly.

4.4. 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 [CSS3SYN].

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.

4.5. Mixing Percentages and Dimensions

In cases where a <percentage> can represent the same quantity as a dimension or number 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>.

<number-percentage>

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

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

5. 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.

5.1. 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.

5.1.1. Font-relative lengths: the em, ex, ch, rem units

The font-relative lengths refer to the font metrics of the element on which they are used—or, in the case of rem, the metrics of the root element).

em unit

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.

ex unit

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.

ch unit

Equal to the used advance measure of the "0" (ZERO, U+0030) glyph found 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: 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).

rem unit

Equal to the computed value of font-size on the root element.

If used in the font-size property of the root element, or in a document with no root element, 1rem is equal to the initial value of the font-size property.

When used in the value of the font-size property on the element they refer to, these units refer to the computed font metrics of the parent element (or the computed font metrics corresponding to the initial values of the font property, if the element has no parent). When used outside the context of an element (such as in media queries), these units refer to the computed font metrics corresponding to the initial values of the font property.

5.1.2. Viewport-percentage lengths: the vw, vh, vmin, vmax units

The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. However, any scrollbars are assumed not to exist.

For paged media, the exact definition of the viewport-percentage lengths is deferred to [CSS3PAGE].

vw unit

Equal to 1% of the width of the initial containing block.

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 unit

Equal to 1% of the height of the initial containing block.

vmin unit

Equal to the smaller of vw or vh.

vmax unit

Equal to the larger of vw or vh.

5.2. 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 (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 */

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 standard 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 previous versions of CSS. 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 change was made because too much existing content relies on the assumption of 96dpi, and breaking that assumption broke the content.)

Note: Values are 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 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).

6. Other Quantities

6.1. 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.

6.2. 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.

6.3. 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: Values are case-insensitive and serialize as lower case, for example 1Hz serializes as 1hz.

6.4. 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

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.

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) { ... }

7. 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.

7.1. Colors: the <color> type

The <color> data type is defined in [CSS3COLOR]. UAs that support CSS Color Level 3 or its successor must interpret <color> as defined therein.

7.2. Images: the <image> type

The <image> data type is defined in [CSS3-IMAGES]. UAs that support CSS Image Values 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>.

7.3. 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.

The canonical order when serializing is the horizontal component followed by the vertical component.

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. 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.

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);

8.1. Mathematical Expressions: calc()

The calc() function allows mathematical expressions with addition (+), subtraction (-), multiplication (*), and division (/) to be used as component values. The calc() expression represents the result of the mathematical calculation it contains, using standard operator precedence rules. It can be used wherever <length>, <frequency>, <angle>, <time>, <percentage>, <number>, or <integer> values are allowed. Components of a calc() expression can be literal values or attr() or calc() expressions.

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

p {
  margin: calc(1rem - 2px) calc(1rem - 1px);
}

:root {
  font-size: calc(100vw / 40);
}

.foo {
  background: url(top.png), url(bottom.png);
  background-repeat: no-repeat;
  background-position: 50% 50%, calc(50% + 20px) calc(50% + 20px);
}

.foo {
  background-image: linear-gradient(to right, silver,
                                              white 50px,
                                              white calc(100% - 50px),
                                              silver);
}

8.1.1. Syntax

The syntax of a calc() function is:

<calc()> = calc( <calc-sum> )
<calc-sum> = <calc-product> [ [ '+' | '-' ] <calc-product> ]*
<calc-product> = <calc-value> [ '*' <calc-value> | '/' <calc-number-value> ]*
<calc-value> = <number> | <dimension> | <percentage> | ( <calc-sum> )
<calc-number-sum> = <calc-number-product> [ [ '+' | '-' ] <calc-number-product> ]*
<calc-number-product> = <calc-number-value> [ '*' <calc-number-value> | '/' <calc-number-value> ]*
<calc-number-value> = <number> | ( <calc-number-sum> )

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

UAs must support calc() expressions of at least 20 terms, where each NUMBER, DIMENSION, or PERCENTAGE is a term. If a calc() expression contains more than the supported number of terms, it must be treated as if it were invalid.

