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.
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When filing an issue, please put the text “css-color” in the title,
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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.
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:
sRGB red (#F00) is lch(54.2917 106.8390 40.8526)
sRGB yellow (#FF0) is lch(97.6071 94.7077 99.5746)
which is a red-orange: rgb(75.3600% 65.6304% 16.9796%)
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.
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",
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 to this specification
is defined for three conformance classes:
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.
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
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