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Refactoring of the forced-colors and prefers-contrast media feature (#…
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…5434)

This editorial reorganization of the text is intended to make the
overlap between forced-colors:active and prefers-contrast:forced
more readily understandable: one being the syntax that the CSS Working
Group finds more desirable, the other being retained for compatibility
with content issued against an earlier draft (see #3856).

Prior to this refactoring, the definition of this mode was also spread
between the two media features, causing confusion. This commits
consolidates the whole thing in one place.

Related to #5433
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@frivoal
frivoal authored Oct 1, 2020
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113 changes: 69 additions & 44 deletions mediaqueries-5/Overview.bs
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Expand Up @@ -2624,7 +2624,7 @@ Detecting the desire for reduced transparency on the page: the 'prefers-reduced-
ISSUE: How does this interact with preferences around e.g. pattern fills and backgrounds?
They're not about transparency, but they also interfere with shape recognition.

<h3 id="prefers-contrast">
<h3 id="prefers-contrast" oldids="forced-colors">
Detecting the desire for increased or decreased color contrast from elements on the page: the 'prefers-contrast' feature</h3>

<pre class='descdef mq'>
Expand All @@ -2634,11 +2634,12 @@ Detecting the desire for increased or decreased color contrast from elements on
Type: discrete
</pre>

The 'prefers-contrast' media feature is used to detect if the user
has requested the system increase or decrease the amount of contrast
between adjacent colors. For example, many users have difficulty
reading text that has a small difference in contrast to the text
background and would prefer a larger contrast.
The 'prefers-contrast' media feature is used to detect
if the user has requested more or less contrast in the page.
This could be responded to, for example,
by adjusting the contrast ratio between adjacent colors,
or by changing how much elements stand out visually,
such as by adjusting their borders.

<dl dfn-type=value dfn-for="@media/prefers-contrast">
<dt><dfn>no-preference</dfn>
Expand All @@ -2659,10 +2660,12 @@ Detecting the desire for increased or decreased color contrast from elements on

<dt><dfn>forced</dfn>
<dd>
Indicates that <a>forced colors mode</a> is active,
identically to ''forced-colors: active''.
See [[#forced-colors]] and
[[css-color-adjust-1#forced]].
Indicates that <a>forced colors mode</a> is active:
the user agent enforces a user-chosen limited color palette on the page,
in effect also implying a strong preference for the associated contrast.
The UA will provide the color palette to authors
through the CSS system color keywords.
See [[css-color-adjust-1#forced]] for details.

<div class=advisement>
This does <em>not</em> necessarily indicates
Expand All @@ -2673,10 +2676,18 @@ Detecting the desire for increased or decreased color contrast from elements on
or some other arrangement that is neither particularly low or high contrast.
</div>

Note: When the User Agent can determine whether the particular forced color scheme
chosen by the user is high or low contrast,
one of ''prefers-contrast: more'' or ''prefers-contrast: less'' should match,
Additionally, when the user agent can determine
whether the forced color palette chosen by the user
has a high or low contrast,
one of ''prefers-contrast: more'' or ''prefers-contrast: less'' must match
in addition to ''prefers-contrast: forced''.

Note:
Similarly,
if the forced color palette chosen by the user
fits within one of the color schemes described by 'prefers-color-scheme',
the corresponding value will also match.
See [[css-color-adjust-1#forced]] for details.
</dl>

<div class=note>
Expand All @@ -2702,6 +2713,51 @@ Detecting the desire for increased or decreased color contrast from elements on
result in a high or low contrast (or something else).
</div>

<div class=example>
Preference for more or less contrast may arise
from a variety of different situations.
Here are some examples:

<ul>
<li>
Many users have difficulty
reading text that has a small difference in contrast to the text
background and would prefer a larger contrast.

<li>
People suffering from migraine
may find strongly contrasting pages
to be visually painful
and would prefer a low contrast.

<li>
Some people with dyslexia find high contrast text hard to read,
as they feel that the letters shine / sparkle as if backlit by too bright a light,
and find low contrast to be more comfortable.

<li>
Environmental factors may also lead
a user to prefer more or less contrast.
See also [[#auto-pref]].
</ul>

Issue: This list should be refined and expanded.
</div>

For compatibility with legacy content,
the following syntax is also supported:

<pre class='descdef mq'>
Name: forced-colors
Value: none | active
For: @media
Type: discrete
</pre>

<dfn type=value dfn-for="@media/prefers-color-scheme" export>active</dfn> matches
when ''prefers-contrast: forced'' matches,
otherwise <dfn type=value dfn-for="@media/prefers-color-scheme" export>none</dfn> matches.

<h3 id="prefers-color-scheme">
Detecting the desire for light or dark color schemes: the 'prefers-color-scheme' feature</h3>

Expand Down Expand Up @@ -2763,36 +2819,6 @@ Detecting the desire for light or dark color schemes: the 'prefers-color-scheme'
please contact the CSSWG to discuss this.
</div>

<h3 id="forced-colors">
Detecting a forced color palette: the 'forced-colors' feature</h3>

<pre class='descdef mq'>
Name: forced-colors
Value: none | active
For: @media
Type: discrete
</pre>

The 'forced-colors' media feature is used to detect if
the user agent has enabled a <a>forced colors mode</a>
where it enforces a user-chosen limited color palette on the page.

<dl dfn-type=value dfn-for="@media/prefers-color-scheme">
<dt><dfn>none</dfn>
<dd>
<a>Forced colors mode</a> is not active;
the page's colors are not being forced into a limited palette.

<dt><dfn>active</dfn>
<dd>
Indicates that <a>forced colors mode</a> is active.
The UA will provide the color palette to authors
through the CSS system color keywords
and, if appropriate, trigger the appropriate value of 'prefers-color-scheme'
so that authors can adapt the page.
See [[css-color-adjust-1#forced]] for details.
</dl>

<h3 id="prefers-reduced-data">
Detecting the desire for reduced data usage when loading a page: the 'prefers-reduced-data' feature</h3>

Expand Down Expand Up @@ -3212,4 +3238,3 @@ Comments from
akklesed,
and 張俊芝
improved this specification.

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