CSS Color Module Level 3

Abstract

CSS (Cascading Style Sheets) is a language for describing the rendering of HTML and XML documents on screen, on paper, in speech, etc. It uses color-related properties and values to color the text, backgrounds, borders, and other parts of elements in a document. This specification describes color values and properties for foreground color and group opacity. These include properties and values from CSS level 2 and new values.

Status of This Document

This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.downtownmelody.com/_x/d3d3LnczLm9yZw/TR/.

This document was published by the CSS Working Group as a Recommendation using the Recommendation track.

A W3C Recommendation is a specification that, after extensive consensus-building, is endorsed by W3C and its Members, and has commitments from Working Group members to royalty-free licensing for implementations.

W3C recommends the wide deployment of this specification as a standard for the Web.

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

This document is governed by the 2 November 2021 W3C Process Document.

A separate implementation report shows that each test in the test suite was passed by at least two independent implementations. However, most tests have now been updated for CSS Color 4 (see implementation report).

A complete list of changes to this document is available.

1. Introduction

This section is non-normative.

CSS beyond level 2 is a set of modules, divided up to allow the specifications to develop incrementally, along with their implementations. This specification is one of those modules.

This module describes CSS properties which allow authors to specify the foreground color and opacity of an element. This module also describes in detail the CSS <color> value type.

It not only defines the color-related properties and values that already exist in CSS1 and CSS2, but also defines new properties and values.

The specification is the result of the merging of relevant parts of the following Recommendations and Working Drafts, and the addition of some new features.

• HTML 4.01 [HTML401]
• CSS 2.0 [CSS2]
• SVG 1.0 [SVG10]
• User Interface for CSS3 (16 February 2000) [CSS-UI-3]

2. Dependencies

Additional terminology is defined in the Definitions section of [CSS21]. Examples of document source code and fragments are given in XML [XML10] or HTML [HTML401] syntax.

3. Color properties

3.1. Foreground color: the ‘color’ property

This property describes the foreground color of an element's text content. In addition it is used to provide a potential indirect value (currentColor) for any other properties that accept color values. If the ‘currentColor’ keyword is set on the ‘color’ property itself, it is treated as ‘color: inherit’.

There are different ways to specify lime green:

em { color: lime }               /* color keyword */
em { color: rgb(0,255,0) }       /* RGB range 0-255   */

<color>

Color units are defined in a following section.

3.2. Transparency: the ‘opacity’ property

Opacity can be thought of as a postprocessing operation. Conceptually, after the element (including its descendants) is rendered into an RGBA offscreen image, the opacity setting specifies how to blend the offscreen rendering into the current composite rendering. See simple alpha compositing for details.

<alphavalue>

Syntactically a <number>. The uniform opacity setting to be applied across an entire object. Any values outside the range 0.0 (fully transparent) to 1.0 (fully opaque) will be clamped to this range. If the object has children, then the effect is as if the children were blended against the current background using a mask where the value of each pixel of the mask is <alphavalue>. For SVG, ‘has children’ is equivalent to being a container element [SVG11].

Since an element with opacity less than 1 is composited from a single offscreen image, content outside of it cannot be layered in z-order between pieces of content inside of it. For the same reason, implementations must create a new stacking context for any element with opacity less than 1. If an element with opacity less than 1 is not positioned, then it is painted on the same layer, within its parent stacking context, as positioned elements with stack level 0. If an element with opacity less than 1 is positioned, the ‘z-index’ property applies as described in [CSS21], except that if the used value is ‘auto’ then the element behaves exactly as if it were ‘0’. See section 9.9 and Appendix E of [CSS21] for more information on stacking contexts. The rules in this paragraph do not apply to SVG elements, since SVG has its own rendering model ([SVG11], Chapter 3).

4. Color units

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

4.1. Basic color keywords

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

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

4.2. Numerical color values

4.2.1. 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) }
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]).

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. User agents may perform higher quality mapping of colors from one gamut to another. This specification does not define precise clipping behavior. 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.

4.2.2. RGBA color values

The RGB color model is extended in this specification to include “alpha” to allow specification of the opacity of a color. See simple alpha compositing for details. These examples all specify the same color:

em { color: rgb(255,0,0) }      /* integer range 0 - 255 */
em { color: rgba(255,0,0,1)     /* the same, with explicit opacity of 1 */
em { color: rgb(100%,0%,0%) }   /* float range 0.0% - 100.0% */
em { color: rgba(100%,0%,0%,1) } /* the same, with explicit opacity of 1 */

Unlike RGB values, there is no hexadecimal notation for an RGBA value.

The format of an RGBA value in the functional notation is ‘rgba(’ followed by a comma-separated list of three numerical values (either three integer values or three percentage values), followed by an <alphavalue>, followed by ‘)’. The integer value 255 corresponds to 100%, rgba(255,255,255,0.8) = rgba(100%,100%,100%,0.8). White space characters are allowed around the numerical values.

Implementations must clip the red, green, and blue components of RGBA color values to the device gamut according to the rules for the RGB color value composed of those components.

These examples specify effects that are possible with the rgba() notation:

p { color: rgba(0,0,255,0.5) }        /* semi-transparent solid blue */
p { color: rgba(100%, 50%, 0%, 0.1) } /* very transparent solid orange */

Note. If RGBA values are not supported by a user agent, they should be treated like unrecognized values per the CSS forward compatibility parsing rules ([CSS21], Chapter 4). RGBA values must not be treated as simply an RGB value with the opacity ignored.

