CSS Color Module Level 5

Editor’s Draft,

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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 and design systems developers often use functions to design components. With the increasing usage of design systems that support multiple platforms, and 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 layouts are calculated.

Currently Sass, calc() on HSL values, or PostCSS is used to do this. Preprocessors are unable to work on dynamically adjusted colors, and all current solutions are restricted to the sRGB gamut and to the perceptual limitations of HSL.

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

there are two proposals for color modification. 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>

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>? ]?] )
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 70% of the lightness of yellow. The chroma and hue of red are left unchanged.
mix-color(red, yellow, lightness(30%));

The calculation is as follows:

Instead of a list of color functions, a plain number or percentage can be specified, which applies to all color channels.

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.

mix-color(red, yellow, 65%);

The calculation is as follows:

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 the list of colors the one with highest luminance contrast to the single color.

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


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.
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.


Terms defined by this specification

Terms defined by reference


Normative References

Tantek Çelik; Chris Lilley; David Baron. CSS Color Module Level 3. 19 June 2018. REC. URL: https://www.w3.org/TR/css-color-3/
Tab Atkins Jr.; Chris Lilley. CSS Color Module Level 4. 5 March 2019. WD. URL: https://www.w3.org/TR/css-color-4/
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/
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/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Issues Index

there are two proposals for color modification. 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>