CSS Property Values

Values of properties used in CSS.

Integers and real numbers

Some value types may have integer values (denoted by <integer>) or real number values (denoted by <number>).

Note that many properties that allow an integer or real number as a value actually restrict the value to some range, often to a non-negative value.

Real numbers and integers are specified in decimal notation only. Both integers and real numbers may be preceded by a "-" or "+" to indicate the sign. -0 is equivalent to 0 and is not a negative number.

<integer>

An <integer> value consists of one or more digits "0" to "9".

<number>

A <number> value is real number value. A <number> can either be an <integer>, or it can be zero or more digits followed by a dot (.) followed by one or more digits.

Lengths

Lengths refer to distance measurements.

The format of a length value (denoted by >length< in this specification) is a >number< (with or without a decimal point) immediately followed by a unit identifier (e.g., px, em, etc.). After a zero length, the unit identifier is optional.

Some properties allow negative length values, but this may complicate the formatting model and there may be implementation-specific limits. If a negative length value cannot be supported, it should be converted to the nearest value that can be supported.

If a negative length value is set on a property that does not allow negative length values, the declaration is ignored.

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

Relative Length Units

Types of relative length units

em: the 'font-size' of the relevant font
ex: the 'x-height' of the relevant font

h1 { margin: 0.5em } /* em */

h1 { margin: 1ex } /* ex */

The 'em' unit is equal to the computed value of the 'font-size' property of the element on which it is used. The exception is when 'em' occurs in the value of the 'font-size' property itself, in which case it refers to the font size of the parent element. It may be used for vertical or horizontal measurement. (This unit is also sometimes called the quad-width in typographic texts.)

The 'ex' unit is defined by the element's first available font. The exception is when 'ex' occurs in the value of the 'font-size' property, in which case it refers to the 'ex' of the parent element.

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 should be 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 the "h1" elements. On the other hand:

h1 { font-size: 1.2em }

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

When specified for the root of the document tree (e.g., "HTML" in HTML), 'em' and 'ex' refer to the property's initial value. Child elements do not inherit the relative values specified for their parent; they inherit the computed values.

In the following rules, the computed 'text-indent' value of "h1" elements will be 36px, not 45px, if "h1" is a child of the "body" element.

body { font-size: 12px;
text-indent: 3em; /* i.e., 36px */
}
h1 { font-size: 15px }

Absolute Length Units

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

in: inches — 1in is equal to 2.54cm.
cm: centimeters
mm: millimeters
pt: points — the points used by CSS are equal to 1/72nd of 1in.
pc: picas — 1pc is equal to 12pt.
px: pixel units — 1px is equal to 0.75pt.

For a CSS device, these dimensions are either anchored (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 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 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.

Showing that pixels must become larger if the viewing distance increases [D]

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).

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 [D]

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

Percentages

The format of a percentage value (denoted by <percentage> in this specification) is a <number> immediately followed by '%'.

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). When a percentage value is set for a property of the root element and the percentage is defined as referring to the inherited value of some property, the resultant value is the percentage times the initial value of that property.

Since child elements (generally) inherit the computed values of their parent, in the following example, the children of the P element will inherit a value of 12px for 'line-height', not the percentage value (120%):

p { font-size: 10px }
p { line-height: 120% } /* 120% of 'font-size' */

URLs and URIs

URI values (Uniform Resource Identifiers, see [RFC3986], which includes URLs, URNs, etc) in this specification are denoted by <uri>. The functional notation used to designate URIs in property values is "url()", as in:

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

The format of a URI value is 'url(' followed by optional white space followed by an optional single quote (') or double quote (") character followed by the URI itself, followed by an optional single quote (') or double quote (") character followed by optional white space followed by ')'. The two quote characters must be the same.

An example without quotes:

li { list-style: url(http://www.example.com/redball.png) disc }

Some characters appearing in an unquoted URI, such as parentheses, white space characters, single quotes (') and double quotes ("), must be escaped with a backslash so that the resulting URI value is a URI token: '\(', '\)'.

Depending on the type of URI, it might also be possible to write the above characters as URI-escapes (where "(" = %28, ")" = %29, etc.) as described in [RFC3986].

Note that COMMENT tokens cannot occur within other tokens: thus, "url(/*x*/pic.png)" denotes the URI "/*x*/pic.png", not "pic.png".

In order to create modular style sheets that are not dependent on the absolute location of a resource, authors may use relative URIs. Relative URIs (as defined in [RFC3986]) are resolved to full URIs using a base URI. RFC 3986, section 5, defines the normative algorithm for this process. For CSS style sheets, the base URI is that of the style sheet, not that of the source document.

For example, suppose the following rule:

body { background: url("yellow") }

is located in a style sheet designated by the URI:

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 URI

http://www.example.org/style/yellow

User agents may vary in how they handle invalid URIs or URIs that designate unavailable or inapplicable resources.

Counters

Counters are denoted by case-sensitive identifiers (see the 'counter-increment' and 'counter-reset' properties). To refer to the value of a counter, the notation 'counter(<identifier>)' or 'counter(<identifier>, <'list-style-type'>)', with optional white space separating the tokens, is used. The default style is 'decimal'.

