CSS Values and Units Module Level 3

Group: CSSWG
Shortname: css-values
Level: 3
Status: ED
Work Status: Testing
ED: http://dev.w3.org/csswg/css-values/
TR: http://www.w3.org/TR/css-values/
Previous Version: http://www.w3.org/TR/2013/CR-css3-values-20130730/
Previous Version: http://www.w3.org/TR/2013/CR-css3-values-20130404/
Previous Version: http://www.w3.org/TR/2012/CR-css3-values-20120828/
Previous Version: http://www.w3.org/TR/2012/WD-css3-values-20120308/
Previous Version: http://www.w3.org/TR/2011/WD-css3-values-20110906/
Previous Version: http://www.w3.org/TR/2006/WD-css3-values-20060919
Previous Version: http://www.w3.org/TR/2005/WD-css3-values-20050726
Previous Version: http://www.w3.org/TR/2001/WD-css3-values-20010713/
Editor: Tab Atkins, Google, http://xanthir.com/contact/
Editor: fantasai, http://fantasai.inkedblade.net/contact
Former Editor: Håkon Wium Lie, Opera Software, howcome@opera.com
Abstract: This CSS3 module describes the common values and units that CSS properties accept and the syntax used for describing them in CSS property definitions.
Issue Tracking: Tracker http://www.w3.org/Style/CSS/Tracker/products/33
At Risk: ''toggle()'', ''attr()''
Ignored Terms: , containing block


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 (<> being the most widely used) that can be used by many properties are described in this specification, while more specific data types (e.g., <>) are described in the corresponding modules.

Module Interactions

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

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., <>, <>, 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 <>. Notice the distinction between <> 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 omittable 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.

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 re-orderable 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

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.

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.

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 <> 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''.

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 ''text-align/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''

Textual Data Types

An identifier is a sequence of characters conforming to the <> grammar. [[!CSS3SYN]] Identifiers cannot be quoted; otherwise they would be interpreted as a string.

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 }

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.

Author-defined Identifiers: the <> 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 <>s. The ''default'' keyword is reserved and is also not a valid <>. Specifications using <> must specify clearly what other keywords are excluded from <>, 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 <> 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 <> production belonging to 'animation-timing-function', leaving ''ease-out'' to be claimed by the <> production belonging to 'animation-name'.
Note: When designing grammars with <>, the <> should always be "positionally unambiguous", so that it's impossible to conflict with any keyword values in the property.

Quoted Strings: the <> type

Strings are denoted by <string> and consist of a sequence of characters delimited by double quotes or single quotes. They correspond to the <> 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.)

Resource Locators: the <> type

A URL is a pointer to a resource and is a functional notation denoted by <>. The syntax of a <> is:
<url> = url( <> <>* )
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 <> can sometimes be written in other ways: * For legacy reasons, a <> can be written without quotation marks around the URL itself. This syntax is specially-parsed, and produces a <> rather than a function syntactically. [[!CSS3SYN]] * Some CSS contexts, such as ''@import'', allow a <> to be represented by a <> instead. This behaves identically to writing a ''url()'' function containing that string. Because these alternate ways of writing a <> are not functional notations, they cannot accept any <>s. Note: The special parsing rules for the legacy quotation mark-less <> 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]]. (If written as a normal function containing a string, ordinary string escaping rules apply; only newlines and the character used to quote the string need to be escaped.)

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 <> 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>.

URL Modifiers

The ''url()'' function supports specifying additional <url-modifier>s, which change the meaning or the interpretation of the URL somehow. A <> is either an <> or a function. This specification does not define any <>s, but other specs may do so.

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.

Integers: the <> type

Integer values are denoted by <integer>. An integer is one or more decimal digits ''0'' through ''9'' and corresponds to a subset of the <> 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.

Real Numbers: the <> type

Number values are denoted by <number>. A number is either an <> 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 <> 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.

Percentages: the <> type

A percentage value is denoted by <percentage>, and consists of a <> immediately followed by a percent sign ''%''. It corresponds to the <> production in the CSS Syntax Module [[!CSS3SYN]]. Percentage values are always relative to another value, for example a length. Each property that allows percentages also defines the value to which the percentage refers. The value may be that of another property for the same element, a property for an ancestor element, or a value of the formatting context (e.g., the width of a containing block).

Numbers with Units: dimensions

A dimension is a <> immediately followed by a unit identifier. It corresponds to the <> production in the CSS Syntax Module [[!CSS3SYN]]. Like keywords, unit identifiers are ASCII case-insensitive.

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

Distance Units: the <> 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 <> ''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.

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
unitrelative 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% 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.

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. In the cases where it is impossible or impractical to determine the measure of the "0" glyph, a value of 0.5em must be assumed.
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.

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 ''overflow/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''.

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 */
For a CSS device, these dimensions are anchored either
  • (i) by relating the physical units to their physical measurements, or
  • (ii) 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: Note that 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: Note that 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 breaks 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 low-res one

Other Quantities

Angle Units: the <> type and ''deg'', ''grad'', ''rad'', ''turn'' units

Angle values are dimensions denoted by <angle>. 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''. When an angle denotes a direction, it must always be 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 <> that determines the direction of the gradient is interpreted as a bearing angle.

Duration Units: the <

Time values are dimensions denoted by <time>. The time unit identifiers are:
Milliseconds. There are 1000 milliseconds in a second.
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.

Frequency Units: the <> 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.

Resolution Units: the <> 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 <> 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]]. Note: 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) { ... }

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.

Colors: the <> type

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

Images: the <> type

The <> data type is defined in [[!CSS3-IMAGES]]. UAs that support CSS Image Values Level 3 or its successor must interpret <> as defined therein.

2D Positioning: the <> type

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

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 <>) 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, whitespace 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);

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 <>, <>, <>, <