CSS Color Module Level 5

Editor’s Draft,

Specification Metadata
This version:
https://drafts.csswg.org/css-color-5/
Latest published version:
https://www.w3.org/TR/css-color-5/
Previous Versions:
Issue Tracking:
Inline In Spec
GitHub Issues
Editors:
(W3C)
Una Kravets (Google)
Lea Verou (Invited Expert)
Adam Argyle (Google)
Suggest an Edit for this Spec:
GitHub Editor
Delta Spec:
yes

Abstract

This module extends CSS Color [css-color-4] to add color modification functions.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-color” in the title, preferably like this: “[css-color] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group.

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 1 March 2019 W3C Process Document.

1. Introduction

This section is not normative.

Web developers, design tools and design system developers often use color functions to assist in scaling the design of their component color relations. With the increasing usage of design systems that support multiple platforms and multiple user preferences, like the increased capability of Dark Mode in UI, this becomes even more useful to not need to manually set color, and to instead have a single source from which schemes are calculated.

LC color picker
chloropleth map of the US

Above, a color picker operating in LCH space. Here, a pair of colors are being used to define a color scale on the Chroma-Lightness plane (constant Hue). Below, the color scale in use on a chloropleth map.

Currently Sass, calc() on HSL values, or PostCSS is used to do this. However, preprocessors are unable to work on dynamically adjusted colors; all current solutions are restricted to the sRGB gamut and to the perceptual limitations of HSL (colors are bunched up in the color wheel, and two colors with visually different lightness, like yellow and blue, can have the same HSL lightness).

This module adds three functions: color-mix, color-contrast, and a way to modify colors.

The perceptually uniform ``lch()`` colorspace is used for mixing by default, as this has no gamut restrictions and colors are evenly distributed. However, other colorspaces can be specified, including ``hsl()`` or ``srgb`` if desired.

2. Mixing colors: the color-mix function

This function takes two <color> specifications and returns the result of mixing them, in a given colorspace, by a specified amount.

Unless otherwise specified, the mixing is done in the lch() colorspace.

Multiple color functions can be specified.

color-mix() = color-mix( <color>  <color> [ <number> | <percentage> |
  [ <color-function> <colorspace>? ]?] )

color-function used but not defined <https://github.com/w3c/csswg-drafts/issues/4747>

When a plain number or percentage is specified, it applies to all color channels.

If the plain number or percentage is omitted, it defaults to 50% (an equal mix of the two colors).

This example produces a mixture of 40% peru and 60% lightgoldenrod.
color-mix(peru lightgoldenrod 40%)

The mixing is done in lch() colorspace. Here is a top-down view, looking along the neutral L axis:

Mixtures of peru and lightgoldenrod. Peru has a hue angle, measured from the positive a axis, of 63.677 degrees while lightgoldenrod has a hue angle of 98.821 degrees. Peru has a chroma, or distance from the central neutral axis, of 54.011 while lightgoldenrod has a chroma of 31.415. Mixtures lie along the curve. A 40%/60% mixture is shown.

The calculation is as follows:

This example produces the mixture of red and yellow, in lch colorspace (the default), with each lch channel being 65% of the value for red and 35% of the value for yellow.

Note: interpolating on hue and chroma keeps the intermediate colors as saturated as the endpoint colors.

color-mix(red yellow 65%);
Mixtures of red and yellow. Red has a hue angle, measured from the positive a axis, of 40.8526 degrees while yellow has a hue angle of 99.5746 degrees. Red has a chroma, or distance from the central neutral axis, of 106.8390 while yellow has a chroma of 94.7077. Mixtures lie along the dashed curve. A 65%/35% mixture is shown.

The calculation is as follows:

Instead of mixing all channels equally, color functions can be used to only mix certain channels. The unmixed channels in the result are copied across from the first color.

This example produces the mixture of a deep green and a sky blue, in lch() colorspace (the default), with the lightness being 40% of the lightness of the green (and thus, 60% of the lightness of the blue). The chroma and hue of the green are used unchanged in the mixed result.
color-mix(rgb(0% 42.35% 33.33%) rgb(41.2% 69.88% 96.64%) lightness(40%));

The calculation is as follows:

This example produces the mixture of a red and a sky blue, in lch() colorspace (the default), with the hue being 75.23% of that of the red (and thus, 24.77% of that of the blue). The chroma and lightness of the red are left unchanged.
color-mix(lch(52% 58.1 22.7) lch(56% 49.1 257.1) hue(75.23%));
Mixtures of a red and a sky blue. The red has a hue angle, measured from the positive a axis, of 22.7 degrees while the blue has a hue angle of 257.1 degrees. The red has a chroma, or distance from the central neutral axis, of 58.1 and the chroma is not interpolated here, just the hue. Thus, mixtures lie along the dashed circle.

