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 [CSS2] 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:

Keyword values (such as auto, disc, etc.), which appear literally, without quotes (e.g. auto).
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.
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. Additionally, if the property’s value grammar is a comma-separated repetition, the corresponding type does not include the top-level comma-separated list multiplier. (E.g. if a property named pairing is defined as [ <custom-ident> <integer>? ]#, then <'pairing'> is equivalent to [ <custom-ident> <integer>? ], not [ <custom-ident> <integer>? ]#.)\

Why remove the multiplier?

The top-level multiplier is ripped out of these value types because top-level comma-separated repetitions are mostly used for coordinating list properties, and when a shorthand combines several such properties, it needs the unmultiplied grammar so it can construct its own comma-separated repetition.

Without this special treatment, every such longhand would have to be defined with an ad-hoc production just for the inner value, which makes the grammars harder to understand overall.
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.
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.

For example, if a function can accept three arguments in order, but all of them are optional, the grammar can be written like:

example( first? , second? , third? )

Given this grammar, writing example(first, second, third) is valid, as is example(first, second) or example(first, third) or example(second). However, example(first, , third) is invalid, as one of those commas are no longer separating two options; similarly, example(,second) and example(first,) are invalid. example(first second) is also invalid, as commas are still required to actually separate the options.

If commas were not implicitly omittable, the grammar would have to be much more complicated to properly express the ways that the arguments can be omitted, greatly obscuring the simplicity of the feature.

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:

	in order	any order
zero or more	`A? B? C?`	`A? \|\| B? \|\| C?`
one or more	`[ A? B? C? ]!`	`A \|\| B \|\| C`
all	`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

Property	Value definition field	Example value
orphans	<integer>	3
text-align	left \| right \| center \| justify	center
padding-top	<length> \| <percentage>	5%
outline-color	<color> \| invert	#fefefe
text-decoration	none \| underline \|\| overline \|\| line-through \|\| blink	overline underline
font-family	[ <family-name> \| <generic-family> ]#	"Gill Sans", Futura, sans-serif
border-width	[ <length> \| thick \| medium \| thin ]{1,4}	2px medium 4px
box-shadow	[ inset? && <length>{2,4} && <color>? ]# \| none	3px 3px rgba(50%, 50%, 50%, 50%), lemonchiffon 0 0 4px inset

3. Textual Data Types

The textual data types include various keywords and identifiers as well as strings (<string>) and URLs (<url>).

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.

3.1. 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).

For example, here is the value definition for the border-collapse property:

Value: collapse | separate

And here is an example of its use:

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. These keywords are normatively defined in the CSS Cascading and Inheritance Module.

Tests

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.

For example, the shorthand declaration animation: ease-in ease-out is equivalent to the longhand declarations animation-timing-function: ease-in; animation-name: ease-out;. ease-in is claimed by the <easing-function> production belonging to animation-timing-function, leaving ease-out to be claimed by the <custom-ident> production belonging to animation-name.

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>. 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].

Double quotes cannot occur inside double quotes, unless escaped (as "\"" or as "\22"). Analogously for single quotes ('\'' or '\27').

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>* )

This example shows a URL being used as a background image:

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

A url() can alternatively be written be written without quotation marks around the URL value, in which case it is specially-parsed as a <url-token>; see CSS Syntax 3 § 4.3.6 Consume a url token. [CSS-SYNTAX-3]

For example, the following declarations are identical:

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

Note: The unquoted url() syntax 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 function wrapper. In such cases the string behaves identically to a url() function containing that string.

For example, the following statements act identically:

@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.

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.

For example, suppose the following rule:

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

is located in a style sheet designated by the URL:

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

The background of the source document’s <body> will be tiled with whatever image is described by the resource designated by the URL:

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

The same image will be used regardless of the URL of the source document containing the <body>.

3.4.1.1. Fragment URLs

To work around some common eccentricities 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.

What “browser eccentricities”?

Theoretically, browsers should re-resolve any relative URLs, including fragment-only URLs, whenever the document’s base URL changes (such as through mutation of the base element, or calling pushState()). In many cases they don’t, however, and so without special handling, fragment-only URLs will suddenly become cross-document references (pointing at the previous base URL) and break in many of the places they’re used.

Since fragment-only URLs express a clear semantic of wanting to refer to the current document regardless of what its current URL is, this hack preserves the expected behavior at least in these cases.

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).

Its computed value is url(""), and it must serialize as such.

Tests

empty.html (live test) (source)

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.

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

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.

4.1. 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.

4.2. 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.

Tests

4.3. Real Numbers: the <number> type

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

Tests

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.

