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.
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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.
Currently Sass, calc() on HSL values, or PostCSS is used to do this.
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 two functions: color-mix, color-contrast.
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.
This example produces the mixture of red and white,
in lch() colorspace (the default),
with the lightness being 50% of the lightness of red
(and thus, 50% of the lightness of white).
The chroma and hue of red are left unchanged.
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.
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 very light red (and outside the gamut of sRGB: rgb(140.4967% 51.2654% 32.6891%))
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:
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 colors;
it selects from that list
the color with highest luminance contrast
to the single color.
wheat (#f5deb3), the background, has relative luminance 0.749
tan (#d2b48c) has relative luminance 0.482 and contrast ratio 1.501
sienna (#a0522d) has relative luminance 0.137 and contrast ratio 4.273
suppose myAccent has the value #b22222
#b22222 has relative luminance 0.107 and contrast ratio 5.081
#d2691e has relative luminance 0.305 and contrast ratio 2.249
The highest contrast ratio is 5.081 so var(--myAccent) wins
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:
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
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/.
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