4.2.3. ‘transparent’ color keyword

CSS1 introduced the ‘transparent’ value for the background-color property. CSS2 allowed border-color to also accept the ‘transparent’ value. The Open eBook(tm) Publication Structure 1.0.1 [OEB101] extended the ‘color’ property to also accept the ‘transparent’ keyword. CSS3 extends the color value to include the ‘transparent’ keyword to allow its use with all properties that accept a <color> value. This simplifies the definition of those properties in CSS3.

transparent

Fully transparent. This keyword can be considered a shorthand for transparent black, rgba(0,0,0,0), which is its computed value.

4.2.4. HSL color values

CSS3 adds numerical hue-saturation-lightness (HSL) colors as a complement to numerical RGB colors. It has been observed that RGB colors have the following limitations:

• RGB is hardware-oriented: it reflects the use of CRTs.
• RGB is non-intuitive. People can learn how to use RGB, but actually by internalizing how to translate hue, saturation and lightness, or something similar, to RGB.

There are several other color schemes possible. Some advantages of HSL are that it is symmetrical to lightness and darkness (which is not the case with HSV for example), and it is trivial to convert HSL to RGB.

HSL colors are encoding as a triple (hue, saturation, lightness). Hue is represented as an angle of the color circle (i.e. the rainbow represented in a circle). This angle is so typically measured in degrees that the unit is implicit in CSS; syntactically, only a <number> is given. By definition red=0=360, and the other colors are spread around the circle, so green=120, blue=240, etc. As an angle, it implicitly wraps around such that -120=240 and 480=120. One way an implementation could normalize such an angle x to the range [0,360) (i.e. zero degrees, inclusive, to 360 degrees, exclusive) is to compute (((x mod 360) + 360) mod 360). Saturation and lightness are represented as percentages. 100% is full saturation, and 0% is a shade of gray. 0% lightness is black, 100% lightness is white, and 50% lightness is “normal”.

So for instance:

* { color: hsl(0, 100%, 50%) }   /* red */
* { color: hsl(120, 100%, 50%) } /* lime */
* { color: hsl(120, 100%, 25%) } /* dark green */
* { color: hsl(120, 100%, 75%) } /* light green */
* { color: hsl(120, 75%, 75%) }  /* pastel green, and so on */

The advantage of HSL over RGB is that it is far more intuitive: you can guess at the colors you want, and then tweak. It is also easier to create sets of matching colors (by keeping the hue the same and varying the lightness/darkness, and saturation)

If saturation is less than 0%, implementations must clip it to 0%. If the resulting value is outside the device gamut, implementations must clip it to the device gamut. This clipping should preserve the hue when possible, but is otherwise undefined. (In other words, the clipping is different from applying the rules for clipping of RGB colors after applying the algorithm below for converting HSL to RGB.)

Converting an HSL color to sRGB is straightforward mathematically. Here's a sample implementation of the conversion algorithm in JavaScript. It returns an array of three numbers representing the red, green, and blue channels of the colors, normalized to the range [0, 1].

    function hslToRgb (hue, sat, light) {
      hue = hue % 360;

      if (hue < 0) {
          hue += 360;
      }

      sat /= 100;
      light /= 100;

      function f(n) {
          let k = (n + hue/30) % 12;
          let a = sat * Math.min(light, 1 - light);
          return light - a * Math.max(-1, Math.min(k - 3, 9 - k, 1));
      }

      return [f(0), f(8), f(4)];
  }
  

4.2.4.1. HSL examples

Each table below represents one hue. Twelve equally spaced colors (i.e. at 30° intervals) have been chosen from the color circle: red, yellow, green, cyan, blue, magenta, with all the intermediate colors (the last is the color between magenta and red).

In each table, the X axis represents the saturation while the Y axis represents the lightness.

4.2.5. HSLA color values

Just as the ‘rgb()’ functional notation has the ‘rgba()’ alpha counterpart, the ‘hsl()’ functional notation has the ‘hsla()’ alpha counterpart. See simple alpha compositing for details. These examples specify the same color:

em { color: hsl(120, 100%, 50%) }     /* green */
em { color: hsla(120, 100%, 50%, 1) } /* the same, with explicit opacity of 1 */

The format of an HSLA color value in the functional notation is ‘hsla(’ followed by the hue in degrees, saturation and lightness as a percentage, and an <alphavalue>, followed by ‘)’. White space characters are allowed around the numerical values.

Implementations must clip the hue, saturation, and lightness components of HSLA color values to the device gamut according to the rules for the HSL color value composed of those components.

These examples specify effects that are possible with the hsla() notation:

p { color: hsla(240, 100%, 50%, 0.5) } /* semi-transparent solid blue */
p { color: hsla(30, 100%, 50%, 0.1) }  /* very transparent solid orange */

4.3. Extended color keywords

The table below provides a list of the X11 colors [X11COLORS] supported by popular browsers with the addition of gray/grey variants from SVG 1.0. The resulting list is precisely the same as the SVG 1.0 color keyword names. This specification extends their definition beyond SVG. 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 respective numeric color value.