The calculation is as follows:

When mixing hue, there are two ways round the hue range

This example produces the mixture of the same two colors as the previous example (a red and a sky blue), in lch() colorspace (the default), with the hue being 75.23% of that of the red (and thus, 24.77% of that of the blue). This time, the lightness is set to 68.4% and the chroma is left unchanged.
color-mix(rgb(82.02% 30.21% 35.02%) rgb(5.64% 55.94% 85.31%) hue(75.23%) lightness(68.4%));

The calculation is as follows:

This example produces the mixture of red and yellow, in lch() colorspace (the default), with the lightness being 30% of the lightness of red (and thus, 70% of the lightness of yellow). The chroma and hue of red are left unchanged.
color-mix(red yellow lightness(30%));

The calculation is as follows:

This example demonstrates that not all colors which can be mixed, can be displayed on current devices.

This example produces the mixture of the same two colors as the previous example (a red and a sky blue), in Lab colorspace, with 38% of a and 38% of b. The lightness of the red is left unchanged.

This shows a desaturated result, compared to LCH interpolation, because interpolation is in a stright line on the a,b plane and can thus pass close to the central, neutral axis.

color-mix(rgb(82.02% 30.21% 35.02%) rgb(5.64% 55.94% 85.31%) lab() a(38%) b(38%));

The calculation is as follows:

color-mix to allow more than two colors?

3. Selecting the most contrasting color: the color-contrast() function

This function takes, firstly, a single color (typically a background, but not necessarily), and then second, a list of two or more colors; it selects from that list the color with highest luminance contrast [WCAG21] to the single color.

color-contrast() = color-contrast( <color>  <color>#  )

add target contrast ratio to color-contrast?

color-contrast(wheat tan, sienna, var(--myAccent), #d2691e)

The calculation is as follows:

Suppose myAccent has the value #b22222:

The highest contrast ratio is 5.081 so var(--myAccent) wins

The colors in the list are tested sequentially, from left to right; a color is the temporary winner if it has the highest contrast of all those tested so far, and once the end of the list is reached, the current temporary winner is the overall winner. Thus, if two colors in the list happen to have the same contrast, the earlier in the list wins because the later one has the same contrast, not higher.

foo {
  --bg: hsl(200 50% 80%);
  --purple-in-hsl: hsl(300 100% 25%);
  color: color-contrast(var(--bg) hsl(200 83% 23%), purple, var(--purple-in-hsl));
  }

The calculation is as follows:

The calculated values here are shown to six significant figures, to demonstrate that early rounding to a lower precision would have given the wrong result (0.061575 is very close to 0.061487; 6.08409 is very close to 6.08889).

4. Modifying colors

Note: There are currently two proposals for modifying colors: color-adjust and Relative color syntax.

there are two proposals for color modification (proposal 1, proposal 2). The CSS WG expects that the best aspects of each will be chosen to produce a single eventual solution. <https://github.com/w3c/csswg-drafts/issues/3187>

4.1. Adjusting colors: the color-adjust function

This function takes one <color> specification and returns the result of adjusting that color, in a given colorspace, by a specified transform function.

Unless otherwise specified, the adjustment is done in the lch() colorspace.

Multiple color functions can be specified.

color-adjust() = color-adjust( <color> [ color-function <colorspace>? ]?] )
This example produces the adjustment of peru (#CD853F), in lch() colorspace (the default), with the lightness being reduced by 20%. The chroma and hue of red are left unchanged.
color-adjust(peru lightness(-20%));

The calculation is as follows:

4.2. Relative color syntax

Besides specifying absolute coordinates, all color functions can also be used with a *relative syntax* to produce colors in the function’s target color space, based on an existing color (henceforth referred to as "origin color"). This syntax consists of the keyword from, a <color> value, and optionally numerical coordinates specific to the color function. To allow calculations on the original color’s coordinates, there are single-letter keywords for each coordinate and `alpha` that corresponds to the color’s alpha. If no coordinates are specified, the function merely converts the origin color to the target function’s color space.

The following sections outline the relative color syntax for each color function.

A future version of this specification may define a relative syntax for color() as well.

4.2.1. Relative RGB colors

The grammar of the rgb() function is extended as follows:

rgb() = rgb([from <color>]? <percentage>{3} [ / <alpha-value> ]? ) |
    rgb([from <color>]? <number>{3} [ / <alpha-value> ]? )
<alpha-value> = <number> | <percentage>

When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:

To manipulate color channels in the sRGB colorspace:
rgb(from  indianred 255 g b)

This takes the sRGB value of indianred (205 92 92) and replaces the red channel with 255 to give rgb(255 92 92).

