CSS Values and Units Module Level 3

Editor’s Draft,

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This CSS module describes the common values and units that CSS properties accept and the syntax used for describing them in CSS property definitions.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-values” in the title, preferably like this: “[css-values] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 September 2015 W3C Process Document.

The following features are at-risk, and may be dropped during the CR period:

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

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, 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.
  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:

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 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:

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:

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
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
text-shadow [ inset? && [ <length>{2,4} && <color>? ] ]# | none 3px 3px rgba(50%, 50%, 50%, 50%), lemonchiffon 0 0 4px inset

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).

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.

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.

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 <single-timing-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> 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].

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

A URL is a pointer to a resource and is a functional notation denoted by <url>. The syntax of a <url> is:

<url> = url( <string> <url-modifier>* )
Below is an example of a URL being used as a background image:
body { background: url("http://www.example.com/pinkish.gif") }

In addition to the syntax defined above, a <url> can sometimes be written in other ways:

Note: The special parsing rules for the legacy quotation-mark–less <url> syntax means that 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). 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]. Alternately, the URL can be quoted as a string, in which case only newlines and the character used to quote the string need to be escaped.

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.

For example, suppose the following rule:
body { background: url("tile.png") }

is located in a style sheet designated by the URL:


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


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

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).

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, 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. 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.

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:


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


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


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


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


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

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.

4.4. Numbers with Units: dimensions

The general term "dimension" refers to a number with a unit attached to it, and are 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.

4.5. 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.

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. However, for zero lengths the unit identifier is optional (i.e. can be syntactically represented as the <number> 0).

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:

Informative Summary of Relative Units
unit relative to
em font size of the element
ex x-height of the element’s font
ch width 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

Aside from rem (which refers to the font-size of the root element), the font-relative lengths refer to the font metrics of the element on which they are used. The exception is when they occur in the value of the font-size property itself, in which case they 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).

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. When specified on the font-size property of the root element, the rem units refer to the property’s initial value.

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, when the value of overflow on the root element is auto, any scroll bars are assumed not to exist. Note that the initial containing block’s size is affected by the presence of scrollbars on the viewport.

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):

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 = 1/72th 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 */

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 and similar high-resolution devices, the anchor unit should be one of the standard physical units (inches, centimeters, etc). For lower-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.)

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 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)

6. Other Quantities

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

Angle values are <dimension>s denoted by <angle>. However, for zero angles the unit identifier is optional (i.e. can be syntactically represented as the number 0). The angle unit identifiers are:

Degrees. There are 360 degrees in a full circle.
Gradians, also known as "gons" or "grades". There are 400 gradians in a full circle.
Radians. There are 2π radians in a full circle.
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.

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

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

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:

Hertz. It represents the number of occurrences per second.
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.

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

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

Dots per inch.
Dots per centimeter.
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 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 Image Values Level 3 or its successor must interpret <image> as defined therein.

7.3. 2D Positioning: the <position> type

The <position> data type is defined in [CSS3BG]. UAs that support CSS Backgrounds & Borders Level 3 or its successor must interpret <position> as defined therein.

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);
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 centered background images, with one offset slightly from the other.
.foo {
  background: url(top.png), url(bottom.png);
  background-repeat: no-repeat;
  background-position: calc(50% + 20px) calc(50% + 20px), 50% 50%;
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),

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> | '/' <number> ]*
<calc-value> = <number> | <dimension> | <percentage> | ( <calc-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.

If percentages are accepted in the context in which the expression is placed, a <percentage-token> has the type of the value that percentages are relative to. For example, in the width property, percentages have the <length> type, and in opacity they have the <number> 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.

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):

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.

For example, background-position has special behavior for percentage values, different from lengths:
.foo {
  width: 200px;
  background-image: url(bg.png);
  background-position: left 50%;
  /* different than: */
  background-position: left 100px;
  /* despite 50% of 200px being 100px */

Due to this, background-position preserves the percentage in a calc() rather than resolving it directly into a length, so that an expression like background-position: left calc(50% + 20px) center properly centers the background and then shifts it 20px to the right, rather than placing its left edge 20px off of center.

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 used value resulting from an expression must be clamped to the range allowed in the target context.

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 synactically invalid, and cause the entire declaration to be dropped.

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:

Note that the default value need not be of the type given, if the attr() expression is the entire property value. For instance, if the type required of the attribute by the author is px, the default could still be auto, like in width: attr(size px, auto);.

If the attr() is used alongside other values to form the full property value, however, then the default value must match the attr()'s type. For example, box-shadow: attr(size px, inset) 5px 10px blue; is invalid, even though it would create a valid declaration if you substituted the attr() expression with either a px length or the inset keyword.

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:

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).

