Formatting Text with
Cascading Style Sheets
CSS1 properties
Notation for property values
Font
properties
Font matching
'font-style'
'font-variant'
'font-weight'
'font-size'
'font'
Color
and background properties
'color'
'background-color'
'background-image'
'background-repeat'
'background-attachment'
'background-position'
'background'
Text
properties
'word-spacing'
'letter-spacing'
'text-decoration'
'vertical-align'
'text-transform'
'text-align'
'text-indent'
'line-height'
Style sheets influence the presentation of documents by assigning values to style properties. This section lists the defined style properties, and their corresponding list of possible values, of CSS.
In the text below, the allowed values for each property are listed with a syntax like the following:
Value: N | NW | NE
Value: [ <length> | thick | thin ]{1,4}
Value: [<family-name> , ]* <family-name>
Value: <url>? <color> [ / <color> ]?
Value: <url> || <color>
The words between "<" and ">" give a type of value. The most common types are <length>, <percentage>, <url>, <number> and <color>. The more specialized types (e.g. <font-family> and <border-style>) are described under the corresponding property.
Other words are keywords that must appear literally, without quotes. The slash (/) and the comma (,) must also appear literally.
Several things juxtaposed mean that all of them must occur, in the given order. A bar (|) separates alternatives: one of them must occur. A double bar (A || B) means that either A or B or both must occur, in any order. Brackets ([]) are for grouping. Juxtaposition is stronger than the double bar, and the double bar is stronger than the bar. Thus "a b | c || d e" is equivalent to "[ a b ] | [ c || [ d e ]]".
Every type, keyword, or bracketed group may be followed by one of the following modifiers:
Setting font properties will be among the most common uses of style sheets. Unfortunately, there exists no well-defined and universally accepted taxonomy for classifying fonts, and terms that apply to one font family may not be appropriate for others. E.g. 'italic' is commonly used to label slanted text, but slanted text may also be labeled as being Oblique, Slanted, Incline, Cursive or Kursiv. Therefore it is not a simple problem to map typical font selection properties to a specific font.
CSS1 defines the properties 'font-family', 'font-style', 'font-variant' and 'font-weight', 'font-size', 'font'.
Because there is no accepted, universal taxonomy of font properties, matching of properties to font faces must be done carefully. The properties are matched in a well-defined order to insure that the results of this matching process are as consistent as possible across UAs (assuming that the same library of font faces is presented to each of them).
(The above algorithm can be optimized to avoid having to revisit the CSS1 properties for each character.)
The per-property matching rules from (2) above are as follows:
Value: [[<family-name>
| <generic-family>],]* [<family-name> | <generic-family>]
Initial: UA specific
Applies to: all elements
Inherited: yes
Percentage values: N/A
The value is a prioritized list of font family names and/or generic family names. Unlike most other CSS1 properties, values are separated by a comma to indicate that they are alternatives:
BODY { font-family: gill, helvetica, sans-serif }
There are two types of list values:
Style sheet designers are encouraged to offer a generic font family as a last alternative.
Font names containing whitespace should be quoted:
BODY { font-family: "new century schoolbook", serif }
<BODY STYLE="font-family: 'My own font', fantasy">
If quoting is omitted, any whitespace characters before and after the font name are ignored and any sequence of whitespace characters inside the font name is converted to a single space.
Value: normal | italic
| oblique
Initial: normal
Applies to: all elements
Inherited: yes
Percentage values: N/A
The 'font-style' property selects between normal (sometimes referred to as "roman" or "upright"), italic and oblique faces within a font family.
A value of 'normal' selects a font that is classified as 'normal' in the UA's font database, while 'oblique' selects a font that is labeled 'oblique'. A value of 'italic' selects a font that is labeled 'italic', or, if that is not available, one labeled 'oblique'.
The font that is labeled 'oblique' in the UA's font database may actually have been generated by electronically slanting a normal font.
Fonts with Oblique, Slanted or Incline in their names will typically be labeled 'oblique' in the UA's font database. Fonts with Italic, Cursive or Kursiv in their names will typically be labeled 'italic'.
H1, H2, H3 { font-style: italic }
H1 EM { font-style: normal }
In the example above, emphasized text within 'H1' will appear in a normal face.
Value: normal | small-caps
Initial: normal
Applies to: all elements
Inherited: yes
Percentage values: N/A
Another type of variation within a font family is the small-caps. In a small-caps font the lower case letters look similar to the uppercase ones, but in a smaller size and with slightly different proportions. The 'font-variant' property selects that font.
A value of 'normal' selects a font that is not a small-caps font, 'small-caps' selects a small-caps font. It is acceptable (but not required) in CSS1 if the small-caps font is a created by taking a normal font and replacing the lower case letters by scaled uppercase characters. As a last resort, uppercase letters will be used as replacement for a small-caps font.
The following example results in an 'H3' element in small-caps, with emphasized words in oblique small-caps:
H3 { font-variant: small-caps }
EM { font-style: oblique }
There may be other variants in the font family as well, such as fonts with old-style numerals, small-caps numerals, condensed or expanded letters, etc. CSS1 has no properties that select those.
CSS1 core: insofar as this property causes text to be transformed to uppercase, the same considerations as for 'text-transform' apply.
Value: normal | bold
| bolder | lighter | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900
Initial: normal
Applies to: all elements
Inherited: yes
Percentage values: N/A
The 'font-weight' property selects the weight of the font. The values '100' to '900' form an ordered sequence, where each number indicates a weight that is at least as dark as its predecessor. The keyword 'normal' is synonymous with '400', and 'bold' is synonymous with '700'. Keywords other than 'normal' and 'bold' have been shown to be often confused with font names and a numerical scale was therefore chosen for the 9-value list.
P { font-weight: normal } /* 400 */
H1 { font-weight: 700 } /* bold */
The 'bolder' and 'lighter' values select font weights that are relative to the weight inherited from the parent:
STRONG { font-weight: bolder }
Child elements inherit the resultant weight, not the keyword value.
Fonts (the font data) typically have one or more properties whose values are names that are descriptive of the "weight" of a font. There is no accepted, universal meaning to these weight names. Their primary role is to distinguish faces of differing darkness within a single font family. Usage across font families is quite variant; for example a font that you might think of as being bold might be described as being Regular, Roman, Book, Medium, Semi- or DemiBold, Bold, or Black, depending on how black the "normal" face of the font is within the design. Because there is no standard usage of names, the weight property values in CSS1 are given on a numerical scale in which the value '400' (or 'normal') corresponds to the "normal" text face for that family. The weight name associated with that face will typically be Book, Regular, Roman, Normal or sometimes Medium.
The association of other weights within a family to the numerical weight values is intended only to preserve the ordering of darkness within that family. However, the following heuristics tell how the assignment is done in typical cases:
The following two examples illustrate the process. Assume four weights in the "Example1" family, from lightest to darkest: Regular, Medium, Bold, Heavy. And assume six weights in the "Example2" family: Book, Medium, Bold, Heavy, Black, ExtraBlack. Note how in the second example it has been decided not to assign "Example2 ExtraBlack" to anything.
Available faces | Assignments | Filling the holes
----------------------+---------------+-------------------
"Example1 Regular" | 400 | 100, 200, 300
"Example1 Medium" | 500 |
"Example1 Bold" | 700 | 600
"Example1 Heavy" | 800 | 900
Available faces | Assignments | Filling the holes
----------------------+---------------+-------------------
"Example2 Book" | 400 | 100, 200, 300
"Example2 Medium" | 500 |
"Example2 Bold" | 700 | 600
"Example2 Heavy" | 800 |
"Example2 Black" | 900 |
"Example2 ExtraBlack" | (none) |
Since the intent of the relative keywords 'bolder' and 'lighter' is to darken or lighten the face within the family and because a family may not have faces aligned with all the symbolic weight values, the matching of 'bolder' is to the next darker face available on the client within the family and the matching of 'lighter' is to the next lighter face within the family. To be precise, the meaning of the relative keywords 'bolder' and 'lighter' is as follows:
There is no guarantee that there will be a darker face for each of the 'font-weight' values; for example, some fonts may have only a normal and a bold face, others may have eight different face weights. There is no guarantee on how a UA will map font faces within a family to weight values. The only guarantee is that a face of a given value will be no less dark than the faces of lighter values.
Value: <absolute-size>
| <relative-size> | <length> | <percentage>
Initial: medium
Applies to: all elements
Inherited: yes
Percentage values: relative to parent element's font size
Length and percentage values should not take the font size table into account when calculating the font size of the element.
Negative values are not allowed.
On all other properties, 'em' and 'ex' length values refer to the font size of the current element. On the 'font-size' property, these length units refer to the font size of the parent element.
Note that an application may reinterpret an explicit size, depending on the context. E.g., inside a VR scene a font may get a different size because of perspective distortion.
Examples:
P { font-size: 12pt; }
BLOCKQUOTE { font-size: larger }
EM { font-size: 150% }
EM { font-size: 1.5em }
Value: [ <font-style>
|| <font-variant> || <font-weight> ]? <font-size> [ / <line-height>
]? <font-family>
Initial: not defined for shorthand properties
Applies to: all elements
Inherited: yes
Percentage values: allowed on <font-size> and <line-height>
The 'font' property is a shorthand property for setting 'font-style' 'font-variant' 'font-weight' 'font-size', 'line-height' and 'font-family' at the same place in the style sheet. The syntax of this property is based on a traditional typographical shorthand notation to set multiple properties related to fonts.
For a definition of allowed and initial values, see the previously defined properties. Properties for which no values are given are set to their initial value.
P { font: 12pt/14pt sans-serif }
P { font: 80% sans-serif }
P { font: x-large/110% "new century schoolbook", serif }
P { font: bold italic large Palatino, serif }
P { font: normal small-caps 120%/120% fantasy }
In the second rule, the font size percentage value ('80%') refers to the font size of the parent element. In the third rule, the line height percentage refers to the font size of the element itself.
In the first three rules above, the 'font-style', 'font-variant' and 'font-weight' are not explicitly mentioned, which means they are all three set to their initial value ('normal'). The fourth rule sets the 'font-weight' to 'bold', the 'font-style' to 'italic' and implicitly sets 'font-variant' to 'normal'.
The fifth rule sets the 'font-variant' ('small-caps'), the 'font-size' (120% of the parent's font), the 'line-height' (120% times the font size) and the 'font-family' ('fantasy'). It follows that the keyword 'normal' applies to the two remaining properties: 'font-style' and 'font-weight'.
These properties describe the color (often called foreground color) and background of an element (i.e. the surface onto which the content is rendered). One can set a background color and/or a background image. The position of the image, if/how it is repeated, and whether it is fixed or scrolled relative to the canvas can also be set.
The 'color' property inherits normally. The background properties do not inherit, but the parent element's background will shine through by default because of the initial 'transparent' value on 'background-color'.
Value: <color>
Initial: UA specific
Applies to: all elements
Inherited: yes
Percentage values: N/A
This property describes the text color of an element (often referred to as the foreground color). There are different ways to specify red:
EM { color: red } /* natural language */
EM { color: rgb(255,0,0) } /* RGB range 0-255 */
Value: <color>
| transparent
Initial: transparent
Applies to: all elements
Inherited: no
Percentage values: N/A
This property sets the background color of an element.
H1 { background-color: #FFCC00 }
Value: <url> |
none
Initial: none
Applies to: all elements
Inherited: no
Percentage values: N/A
This property sets the background image of an element. When setting a background image, one should also set a background color that will be used when the image is unavailable. When the image is available, it is overlaid on top of the background color.
BODY { background-image: url(marble.gif) }
P { background-image: none }
Value: repeat | repeat-x
| repeat-y | no-repeat
Initial: repeat
Applies to: all elements
Inherited: no
Percentage values: N/A
If a background image is specified, the value of 'background-repeat' determines how/if the image is repeated.
A value of 'repeat' means that the image is repeated both horizontally and vertically. The 'repeat-x' ('repeat-y') value makes the image repeat horizontally (vertically), to create a single band of images from one side to the other. With a value of 'no-repeat', the image is not repeated.
BODY {
background: red url(pendant.gif);
background-repeat: repeat-y;
}
In the example above, the image will only be repeated vertically.
Value: scroll | fixed
Initial: scroll
Applies to: all elements
Inherited: no
Percentage values: N/A
If a background image is specified, the value of 'background-attachment' determines if it is fixed with regard to the canvas or if it scrolls along with the content.
BODY {
background: red url(pendant.gif);
background-repeat: repeat-y;
background-attachment: fixed;
}
CSS1 core: UAs may treat 'fixed' as 'scroll'. However, it is recommended they interpret 'fixed' correctly, at least on the HTML and BODY elements, since there is no way for an author to provide an image only for those browsers that support 'fixed'.
Value: [<percentage>
| <length>]{1,2} | [top | center | bottom] || [left | center | right]
Initial: 0% 0%
Applies to: block-level and replaced elements
Inherited: no
Percentage values: refer to the size of the element itself
If a background image has been specified, the value of 'background-position' specifies its initial position.
With a value pair of '0% 0%', the upper left corner of the image is placed in the upper left corner of the box that surrounds the content of the element (i.e., not the box that surrounds the padding, border or margin). A value pair of '100% 100%' places the lower right corner of the image in the lower right corner of the element. With a value pair of '14% 84%', the point 14% across and 84% down the image is to be placed at the point 14% across and 84% down the element.
With a value pair of '2cm 2cm', the upper left corner of the image is placed 2cm to the right and 2cm below the upper left corner of the element.
If only one percentage or length value is given, it sets the horizontal position only, the vertical position will be 50%. If two values are given, the horizontal position comes first. Combinations of length and percentage values are allowed, e.g. '50% 2cm'. Negative positions are allowed.
One can also use keyword values to indicate the position of the background image. Keywords cannot be combined with percentage values, or length values. The possible combinations of keywords and their interpretations are as follows:
examples:
BODY { background: url(banner.jpeg) right top } /* 100% 0% */
BODY { background: url(banner.jpeg) top center } /* 50% 0% */
BODY { background: url(banner.jpeg) center } /* 50% 50% */
BODY { background: url(banner.jpeg) bottom } /* 50% 100% */
If the background image is fixed with regard to the canvas (see the 'background-attachment' property above), the image is placed relative to the canvas instead of the element. E.g.:
BODY {
background-image: url(logo.png);
background-attachment: fixed;
background-position: 100% 100%;
}
In the example above, the image is placed in the lower right corner of the canvas.
Value: <background-color>
|| <background-image> || <background-repeat> || <background-attachment>
|| <background-position>
Initial: not defined for shorthand properties
Applies to: all elements
Inherited: no
Percentage values: allowed on <background-position>
The 'background' property is a shorthand property for setting the individual background properties (i.e., 'background-color', 'background-image', 'background-repeat', 'background-attachment' and 'background-position') at the same place in the style sheet.
Possible values on the 'background' properties are the set of all possible values on the individual properties.
BODY { background: red }
P { background: url(chess.png) gray 50% repeat fixed }
The 'background' property always sets all the individual background properties. In the first rule of the above example, only a value for 'background-color' has been given and the other individual properties are set to their initial value. In the second rule, all individual properties have been specified.
Value: normal | <length>
Initial: normal
Applies to: all elements
Inherited: yes
Percentage values: N/A
The length unit indicates an addition to the default space between words. Values can be negative, but there may be implementation-specific limits. The UA is free to select the exact spacing algorithm. The word spacing may also be influenced by justification (which is a value of the 'align' property).
H1 { word-spacing: 0.4em }
Here, the word-spacing between each word in 'H1' elements would be increased by '1em'.
Value: normal | <length>
Initial: normal
Applies to: all elements
Inherited: yes
Percentage values: N/A
The length unit indicates an addition to the default space between characters. Values can be negative, but there may be implementation-specific limits. The UA is free to select the exact spacing algorithm. The letter spacing may also be influenced by justification (which is a value of the 'align' property).
BLOCKQUOTE { letter-spacing: 0.1em }
Here, the letter-spacing between each character in 'BLOCKQUOTE' elements would be increased by '0.1em'.
With a value of 'normal', the UAs may change the space between letters to justify text. This will not happen if 'letter-spacing' is explicitly set to a <length> value:
BLOCKQUOTE { letter-spacing: 0 }
BLOCKQUOTE { letter-spacing: 0cm }
When the resultant space between two letters is not the same as the default space, UAs should not use ligatures.
Value: none | [ underline
|| overline || line-through || blink ]
Initial: none
Applies to: all elements
Inherited: no, but see clarification below
Percentage values: N/A
This property describes decorations that are added to the text of an element. If the element has no text (e.g. the 'IMG' element in HTML) or is an empty element (e.g. '<EM></EM>'), this property has no effect. A value of 'blink' causes the text to blink.
The color(s) required for the text decoration should be derived from the 'color' property value.
This property is not inherited, but elements should match their parent. E.g., if an element is underlined, the line should span the child elements. The color of the underlining will remain the same even if descendant elements have different 'color' values.
A:link, A:visited, A:active { text-decoration: underline }
The example above would underline the text of all links (i.e., all 'A' elements with a 'HREF' attribute).
UAs must recognize the keyword 'blink', but are not required to support the blink effect.
Value: baseline | sub
| super | top | text-top | middle | bottom | text-bottom | <percentage>
Initial: baseline
Applies to: inline elements
Inherited: no
Percentage values: refer to the 'line-height' of the element itself
The property affects the vertical positioning of the element. One set of keywords is relative to the parent element:
Another set of properties are relative to the formatted line that the element is a part of:
Using the 'top' and 'bottom' alignment, unsolvable situations can occur where element dependencies form a loop.
Percentage values refer to the value of the 'line-height' property of the element itself. They raise the baseline of the element (or the bottom, if it has no baseline) the specified amount above the baseline of the parent. Negative values are possible. E.g., a value of '-100%' will lower the element so that the baseline of the element ends up where the baseline of the next line should have been. This allows precise control over the vertical position of elements (such as images that are used in place of letters) that don't have a baseline.
It is expected that a future version of CSS will allow <length&t; as a value on this property.
Value: capitalize | uppercase
| lowercase | none
Initial: none
Applies to: all elements
Inherited: yes
Percentage values: N/A
The actual transformation in each case is human language dependent. See [4] for ways to find the language of an element.
H1 { text-transform: uppercase }
The example above would put 'H1'
elements in uppercase text.
CSS1 core: UAs may ignore
'text-transform' (i.e., treat it as 'none') for characters that are not from
the Latin-1 repertoire and for elements in languages for which the transformation
is different from that specified by the case-conversion tables of Unicode
[8].
Value: left | right |
center | justify
Initial: UA specific
Applies to: block-level elements
Inherited: yes
Percentage values: N/A
This property describes how text is aligned within the element. The actual justification algorithm used is UA and human language dependent.
Example:
DIV.center { text-align: center }
Since 'text-align' inherits, all block-level elements inside the 'DIV' element with 'CLASS=center' will be centered. Note that alignments are relative to the width of the element, not the canvas. If 'justify' is not supported, the UA will supply a replacement. Typically, this will be 'left' for western languages.
CSS1 core: UAs may treat 'justify' as 'left' or 'right', depending on whether the element's default writing direction is left-to-right or right-to-left, respectively.
Value: <length>
| <percentage>
Initial: 0
Applies to: block-level elements
Inherited: yes
Percentage values: refer to parent element's width
The property specifies the indentation that appears before the first formatted line. The value of 'text-indent' may be negative, but there may be implementation-specific limits. An indentation is not inserted in the middle of an element that was broken by another (such as 'BR' in HTML).
Example:
P
{ text-indent: 3em }
Value: normal | <number>
| <length> | <percentage>
Initial: normal
Applies to: all elements
Inherited: yes
Percentage values: relative to the font size of the element itself
The property sets the distance between two adjacent lines' baselines.
When a numerical value is specified, the line height is given by the font size of the current element multiplied with the numerical value. This differs from a percentage value in the way it inherits: when a numerical value is specified, child elements will inherit the factor itself, not the resultant value (as is the case with percentage and other units).
Negative values are not allowed.
The three rules in the example below have the same resultant line height:
DIV { line-height: 1.2; font-size: 10pt } /* number */
DIV { line-height: 1.2em; font-size: 10pt } /* length */
DIV { line-height: 120%; font-size: 10pt } /* percentage */
A value of 'normal' sets the 'line-height' to a reasonable value for the element's font. It is suggested that UAs set the 'normal' value to be a number in the range of 1.0 to 1.2.
Value: normal | pre |
nowrap
Initial: normal
Applies to: block-level elements
Inherited: yes
Percentage values: N/A
This property declares how whitespace inside the element is handled: the 'normal' way (where whitespace is collapsed), as 'pre' (which behaves like the 'PRE' element in HTML) or as 'nowrap' (where wrapping is done only through BR elements):
PRE { white-space: pre }
P { white-space: normal }
The initial value of 'white-space' is 'normal', but a UA will typically have default values for all HTML elements according to the suggested rendering of elements in the HTML specification.