8.1.2. Type Checking

A math expression has a resolved type, which is one of <length>, <frequency>, <angle>, <time>, <percentage>, <number>, or <integer>. The resolved type must be valid for where the expression is placed; otherwise, the expression is invalid. The resolved type of the expression is determined by the types of the values it contains. <number-token>s are of type <number> or <integer>. A <dimension-token>’s type is given by its unit (cm is <length>, deg is <angle>, etc.). An attr() expression’s type is given by its <type-or-unit> argument.

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

If percentages are accepted in the context in which the expression is placed, and they are defined to be relative to another type besides <number>, a <percentage-token> is treated as that type. For example, in the width property, percentages have the <length> type. A percentage only has the <percentage> type if in that context <percentage> values are not used-value compatible with any other type. If percentages are not normally allowed in place of the calc(), then a calc() expression containing percentages is invalid in that context.

Note: Note that <percentage>s relative to <number>s, such as in opacity, are not allowed in calc(). Allowing this would cause 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.)

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.

Operators form sub-expressions, which gain types based on their arguments. To make expressions simpler, operators have restrictions on the types they accept. At each operator, the types of the left and right argument are checked for these restrictions. If compatible, the type resolves as described below (the following ignores precedence rules on the operators for simplicity):

• At + or -, check that both sides have the same type, or that one side is a <number> and the other is an <integer>. If both sides are the same type, resolve to that type. If one side is a <number> and the other is an <integer>, resolve to <number>.
• At *, check that at least one side is <number>. If both sides are <integer>, resolve to <integer>. Otherwise, resolve to the type of the other side.
• At /, check that the right side is <number>. If the left side is <integer>, resolve to <number>. Otherwise, resolve to the type of the left side.

If an operator does not pass the above checks, the expression is invalid. Also, division by zero is invalid. This includes both dividing by the literal number zero, as well as any numeric expression that evaluates to zero (as purely-numeric expressions can be evaluated without any additional information at parse time).

Note: Algebraic simplifications do not affect the validity of the calc() expression 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.

8.1.3. Computed Value

The computed value of a calc() expression is the expression with all components computed.

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

Given the complexities of width and height calculations on table cells and table elements, math expressions involving percentages 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 MAY be treated as if auto had been specified.

8.1.4. Range Checking

Parse-time range-checking of values is not performed within calc(), and therefore out-of-range values do not cause the declaration to become invalid. However, the value resulting from an expression 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.

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

8.1.5. Serialization

The serialization of calc() values is undefined in this level.

8.2. Attribute References: attr()

The attr() function is allowed as a component value in properties applied to an element or pseudo-element. It returns the value of an attribute on the element. If used on a pseudo-element, it returns the value of the attribute on the pseudo-element’s originating element.

The computed value of the attr() expression is the value of the attribute with the specified name on the element, according to the rules given below.

Note: In CSS2.1 [CSS21], the attr() expression always returns a string. In CSS3, the attr() expression can return many different types. The attr() expression cannot return everything, for example it cannot do counters, named strings, quotes, or keyword values such as auto, nowrap, or baseline. This is intentional, as the intent of the attr() expression is not to make it possible to describe a presentational language’s formatting using CSS, but to enable CSS to take semantic data into account.

The new syntax for the attr() expression is:

attr( <attr-name> <type-or-unit>? [ , <attr-fallback> ]? )

where <attr-name> is a CSS qualified name (the qname production in [CSS3NAMESPACE]) that represents an attribute name. (In the absence of namespacing, this will just be a CSS identifier.) As with attribute selectors, the case-sensitivity of <attr-name> depends on the document language.

The optional <type-or-unit> argument is a keyword drawn from the list below that tells the UA how to interpret the attribute value, and defines a type for the attr() expression. If omitted, string is implied.

The optional <attr-fallback> argument represents a fallback value, which is used if the named attribute is missing, or its value cannot be parsed into the given type or is invalid/out-of-range for the property. If it’s absent, the default value for the given <type-or-unit> (from the list below) is implied.