The attribute value must parse as a HASH or IDENT CSS token, and be successfully interpreted as a <color>. The default is currentcolor.
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.
The attribute value must parse as a NUMBER CSS 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.
The attribute value must parse as a NUMBER CSS 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.
The attribute value must parse as a DIMENSION CSS 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 CSS 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.
This example shows the use of attr() to visually illustrate data in an XML file:
  <wood length="12"/>
  <wood length="5"/>
  <metal length="19"/>
  <wood length="4"/>

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);
All of the following examples are invalid and would cause a parse-time error, and thus cause the relevant declaration—in this case all of them—to be ignored:
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.

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 resolveable as any type of document.
Contact/Change controller CSS WG <www-style@w3.org> (on behalf of W3C)
Specification CSS Values and Units Module Level 3


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 since the 11 June 2015 Candidate Recomendation are:

Changes since the 30 July 2013 Candidate Recommendation are:

A Disposition of Comments is available.

Changes since the 28 August 2012 Candidate Recommendation are:

Changes since the 4 April 2013 Candidate Recommendation are:

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.


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:

This is an example of an informative example.

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

Note, this is an informative note.

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

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

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.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 property 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

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

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.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.


Terms defined by this specification

Terms defined by reference


Normative References

Rik Cabanier; Nikos Andronikos. Compositing and Blending Level 1. 13 January 2015. CR. URL: http://dev.w3.org/fxtf/compositing-1/
CSS Backgrounds and Borders Module Level 3 URL: https://drafts.csswg.org/css-backgrounds-3/
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. 14 January 2016. CR. URL: http://dev.w3.org/csswg/css-cascade/
CSS Color Module Level 3 URL: https://drafts.csswg.org/css-color-3/
Tab Atkins Jr.; Chris Lilley. CSS Color Module Level 4. 5 July 2016. WD. URL: https://drafts.csswg.org/css-color/
Tab Atkins Jr.. CSS Counter Styles Level 3. 11 June 2015. CR. URL: http://dev.w3.org/csswg/css-counter-styles/
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 15 October 2015. WD. URL: http://dev.w3.org/csswg/css-display/
CSS Image Values and Replaced Content Module Level 3 URL: https://drafts.csswg.org/css-images-3/
CSS Overflow Module Level 4 URL: https://drafts.csswg.org/css-overflow-4/
Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. 15 December 2015. CR. URL: http://dev.w3.org/csswg/css-writing-modes-3/
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS2
John Daggett. CSS Fonts Module Level 3. 3 October 2013. CR. URL: http://dev.w3.org/csswg/css-fonts/
Elika Etemad; Tab Atkins Jr.. CSS Image Values and Replaced Content Module Level 3. 17 April 2012. CR. URL: https://www.w3.org/TR/css3-images/
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 9 September 2014. CR. URL: http://dev.w3.org/csswg/css3-background/
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 3. 19 May 2016. CR. URL: https://drafts.csswg.org/css-cascade-3/
Tantek Çelik; Chris Lilley; David Baron. CSS Color Module Level 3. 7 June 2011. REC. URL: https://www.w3.org/TR/css3-color
Elika Etemad. CSS Namespaces Module Level 3. 20 March 2014. REC. URL: http://dev.w3.org/csswg/css-namespaces/
Melinda Grant; et al. CSS Paged Media Module Level 3. 14 March 2013. WD. URL: https://www.w3.org/TR/css3-page/
Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. 20 February 2014. CR. URL: http://dev.w3.org/csswg/css-syntax/
Simon Pieters; Glenn Adams. CSS Object Model (CSSOM). 17 March 2016. WD. URL: https://drafts.csswg.org/cssom/
Ian Hickson. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
Selectors Level 4 URL: https://drafts.csswg.org/selectors-4/
Anne van Kesteren. URL Standard. Living Standard. URL: https://url.spec.whatwg.org/

Informative References

CSS Animations Module Level 1 URL: https://drafts.csswg.org/css-animations-1/
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 3. 14 January 2016. CR. URL: http://dev.w3.org/csswg/css-break/
Elika Etemad; Koji Ishii. CSS Text Module Level 3. 10 October 2013. LCWD. URL: http://dev.w3.org/csswg/css-text-3/
Elika Etemad; Koji Ishii. CSS Text Decoration Module Level 3. 1 August 2013. CR. URL: http://dev.w3.org/csswg/css-text-decor-3/
CSS Transitions Module Level 1 URL: https://drafts.csswg.org/css-transitions-1/
Tantek Çelik; Florian Rivoal. CSS Basic User Interface Module Level 3 (CSS3 UI). 7 July 2015. CR. URL: http://dev.w3.org/csswg/css-ui/
Florian Rivoal; et al. Media Queries. 19 June 2012. REC. URL: https://www.w3.org/TR/css3-mediaqueries/
S. Moonesamy, Ed.. The "about" URI Scheme. August 2012. Informational. URL: https://tools.ietf.org/html/rfc6694