4.4. 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.

Tests

angle-units-003.html (live test) (source)

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

4.4.1. 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.

Tests

calc-serialization-002.html (live test) (source)

4.5. 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.

Tests

4.6. 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>.

For example, the width property can accept a <length> or a <percentage>, both representing a measure of distance. This means that width: calc(500px + 50%); is allowed—both values are converted to absolute lengths and added. If the containing block is 1000px wide, then width: 50%; is equivalent to width: 500px, and width: calc(50% + 500px) thus ends up equivalent to width: calc(500px + 500px) or width: 1000px.

On the other hand, the second and third arguments of the hsl() function can only be expressed as <percentage>s. Although calc() productions are allowed in their place, they can only combine percentages with themselves, as in calc(10% + 20%).

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. 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.

Tests

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.

Tests

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:

Informative Summary of Relative Units
unit	relative to
em	font size of the element
ex	x-height of the element’s font
ch	character advance of the “0” (ZERO, U+0030) glyph in the element’s font
rem	font size of the root element
vw	1% of viewport’s width
vh	1% of viewport’s height
vmin	1% of viewport’s smaller dimension
vmax	1% of viewport’s larger dimension

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

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.

Tests

calc-rem-lang.html (live test) (source)

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.

Tests

ch unit

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).

Tests

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. 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).

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

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—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. 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.

Tests

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 (pixel unit) (px):

unit	name	equivalence
cm	centimeters	1cm = 96px/2.54
mm	millimeters	1mm = 1/10th of 1cm
Q	quarter-millimeters	1Q = 1/40th of 1cm
in	inches	1in = 2.54cm = 96px
pc	picas	1pc = 1/6th of 1in
pt	points	1pt = 1/72nd of 1in
px	pixels	1px = 1/96th of 1in

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 § 8.1 Mathematical Expressions: calc()).

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

For a CSS device, these dimensions are anchored either

by relating the physical units to their physical measurements, or
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 lowercase, for example 1Q serializes as 1q.

Tests

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 diagram illustrates how the definition of a pixel
depends on the users distance from the viewing surface
(paper or screen).
The image depicts the user looking at two planes,
one 28 inches (71 cm) from the user,
the second 140 inches (3.5 m) from the user.
An expanding cone is projected from the user's eye onto each plane.
Where the cone strikes the first plane,
the projected pixel is 0.26 mm high.
Where the cone strikes the second plane,
the projected pixel is 1.4 mm high. — Showing that pixels must become larger if the viewing distance increases

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).

This diagram illustrates the relationship between
the reference pixel and device pixels (called “dots” below).
The image depicts
a high resolution (large dot density) laser printer output on the left
and a low resolution monitor screen on the right.
For the laser printer, one square reference pixel is implemented by 16 dots.
For the monitor screen, one square reference pixel is implemented by a single dot. — Showing that more device pixels (dots) are needed to cover a 1px by 1px area on a high-resolution device than on a lower-resolution one (of the same approximate viewing distance)

Tests

absolute-length-units-001.html (live test) (source)

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.

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.

By convention, when an angle denotes a direction in CSS, it is typically interpreted as a bearing angle, where 0deg is "up" or "north" on the screen, and larger angles are more clockwise (so 90deg is "right" or "east").

For example, in the linear-gradient() function, the <angle> that determines the direction of the gradient is interpreted as a bearing angle.

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.

Tests

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.

Tests

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: Units are ASCII case-insensitive and serialize as lowercase, 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.

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.

The following @media rule uses Media Queries [MEDIAQ] to assign some special style rules to devices that use two or more device pixels per CSS px unit:

@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 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>.

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 computed and interpreted as specified for background-position. [CSS3-BACKGROUND]

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

For example, transform-origin defines a 3D position as (effectively) <position> <length>?. A value such as left 50px will be parsed as a 2-value <position>, with an omitted z-component; on the other hand, a value such as top 50px will be parsed as a single-value <position> followed by a <length>.

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. Like keywords, function names are ASCII case-insensitive. White space is allowed, but optional, immediately inside the parentheses. Functions can take multiple arguments, which are formatted similarly to a CSS property value.

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);

8.1. Mathematical Expressions: calc()

The calc() function allows a numeric CSS component value to be written as a mathematical expression using addition (+), subtraction (-), multiplication (*), and/or division (/).

The calc() expression represents the result of the mathematical calculation it contains, which is evaluated using standard operator precedence rules (* and / bind tighter than + and -, and operators are otherwise evaluated left-to-right).

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 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);
}

The following sets the font-size so that exactly 40em fits within the viewport, ensuring that roughly the same amount of text always fills the screen no matter the screen size.