To refer to a sequence of nested counters of the same name, the notation is 'counters(<identifier>, <string>)' or 'counters(<identifier>, <string> , <'list-style-type'>)' with optional white space separating the tokens.

See "Nested counters and scope" in the chapter on generated content for how user agents must determine the value or values of the counter. See the definition of counter values of the 'content' property for how it must convert these values to a string.

In CSS 2.1, the values of counters can only be referred to from the 'content' property. Note that 'none' is a possible <'list-style-type'>: 'counter(x, none)' yields an empty string.

Here is a style sheet that numbers paragraphs (p) for each chapter (h1). The paragraphs are numbered with roman numerals, followed by a period and a space:

p {counter-increment: par-num}
h1 {counter-reset: par-num}
p:before {content: counter(par-num, upper-roman) ". "}

Colors

A <color> is either a keyword or a numerical RGB specification.

The list of color keywords is: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, orange, purple, red, silver, teal, white, and yellow. These 17 colors have the following values:

maroon #800000
red #ff0000
orange #ffA500
yellow #ffff00
olive #808000
purple #800080
fuchsia #ff00ff
white #ffffff
lime #00ff00
green #008000
navy #000080
blue #0000ff
aqua #00ffff
teal #008080
black #000000
silver #c0c0c0
gray #808080

In addition to these color keywords, users may specify keywords that correspond to the colors used by certain objects in the user's environment. Please consult the section on system colors for more information.

body {color: black; background: white }
h1 { color: maroon }
h2 { color: olive }

The RGB color model is used in numerical color specifications. These examples all specify the same color:

em { color: #f00 } /* #rgb */
em { color: #ff0000 } /* #rrggbb */
em { color: rgb(255,0,0) }
em { color: rgb(100%, 0%, 0%) }

The format of an RGB value in hexadecimal notation is a '#' immediately followed by either three or six hexadecimal characters. The three-digit RGB notation (#rgb) is converted into six-digit form (#rrggbb) by replicating digits, not by adding zeros. For example, #fb0 expands to #ffbb00. This ensures that white (#ffffff) can be specified with the short notation (#fff) and removes any dependencies on the color depth of the display.

The format of an RGB value in the functional notation is 'rgb(' followed by a comma-separated list of three numerical values (either three integer values or three percentage values) followed by ')'. The integer value 255 corresponds to 100%, and to F or FF in the hexadecimal notation: rgb(255,255,255) = rgb(100%,100%,100%) = #FFF. White space characters are allowed around the numerical values.

All RGB colors are specified in the sRGB color space (see [SRGB]). User agents may vary in the fidelity with which they represent these colors, but using sRGB provides an unambiguous and objectively measurable definition of what the color should be, which can be related to international standards (see [COLORIMETRY]).

Conforming user agents may limit their color-displaying efforts to performing a gamma-correction on them. sRGB specifies a display gamma of 2.2 under specified viewing conditions. User agents should adjust the colors given in CSS such that, in combination with an output device's "natural" display gamma, an effective display gamma of 2.2 is produced. Note that only colors specified in CSS are affected; e.g., images are expected to carry their own color information.

Values outside the device gamut should be clipped or mapped into the gamut when the gamut is known: the red, green, and blue values must be changed to fall within the range supported by the device. Users agents may perform higher quality mapping of colors from one gamut to another. For a typical CRT monitor, whose device gamut is the same as sRGB, the four rules below are equivalent:

em { color: rgb(255,0,0) } /* integer range 0 - 255 */
em { color: rgb(300,0,0) } /* clipped to rgb(255,0,0) */
em { color: rgb(255,-10,0) } /* clipped to rgb(255,0,0) */
em { color: rgb(110%, 0%, 0%) } /* clipped to rgb(100%,0%,0%) */

Other devices, such as printers, have different gamuts than sRGB; some colors outside the 0..255 sRGB range will be representable (inside the device gamut), while other colors inside the 0..255 sRGB range will be outside the device gamut and will thus be mapped.

Note. Mapping or clipping of color values should be done to the actual device gamut if known (which may be larger or smaller than 0..255).

Strings

Strings can either be written with double quotes or with single quotes. Double quotes cannot occur inside double quotes, unless escaped (e.g., as '\"' or as '\22'). Analogously for single quotes (e.g., "\'" or "\27").

"this is a 'string'"
"this is a \"string\""
'this is a "string"'
'this is a \'string\''

A string cannot directly contain a newline. To include a newline in a string, use an escape representing the line feed character in ISO-10646 (U+000A), such as "\A" or "\00000a". This character represents the generic notion of "newline" in CSS. See the 'content' property for an example.

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 (\). 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"] {/*...*/}

Unsupported Values

If a UA does not support a particular value, it should ignore that value when parsing style sheets, as if that value was an illegal value. e.g.

h3{
display: inline;
display: run-in;
}

A UA that supports the 'run-in' value for the 'display' property will accept the first display declaration and then "write over" that value with the second display declaration. A UA that does not support the 'run-in' value will process the first display declaration and ignore the second display declaration.

Sources: