CSS3 module: Color

W3C Working Draft, 5 March 2001

This version:

http://www.w3.org/TR/2001/WD-css3-color-20010305

Latest version:

http://www.w3.org/TR/css3-color

Previous version:

http://www.w3.org/1999/06/WD-css3-iccprof-19990623

Editor:

Tantek Çelik (Microsoft Corporation) <tantekc@microsoft.com>

Contributors:

Chris Lilley (W3C) <chris@w3.org>

Brad Pettit (Microsoft Corporation) <bradp@microsoft.com>

Copyright © 2001 W3C^® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.

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Abstract

CSS (Cascading Style Sheets) is a language for describing the rendering of HTML and XML documents on screen, on paper, in speech, etc. To color elements in a document, it uses color related properties and respective values. This draft describes the properties and values that are proposed for CSS level 3. It includes and extends them from properties and values of CSS level 2.

Status of this document

This document is a draft of one of the "modules" for the upcoming CSS3 specification. It not only describes the color related properties and values that already exist in CSS1 and CSS2, but also proposes new properties and values for CSS3 as well. The Working Group doesn't expect that all implementations of CSS3 will implement all properties or values. Instead, there will probably be a small number of variants of CSS3, so-called "profiles". For example, it may be that only the profile for 32-bit color user agents will include all of the proposed color related properties and values.

The current draft is the successor to "Color Profiles for CSS3" (22 June 1999) and is the result of merging with the relevant parts of the following Recommendations and Working Drafts:

• HTML40
• CSS2
• "User Interface for CSS3" (16 February 2000)
• "Scalable Vector Graphics (SVG) 1.0" (3 March 2000)

This document is a working draft of the CSS & FP working group which is part of the style activity.

Comments on, and discussions of this draft can be sent to the (archived) public mailing list www-style@w3.org (see instructions). W3C Members can also send comments directly to the CSS & FP working group.

It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress." A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR.

Table of contents

• 1. Introduction
• 2. Dependencies
• 3. Color properties
  • 3.1. Foreground color: the 'color' property
    • 3.1.1. Gamma correction
  • 3.2. Transparency: the 'opacity' property
  • 3.3. ICC Color Profile: the 'color-profile' property
  • 3.4. The 'rendering-intent' property
• 4. Color units
  • 4.1. [HTML4] color keywords
  • 4.2. Numerical RGB color values
  • 4.3. X11 color keywords
  • 4.4. CSS System Colors
    • 4.4.1. CSS2 User preferences for colors
    • 4.4.2. 'flavor' system color
  • 4.5. Notes on using colors
• 5. Sample style sheet for HTML 4.0
• Acknowledgments
• References
  • Normative
  • Informative
• Index

1. Introduction

CSS3 is a set of modules, divided up and profiled in order to simplify the specification, and to allow implementors the flexibility of supporting the particular modules appropriate for their implementations.

This module describes CSS properties which allow authors to specify the foreground color and opacity of an element. Additional properties allow specification of the ICC color profile and rendering intent of image content.

2. Dependencies

This CSS3 module depends on the following other CSS3 modules:

• CSS3 Syntax / Grammar
• CSS3 Values, Units

The following CSS3 modules depend on this CSS3 module:

• CSS3 Box Model
• CSS3 Background
• CSS3 User Interface
• CSS3 SVG

3. Color properties

3.1. Foreground color: the 'color' property

This property describes the foreground color of an element's text content. There are different ways to specify red:

EM { color: red }              /* predefined color name */
EM { color: rgb(255,0,0) }     /* RGB range 0-255   */

<color>

Color units are defined in a following section.

attr(X,color)

This function returns as a color the value of attribute X for the subject of the selector. The attribute value is parsed by the CSS processor as a <color>. If the subject of the selector doesn't have an attribute X, or if the value of attribute X is not a valid <color>, the declaration is ignored. The case-sensitivity of attribute names depends on the document language. Note. In CSS3, it is not possible to refer to attribute values for other elements of the selector.

3.1.1. Gamma correction

For information about gamma issues, please consult the the Gamma Tutorial in the PNG specification ([PNG10]).

In the computation of gamma correction, UAs displaying on a CRT may assume an ideal CRT and ignore any effects on apparent gamma caused by dithering. That means the minimal handling they need to do on current platforms is:

PC using MS-Windows

none

Unix using X11

none

Mac using QuickDraw

apply gamma 1.45 [ICC32] (ColorSync-savvy applications may simply pass the sRGB ICC profile to ColorSync to perform correct color correction)

SGI using X

apply the gamma value from /etc/config/system.glGammaVal (the default value being 1.70; applications running on Irix 6.2 or above may simply pass the sRGB ICC profile to the color management system)

NeXT using NeXTStep

apply gamma 2.22

"Applying gamma" means that each of the three R, G and B must be converted to R'=R^gamma, G'=G^gamma, B'=B^gamma, before being handed to the OS.

This may rapidly be done by building a 256-element lookup table once per browser invocation thus:

for i := 0 to 255 do
  raw := i / 255.0;
  corr := pow (raw, gamma);
  table[i] := trunc (0.5 + corr * 255.0)
end

which then avoids any need to do transcendental math per color attribute, far less per pixel.

3.2. Transparency: the 'opacity' property

Opacity can be thought of conceptually as a postprocessing operation. Conceptually, after the element is rendered into an RGBA offscreen image, the opacity setting specifies how to blend the offscreen rendering into the current composite rendering.

<alphavalue>

The uniform opacity setting to be applied across an entire element including its outline, border and background if any. Any values outside the range 0.0 (fully transparent) to 1.0 (fully opaque) will be clipped to this range. If the element is a container element, then the effect is as if the contents of the element were blended against the current rendering composite using a mask where the value of each pixel of the mask is <alphavalue>.

3.3. ICC Color Profile: the 'color-profile' property

This property permits the specification of a source color profile other than the default.

auto

This is the default behavior. All colors are presumed to be defined in the sRGB color space unless a more precise embedded profile is specified within content data. For images that do have a profile built into their data, that profile is used. For images that do not have a profile, the sRGB profile is used so that the colors in these images can be kept "in synch" with the colors specified in CSS and HTML.

sRGB

The source profile is assumed to be sRGB. This differs from auto in that it overrides an embedded profile inside an image.

<uri>

The name or location of a standard ICC profile resource. Just like specifying sRGB, it overrides an embedded profile. Due to the size of profiles, the <uri> may specify a special name representing a standard profile. The name sRGB, being the standard WWW color space, is defined separately because of its significance, although the rules regarding application of any special profile should be identical.

/* use the specified profile, 
   even if the image contains an embedded profile */
IMG { color-profile: "http://www.colors.corp/images/profiles/mah314.icm" }

Note. SVG defines an '@color-profile' rule which permits the author to set both color-profile, and rendering-intent. The editor expects to include the '@color-profile' rule in a future draft of this document.

3.4. The 'rendering-intent' property

This property permits the specification of a color profile rendering intent other than the default. The behavior of values other than auto and inherit are defined by the International Color Consortium standard.

auto

This is the default behavior. The user-agent determines the best intent based on the content type. For image content containing an embedded profile, it should be assumed that the intent specified within the profile is the desired intent. Otherwise, the user agent should use the current profile (based on the color-profile style) and force the intent, overriding any intent that may be stored in the profile itself.

4. Color units

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

4.1. [HTML4] color keywords

The list of HTML4 keyword color names is: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, purple, red, silver, teal, white, and yellow. The color names are case-insensitive.

BODY {color: black; background: white }
H1 { color: maroon }
H2 { color: olive }

4.2. Numerical RGB color values

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) }      /* 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 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. Whitespace 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]).

Values outside the device gamut should be clipped: the red, green, and blue values must be changed to fall within the range supported by the device. For a typical CRT monitor, whose device gamut is the same as sRGB, the three 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%) */

4.3. X11 color keywords

The table below provides a list of the X11 colors supported by popular browsers. The two color swatches on the left illustrate setting the background color of a table cell in two ways: The first column uses the named color value, and the second column uses the numeric color value, which should be the same.

4.4. CSS System Colors

4.4.1. CSS2 User preferences for colors

In addition to being able to assign pre-defined color values to text, backgrounds, etc., CSS3, like CSS2, allows authors to specify colors in a manner that integrates them into the user's graphic environment. Style rules that take into account user preferences thus offer the following advantages:

1. They produce pages that fit the user's defined look and feel.
2. They produce pages that may be more accessible as the current user settings may be related to a disability.

For systems that do not have a corresponding value, the specified value should be mapped to the nearest system attribute, or to a default color.

The following lists additional values for color-related CSS attributes and their general meaning. Any color property can take one of the following names. Although these are case-insensitive, it is recommended that the mixed capitalization shown below be used, to make the names more legible.

ActiveBorder

Active window border.

ActiveCaption

Active window caption.

AppWorkspace

Background color of multiple document interface.

Background

Desktop background.

ButtonFace

Face color for three-dimensional display elements.

ButtonHighlight

Dark shadow for three-dimensional display elements (for edges facing away from the light source).

ButtonShadow

Shadow color for three-dimensional display elements.

ButtonText

Text on push buttons.

CaptionText

Text in caption, size box, and scrollbar arrow box.

GrayText

Grayed (disabled) text. This color is set to #000 if the current display driver does not support a solid gray color.

Highlight

Item(s) selected in a control.

HighlightText

Text of item(s) selected in a control.

InactiveBorder

Inactive window border.

InactiveCaption

Inactive window caption.

InactiveCaptionText

Color of text in an inactive caption.

InfoBackground

Background color for tooltip controls.

InfoText

Text color for tooltip controls.

Menu

Menu background.

MenuText

Text in menus.

Scrollbar

Scroll bar gray area.

ThreeDDarkShadow

Dark shadow for three-dimensional display elements.

ThreeDFace

Face color for three-dimensional display elements.

ThreeDHighlight

Highlight color for three-dimensional display elements.

ThreeDLightShadow

Light color for three-dimensional display elements (for edges facing the light source).

ThreeDShadow

Dark shadow for three-dimensional display elements.

Window

Window background.

WindowFrame

Window frame.

WindowText

Text in windows.

P { color: WindowText; background-color: Window }

4.4.2. 'flavor' system color

'flavor'

The accent color (typically chosen by the user) to customize the user interface of the user agent itself. User agents may default the 'flavor' color to the dominant accent color used on the physical machine/mechanism that the user is interacting with (frequently a mouse, keyboard, monitor and computer case, often just a laptop), if the it is able to retrieve that information from the platform and machine.

:focus {outline: 1px solid flavor}
  /* puts an outline around the currently focused element that color
coordinates with the browser accent color if any */

Note. The computed value of a System Color keyword value is the keyword itself. In addition, in future CSS specifications, the color property may accept additional System Color values.

4.5. Notes on using colors

Although colors can add significant amounts of information to document and make them more readable, please consider the following guidelines when including color in your documents:

• Don't use color combinations that cause problems for people with color blindness in its various forms.
• If you use a background image or set the background color, then be sure to set the various text colors as well.
• When practical, adopt common conventions to minimize user confusion.

5. Sample style sheet for HTML 4.0

This appendix is informative, not normative.

BODY { 
	color: black;
	background: white;
	color-profile: sRGB;
	rendering-intent: auto 
}

:link    { color: blue; }
:visited { color: purple; }

IMG,OBJECT {
 	color-profile: auto;
 	rendering-intent: auto
} 

Acknowledgments

Thanks to Brad Pettit for both writing up color-profiles, and for implementing it. Thanks to feedback from Marc Attinasi, David Baron, Bert Bos, Ian Hickson, Steve Zilles. And thanks to Chris Lilley for being the resident CSS Color expert.

References

Normative

[COLORIMETRY]

Colorimetry, Second Edition. CIE Publication 15.2-1986, ISBN 3-900-734-00-3. Available at http://www.hike.te.chiba-u.ac.jp/ikeda/CIE/publ/abst/15-2-86.html.

[CSS2]

Bert Bos, Håkon Wium Lie, Chris Lilley, Ian Jacobs. Cascading Style Sheets, level 2. 12 May 1998. W3C Recommandation. Available at http://www.w3.org/TR/REC-CSS2.

[HTML40]

D. Raggett, A. Le Hors, I. Jacobs. HTML 4.0 Specification. 8 July 1997. W3C Recommendation. Available at http://www.w3.org/TR/REC-html40/.

[ICC32]

ICC Profile Format Specification, version 3.2. 1995. Available at ftp://sgigate.sgi.com/pub/icc/ICC32.pdf.

[PNG10]

T. Boutell ed. PNG (Portable Network Graphics) Specification, Version 1.0. 1 October 1996. Available at http://www.w3.org/pub/WWW/TR/REC-png-multi.html.

[SRGB]

M. Anderson, R. Motta, S. Chandrasekar, M. Stokes. "Proposal for a Standard Color Space for the Internet - sRGB". Available at http://www.w3.org/Graphics/Color/sRGB.html.

Informative

[CSSICCPROF]

Brad Pettit, Tantek Çelik, Chris Lilley. Color Profiles for CSS3. 22 June 1999. W3C Working Draft. Available at http://www.w3.org/1999/06/WD-css3-iccprof-19990623.

[HTMLICCPROF]

Apple Computer with input from Microsoft Corporation. Proposal for HTML support of ICC profiles. Available at http://www.apple.com/colorsync/benefits/web/icc-profiles.html.

[SVG]

Jon Ferraiolo. Scalable Vector Graphics (SVG) 1.0 Draft. 03 March 2000. W3C Working Draft. Available at http://www.w3.org/TR/2000/03/WD-SVG-20000303.

[UI]

Tantek Çelik. User Interface for CSS3 Draft. 16 February 2000. W3C Working Draft. Available at http://www.w3.org/TR/2000/WD-css3-userint-20000216.

[X11COLORS]

Robert B. Hess. Colors By Name. 02 November 1996. MSDN Online Web Workshop. Available at http://msdn.microsoft.com/workshop/design/color/x11_names.asp.

Index

• color
  • names of, #
• <color>
  • definition of, #