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

If the rest of the design is specified using the rem unit, the entire layout will scale to match the viewport width.

The following example stacks two background images, with the first perfectly centered and the second offset 20px to the bottom and to the left of the first.

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

This example shows how to place color-stops on a gradient an equal distance from either end.

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

Tests

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 / operators 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.).

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: Although 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.

Tests

calc-unit-analysis.html (live test) (source)

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.

For example, whereas font-size computes percentage values at computed value time so that font-relative length units can be computed, background-position has layout-dependent behavior for percentage values, and thus does not resolve percentages until used-value time.

Due to this, background-position computation preserves the percentage in a calc() whereas font-size will compute such expressions directly into a length.

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.

Tests

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.

Since widths smaller than 0px are not allowed, these three declarations are equivalent:

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

Note however that width: -5px is not equivalent to width: calc(-5px)! Out-of-range values outside calc() are syntactically invalid, and cause the entire declaration to be dropped.

Tests

calc-numbers.html (live test) (source)

8.1.5. Serialization

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

Appendix A: 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.

Registered Token	`invalid`
Intended Usage	The `about:invalid` URL references a non-existent document with a generic error condition. It can be used when a URL is necessary, but the default value shouldn’t be resolvable as any type of document.
Contact/Change controller	CSS WG <[email protected]> (on behalf of W3C)
Specification	CSS Values and Units Module Level 3

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 1 December 2022 Candidate Recommenation Snapshot are:

Editorial synchronization with CSS Values and Units Module Level 4.
Added definition for <func()> notation to the value definition syntax.
Clarified stacking of multipliers.
Clarified that <resolution> cannot be negative.

Changes since the 6 June 2019 Candidate Recommendation are:

Dropped the attr() function, since it was punted to Level 5.
Specified serialization of empty urls to be url(""). (Issue 6447)
Clarified that the font-relative lengths are calculated without text shaping. (Issue 5498)
Added a definition for device pixel. (Issue 7287)
Cleaned up interaction of this specification and [CSS-CASCADE-3] in defining the CSS-wide keywords. (Issue 7439)
Removed definition of <length-number>, since it is impossible to combine them with calc(). (Issue 2789)
Fixed definition of numbers to allow decimals in combination with scientific notation, as originally intended and as defined in [CSS-SYNTAX-3]. (Issue 7248)
Editorial improvements.

Changes since the 31 January 2019 Candidate Recommendation are:

Added the new bracketed range notation to the CSS value definition syntax. This has no normative implications on implementations, just allows more routine annotation of ranges in future CSS specifications. (Issue 355)

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.

A Disposition of Comments is available.

Changes since the 11 June 2015 Candidate Recommendation 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 Considerations

This specification presents no new security considerations.

This specification defines the url() function (<url>), which allows 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 introduces 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).

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

CSS Values and Units Module Level 3

Abstract

Status of this document

1. Introduction

1.1. Module Interactions

2. Value Definition Syntax

2.1. Component Value Types

2.2. Component Value Combinators

2.3. Component Value Multipliers

2.4. Combinator and Multiplier Patterns

2.5. Component Values and White Space

2.6. Property Value Examples

3. Textual Data Types

3.1. Pre-defined Keywords

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

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

3.3. Quoted Strings: the <string> type

3.4. Resource Locators: the <url> type

3.4.1. Relative URLs

3.4.1.1. Fragment URLs

3.4.2. Empty URLs

3.4.3. URL Modifiers

4. Numeric Data Types

4.1. Range Restrictions and Range Definition Notation

4.2. Integers: the <integer> type

4.3. Real Numbers: the <number> type

4.4. Numbers with Units: dimension values

4.4.1. Compatible Units

4.5. Percentages: the <percentage> type

4.6. Mixing Percentages and Dimensions

5. Distance Units: the <length> type

5.1. Relative Lengths

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

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

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

6. Other Quantities

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

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

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

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

7. Data Types Defined Elsewhere

7.1. Colors: the <color> type

7.2. Images: the <image> type

7.3. 2D Positioning: the <position> type

8. Functional Notations

8.1. Mathematical Expressions: calc()

8.1.1. Syntax

8.1.2. Type Checking

8.1.3. Computed Value

8.1.4. Range Checking

8.1.5. Serialization

Appendix A: IANA Considerations

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Acknowledgments

Changes

Security Considerations

Privacy Considerations

Conformance

Document conventions

Conformance classes

Partial implementations

Implementations of Unstable and Proprietary Features

Non-experimental implementations

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

Informative References

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