The attr() expression is only valid if:

• the attr() expression’s type is valid where the attr() expression is placed,
• the namespace prefix of the attribute name, if any, is defined,
• the <attr-fallback> is valid where the attr() expression is placed,
• the <attr-fallback> does not contain another attr() expression,
• and, if the attr() expression is not the sole component value of a property, the <attr-fallback> matches the attr()’s type

If the specified attribute exists on the element, the value of the attribute must be parsed as required by the <type-or-unit> argument (as defined in the list below). Unless the type is string, it must first be stripped of leading and trailing white space. The resulting value is the attr() expression’s value. If the value did not parse as required, the attr() expression’s value is its fallback value.

The <type-or-unit> keywords are:

string

The attribute value is taken as the contents of a CSS <string>. The default is the empty string.

Note: This does not reparse the attribute value with the CSS parser. So, for example, an attribute whose value is "\51" will produce a string containing those three characters, not a string containing "Q" (the character that the escape would evaluate to).

color

The attribute value must parse as a <hash-token> or <ident-token>, and be successfully interpreted as a <color>. The default is currentcolor.

url

The attribute value is taken as the contents of a CSS <string>. It is interpreted as a quoted string within the url() notation. The default is about:invalid, which is a URL defined (in Appendix A) to point to a non-existent document with a generic error condition. Relative URLs must be made absolute according to the rules of the document language as applied to URLs originating from the element; they are not relative to the style sheet.

integer

The attribute value must parse as a <number-token>, and be successfully interpreted as an <integer>. The default is 0, or else the property’s minimum value if 0 is not valid for the property. The default must also be used if the property in question only accepts integers within a certain range and the attribute is out of range.

number

The attribute value must parse as a <number-token>, and is interpreted as an <number>. The default is 0, or else the property’s minimum value if 0 is not valid for the property. The default must also be used if the property in question only accepts integers within a certain range and the attribute is out of range.

length

angle

time

frequency

The attribute value must parse as a <dimension-token>, and be successfully interpreted as the specified type. The default is 0 in the relevant units, or else the property’s minimum value if 0 in the relevant units is not valid for the property. The default must also be used if the property in question only accepts values within a certain range (e.g. positive lengths or angles from 0 to 90deg) and the attribute is out of range (e.g. a negative length or 180deg). If the unit is a relative length, it must be computed to an absolute length.

%

A keyword matching one of the <length>, <angle>, <time>, or <frequency> units

The attribute value must parse as a <number-token>, and is interpreted as a dimension with the specified unit. The default is 0 in the relevant units, or else the property’s minimum value if 0 in the relevant units is not valid for the property. The default must also be used if the property in question only accepts values within a certain range (e.g. positive lengths or angles from 0 to 90deg) and the attribute is out of range (e.g. a negative length or 180deg). If the unit is a relative length, it must be computed to an absolute length.

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

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

content: attr(title color); /* 'content' doesn’t accept colors */

content: attr(end-of-quote string, inherit) close-quote;
/* the 'inherit' value is not allowed there, since the result would be
'inherit close-quote', which is invalid. */

margin: attr(vertical length) attr(horizontal deg);
/* deg units are not valid at that point */

color: attr(color); /* 'color' doesn’t accept strings */

Note: The attr() expression cannot currently fall back onto another attribute. Future versions of CSS may extend attr() in this direction.

9. Appendix A: IANA Considerations

9.1. 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.

Acknowledgments

Comments and suggestions from Giovanni Campagna, Christoph Päper, Keith Rarick, Alex Mogilevsky, Ian Hickson, David Baron, Edward Welbourne, Boris Zbarsky, Björn Höhrmann and Michael Day improved this module.

Changes

Changes since the 14 August 2018 Candidate Recommendation are:

• Defined <‘property’> syntax to refer to the property without any top-level #-multiplier, to make the notation usable with common list-valued property patterns. (Issue 3146)
• Clarified that numeric values outside the allowed range are not ignored if a more specific spec defines other handling. (Issue 3270)

  Properties may restrict numeric values to some range. If the value is outside the allowed range, unless otherwise specified, the declaration is invalid and must be ignored.

• Fixed some background-position examples to be less confusing. (Issue 3482)

A Disposition of Comments is available.