CSS1 core: UAs may ignore the 'white-space' property in author's and reader's style sheets, and use the UA's default values instead.
The format of a length value is an optional sign character ('+' or '-', with '+' being the default) immediately followed by a number (with or without a decimal point) immediately followed by a unit identifier (a two-letter abbreviation). After a '0' number, the unit identifier is optional.
Some properties allow negative length units, 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 clipped to the nearest value that can be supported.
There are two types of length units: relative and absolute. Relative units specify a length relative to another length property. Style sheets that use relative units will more easily scale from one medium to another (e.g. from a computer display to a laser printer). Percentage units (described below) and keyword values (e.g. 'x-large') offer similar advantages.
These relative units are supported:
H1 { margin: 0.5em } /* ems, the height of the element's font */
H1 { margin: 1ex } /* x-height, ~ the height of the letter 'x' */
P { font-size: 12px } /* pixels, relative to canvas */
The relative units 'em' and 'ex' are relative to the font size of the element itself. The only exception to this rule in CSS1 is the 'font-size' property where 'em' and 'ex' values refer to the font size of the parent element.
Pixel units, as used in the last rule, are relative to the resolution of the canvas, i.e. most often a computer display. If the pixel density of the output device is very different from that of a typical computer display, the UA should rescale pixel values. The suggested reference pixel is the visual angle of one pixel on a device with a pixel density of 90dpi and a distance from the reader of an arm's length. For a nominal arm's length of 28 inches, the visual angle is about 0.0227 degrees.
Child elements inherit the computed value, not the relative value:
BODY {
font-size: 12pt;
text-indent: 3em; /* i.e. 36pt */
}
H1 { font-size: 15pt }
In the example above, the 'text-indent' value of 'H1' elements will be 36pt, not 45pt.
Absolute length units are only useful when the physical properties of the output medium are known. These absolute units are supported:
H1 { margin: 0.5in } /* inches, 1in = 2.54cm */
H2 { line-height: 3cm } /* centimeters */
H3 { word-spacing: 4mm } /* millimeters */
H4 { font-size: 12pt } /* points, 1pt = 1/72 in */
H4 { font-size: 1pc } /* picas, 1pc = 12pt */
In cases where the specified length cannot be supported, UAs should try to approximate. For all CSS1 properties, further computations and inheritance should be based on the approximated value.
The format of a percentage value is an optional sign character ('+' or '-', with '+' being the default) immediately followed by a number (with or without a decimal point) immediately followed by '%'.
Percentage values are always relative to another value, for example a length unit. Each property that allows percentage units also defines what value the percentage value refer to. Most often this is the font size of the element itself:
P { line-height: 120% } /* 120% of the element's 'font-size' */
In all inherited CSS1 properties, if the value is specified as a percentage, child elements inherit the resultant value, not the percentage value.
A color is a either a keyword or a numerical RGB specification.
The suggested list of keyword color names is: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, purple, red, silver, teal, white, and yellow. These 16 colors are taken from the Windows VGA palette, and their RGB values are not defined in this specification.
BODY {color: black; background: white }
H1 { color: maroon }
H2 { color: olive }
The RGB color model is being 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) } /* integer range 0 - 255 */
EM { color: rgb(100%, 0%, 0%) } /* float range 0.0% - 100.0% */
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 makes sure 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 in the range of 0-255, or three percentage values in the range of 0.0% to 100.0%) followed by ')'. Whitespace characters are allowed around the numerical values.
Values outside the numerical ranges should be clipped. The three rules below are therefore equivalent:
EM { color: rgb(255,0,0) } /* integer range 0 - 255 */
EM { color: rgb(300,0,0) } /* clipped to 255 */
EM { color: rgb(110%, 0%, 0%) } /* clipped to 100% */
RGB colors are specified in the sRGB color space. UAs may vary in the fidelity with which they represent these colors, but use of sRGB provides an unambiguous and objectively measurable definition of what the color should be, which can be related to international standards.
UAs may limit their efforts in displaying colors to performing a gamma-correction on them. sRGB specifies a display gamma of 2.2 under specified viewing conditions. UAs 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. Appendix D gives further details of this. Note that only colors specified in CSS are affected; e.g., images are expected to carry their own color information.