[css-color] Add color-adjust property, per WG resolution.

Author: tabatkins

css-color/Overview.bs

@@ -2326,6 +2326,81 @@ Color Management: the 'color-correction' property</h2>
 	Note: The initial value of this property may change in a future level of this specification.
 
 
+<h2 id='color-adjust'>
+Preserving Colors in Different-Capability Devices: the 'color-adjust' property</h2>
+
+	On most monitors,
+	the color choices that authors make have no significant difference
+	in terms of how the device performs;
+	displaying a document with a white background or a black background is approximately equally easy.
+
+	However, some devices have limitations and other qualities that make this assumption untrue.
+	For example,
+	printers tend to print on white paper;
+	a document with a white background thus has to spend no ink on drawing that background,
+	while a document with a black background will have to expend a large amount of ink filling in the background color.
+	This tends to look fairly bad,
+	and sometimes has deleterious physical effects on the paper,
+	not to mention the vastly increased printing cost from expending the extra ink.
+	Even fairly small differences,
+	such as coloring text black versus dark gray,
+	can be quite different when printing,
+	as it switches from using a single black ink
+	to a mixture of cyan, magenta, and yellow ink,
+	resulting in higher ink usage and lower resolution.
+
+	As a result, in some circumstances user agents will alter the styles an author specifies in some particular context,
+	adjusting them to be more appropriate for the output device
+	and to accommodate what they assume the user would prefer.
+	However, in some cases the document may be using colors in important, well-thought-out ways that the user would appreciate,
+	and so the document would like some way to hint to the user agent that it might want to respect the page's color choices.
+	The 'color-adjust' property controls this.
+
+	<pre class='propdef'>
+	Name: color-adjust
+	Value: economy | exact
+	Initial: economy
+	Applies to: all elements
+	Inherited: yes
+	Media: visual
+	Computed value: as specified
+	Percentages: N/A
+	</pre>
+
+	The 'color-adjust' property provides a hint to the user-agent about how it should treat color and style choices
+	that might be expensive or generally unwise on a given device,
+	such as using light text on a dark background in a printed document.
+	If user agents allow users to control this aspect of the document's display,
+	the user preference <strong>must</strong> be respected more strongly
+	than the hint provided by 'color-adjust'.
+	It has the following values:
+
+	<dl dfn-type=value dfn-for=color-adjust>
+		<dt><dfn>economy</dfn>
+		<dd>
+			The user agent should make adjustments to the page's styling
+			as it deems necessary and prudent for the output device.
+
+			For example, if the document is being printed,
+			a user agent might ignore any backgrounds
+			and adjust text color to be sufficiently dark,
+			to minimize ink usage.
+
+		<dt><dfn>exact</dfn>
+		<dd>
+			This value indicates that the page is using color and styling on the specified element
+			in a way which is important and significant,
+			and which should not be tweaked or changed except at the user's request.
+
+			For example,
+			a mapping website offering printed directions
+			might "zebra-stripe" the steps in the directions,
+			alternating between white and light gray backgrounds.
+			Losing this zebra-striping and having a pure-white background
+			would make the directions harder to read with a quick glance
+			when distracted in a car.
+	</dl>
+
 <h2 id="sample">
 Sample style sheet for (X)HTML</h2>