4.2.2. Relative HSL colors

The grammar of the hsl() function is extended as follows:

hsl() = hsl([from <color>]? <hue> <percentage> <percentage> [ / <alpha-value> ]? )
<hue> = <number> | <angle>

When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:

This adds 180 degrees to the hue angle, giving a complementary color.
--accent:  lightseagreen;
--complement:   hsl(from var(--accent) calc(h+180) s l);

lightseagreen is hsl(177deg 70% 41%), so --complement is hsl(357deg 70% 41%)

4.2.3. Relative HWB colors

The grammar of the hwb() function is extended as follows:

hwb() = hwb([from <color>]? <hue> <percentage> <percentage> [ / <alpha-value> ]? )

When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:

4.2.4. Relative Lab colors

The grammar of the lab() function is extended as follows:

lab() = lab([from <color>]? <percentage> <number> <number> [ / <alpha-value> ]? )

When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:

Multiple ways to adjust the transparency of a base color:

Note that all the adjustments are lossless in the sense that no gamut clipping occurs, since lab() encompasses all visible color. This is not true for the alpha adjustments in the sRGB based functions (such as’rgb()', 'hsl()', or 'hwb()'), which would also convert to sRGB in addition to adjusting the alpha transparency.

Fully desaturating a color to gray, keeping the exact same lightness:
--mycolor:  orchid;
// orchid is lab(62.753% 52.460 -34.103)
--mygray:  lab(from var(--mycolor) l 0 0)
// mygray is lab(62.753% 0 0) which is rgb(59.515% 59.515% 59.515%)

4.2.5. Relative LCH colors

The grammar of the lch() function is extended as follows:

lch() = lch([from <color>]? <percentage> <number> <hue> [ / <alpha-value> ]? )

When an origin color is present, the following keywords can also be used in this function (provided the end result conforms to the expected type for the parameter) and correspond to:

lch(from peru calc(l * 0.8) c h) produces a color that is 20% darker than peru or lch(62.2532% 54.0114 63.6769), with its chroma and hue left unchanged. The result is lch(49.80256% 54.0114 63.6769)
This adds 180 degrees to the hue angle, giving the complementary color.
--accent:  lightseagreen;
--complement:   LCH(from var(--accent) l c calc(h+180));

lightseagreen is LCH(65.4937% 39.4484 190.1013), so --complement is LCH(65.4937% 39.4484 370.1013)

Fully desaturating a color to gray, keeping the exact same lightness:
--mycolor:  orchid;
// orchid is lch(62.753% 62.571 326.973)
--mygray:  lch(from var(--mycolor) l 0 h)
// mygray is lch(62.753% 0 326.973) which is rgb(59.515% 59.515% 59.515%)

But now (since the hue was preserved) re-saturating again

--mymuted:  lch(from var(--mygray) l 30 h);
// mymuted is lch(62.753% 30 326.973) which is rgb(72.710% 53.293% 71.224%)

5. Security and Privacy Considerations

This specification introduces no new security or privacy considerations.

6. Acessibility Considerations

This specification introduces a new feature to help stylesheet authors write stylesheets which conform to WCAG 2.1 section 1.4.3 Contrast (Minimum).

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-COLOR-3]
Tantek Çelik; Chris Lilley; David Baron. CSS Color Module Level 3. 19 June 2018. REC. URL: https://www.w3.org/TR/css-color-3/
[CSS-COLOR-4]
Tab Atkins Jr.; Chris Lilley. CSS Color Module Level 4. 5 November 2019. WD. URL: https://www.w3.org/TR/css-color-4/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 6 June 2019. CR. URL: https://www.w3.org/TR/css-values-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 31 January 2019. WD. URL: https://www.w3.org/TR/css-values-4/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[SVG2]
Amelia Bellamy-Royds; et al. Scalable Vector Graphics (SVG) 2. 4 October 2018. CR. URL: https://www.w3.org/TR/SVG2/
[WCAG21]
Andrew Kirkpatrick; et al. Web Content Accessibility Guidelines (WCAG) 2.1. 5 June 2018. REC. URL: https://www.w3.org/TR/WCAG21/

Issues Index

color-function used but not defined <https://github.com/w3c/csswg-drafts/issues/4747>
When mixing hue, there are two ways round the hue range
color-mix to allow more than two colors?
add target contrast ratio to color-contrast?
there are two proposals for color modification (proposal 1, proposal 2). The CSS WG expects that the best aspects of each will be chosen to produce a single eventual solution. <https://github.com/w3c/csswg-drafts/issues/3187>
A future version of this specification may define a relative syntax for color() as well.