Changes since the 29 September 2016 Candidate Recommendation are:

• Removed consideration of scrollbars in computing viewport units due to lack of implementations. (Issue 15)
• Inlined the <position> definition and dropped the three-value syntaxes to allow for unambiguous combination in complex grammars. This effectively removes that syntax from object-position, but allows <position> to be re-used e.g. in [CSS-TRANSFORMS-1] for 3D positions. (See discussion.)
• Reverted previous change to allow zero angles to drop their unit; this will instead be special-cased where needed for backwards-compatibility. (See discussion)
• Defined that range checking, and any resulting clamping, of calc() values is performed both at computed time and at used time. (Issue #434)
• Fixed grammar error that disallowed numeric expressions as denominators in calc(). (Issue 12)
• Defined handling of font-relative units outside the context of an element. (Issue 9)
• Defined that 0 parses as <number> if it’s ambiguous whether it’s a <number> or a <length>. (Issue 489)
• Defined empty url()s to refer to an invalid URL, rather than resolving to the URL of the style sheet. (Issue 2211)
• Removed (unused) ability for percentages to be treated as a <number> type in calc(). (Issue 1463)
• Clarified that high-resolution screens should anchor on device pixels, not physical units. (Issue 8)
• Clarified definition of url() to normatively say that it accepts unquoted syntax.
• Defined that fragment-only url() are specially handled to always be page-local links, regardless of base-url shenanigans. (See §3.4.1.1 Fragment URLs.)
• Defined attr() parsing in terms of the Syntax spec, not CSS2.1 grammar. (See §8.2 Attribute References: attr().)

A Disposition of Comments is available.

Changes since the 11 June 2015 Candidate Recomendation are:

• Dropped toggle() for lack of implementations.
• Allow zero angles to be represented as 0. (Change due to Web-compatibility constraints in transform and gradient syntaxes.)
• Defined special handling for fragment URLs.
• Defined an empty <url> resolves to an invalid resource.
• Defined compatible units and canonical units for serialization.
• Defined case-sensitivity of url() attribute argument to match attribute selectors.
• Added definition of <ident> notation to definition of identifiers.
• Added <length-percentage> as a shorthand for <length> | <percentage>, along with equivalent productions for angles, numbers, times, and frequencies.
• Allowed <percentage>s inside calc() to resolve as their own type, if they occur in some (as yet theoretical) context where they are not compatible with any other type.
• Various clarifications and editorial improvements.

Changes since the 30 July 2013 Candidate Recommendation are:

• Specified that, in the absence of font information, 1ch equal .5em.
• Added Q unit.
• Relaxed unnecessary restrictions on <custom-ident>. Require specs referencing it to be clear about excluded keywords, because the new rule isn’t as simple.
• Clarified relative URL resolution for embedded style sheets.
• Clarified {A} variant of {A,B} notation.
• Added notation for restricting the length of comma-separated lists specified with the # notation.
• Clarified handling of toggle() when used in shorthand declarations.
• Clarified that stringing together reorderable combinations allows interleaving.
• Changed syntax references from the 2.1 grammar to the Syntax spec.

A Disposition of Comments is available.

Changes since the 28 August 2012 Candidate Recommendation are:

• Corrected wqname in the attr() syntax to qname
• Made undefined namespace prefixes in attr() invalidate the function.
• Per WG resolution, made viewport-percentage units respect scrollbars on the viewport unless overflow is auto (in which case they ignore the presence of scrollbars).
• Deferred exact definition of viewport-percentage units in paged media to CSS Paged Media.
• Added back the <custom-ident> term as a convenience notation, so that other specs can refer to it.

Changes since the 4 April 2013 Candidate Recommendation are:

• Noted that the list of CSS-wide keywords may be expanded by other specs.
• Clarified definition of ex to refer to the “first available font”.
• Specified that attr() with string or url types doesn’t reparse the attribute contents, just takes the value literally as the value of a <string>.

Security and Privacy Considerations

This specification mostly just defines units that are common to CSS specifications, and which present no security concerns.

Note: Does URL handling have a security concern? Probably.

This specification defines units that expose the user’s screen size and default font size, but both are trivially observable from JS, so they do not constitutate a new privacy risk.