[css-color] Define "resolving color values"

This is a first pass at providing a centralized definition of how all
<<color>> values are treated at computed value and used value time.

This relates to w3c#741, w3c#480, w3c#867

Author: frivoal

css-color/Overview.bs

@@ -60,12 +60,11 @@ Foreground Color: the 'color' property</h2>
 	Inherited: yes
 	Percentages: N/A
 	Media: visual
-	Computed value: an RGBA color
+	Computed value: See <a>resolving color values</a>
 	</pre>
 
 	This property describes the foreground fill color of an element's text content.
-	In addition, it provides the value that ''currentcolor'' resolves to.
-	If the ''currentcolor'' keyword is set on the 'color' property itself, it is treated as ''color: inherit''.
+	In addition, it provides the value that ''<color>/currentcolor'' resolves to.
 
 	The initial value of this property is ''black''.
 
@@ -161,6 +160,67 @@ Notes On Using Colors</h3>
 		Really?  Isn't HD video in Rec.709? Shouldn't video be consistent with images?
 		Or do implementations really do this differently?
 
+<h2 id='resolving-color-values'>
+Resolving Color values</h2>
+
+	Various properties accept <<color>> values.
+	Unless an exception is explicitly defined,
+	all such properties must <dfn lt="resolve color values|resolving color values">resolve color values</dfn> as defined below
+	to determine the <a>computed value</a> and the <a>used value</a> for <<color>>.
+
+	<ul>
+		<li>
+		The computed value of the ''<color>/currentcolor'' keyword is ''<color>/currentcolor''.
+
+		If ''<color>/currentcolor'' is used as the value of the 'color' property,
+		the used value is the same as if ''inherit'' had been specified instead.
+		For all other properties that accept a <<color>>,
+		the used value of ''<color>/currentcolor'' is the same as
+		the used value of the 'color' property on the same element.
+
+		Note: This means that if the ''<color>/currentcolor'' value is inherited,
+		it’s inherited as a keyword,
+		not as the value of the 'color' property,
+		so descendants will use their own 'color' property to resolve it.
+
+		<li>
+		The computed value and used value of ''<color>/transparent'' is ''rgba(0,0,0,0)''.
+
+		<li>
+		The computed value and used value of
+		<a>named colors</a> (including <<deprecated-system-color>> colors),
+		3 and 6 digits <a>hex colors</a>,
+		comma separated ''rgb()'' colors without an alpha channel,
+		and comma separated ''hsl()'' colors without an alpha channel,
+		is the equivalent numeric value in comma separated ''rgb()'' notation omitting the alpha value.
+
+		<li>
+		The computed value and used value of
+		4 and 8 digits <a>hex colors</a>,
+		comma separated ''rgba()'' colors with an alpha channel,
+		and comma separated ''hsla()'' colors with an alpha channel,
+		is the equivalent numeric value in comma separated ''rgba()'' notation with the alpha value.
+
+	</ul>
+
+	<div class=issue>
+		The above defines values that are already implemented
+		based on the most interoperable behavior.
+		We still need to define how newer syntaxes work:
+		* comma-less with slash separated optional alpha variants of ''rgb()'', ''rgba()'', ''hsl()'', ''hsla()''
+		* comma separated with optional alpha variants of ''rgb()'', ''rgba()'', ''hsl()'', ''hsla()''
+		* ''device-cmyk()''
+		* ''hwb()''
+		* ''lab()'' and ''lch()''
+		* ''gray()''
+		* ''color()''
+		* ''color-mod()''
+	</div>
+
+	Issue: Define if changing the working color space should have any impact on the above.
+
+	Any future specification extending the syntax of <<color>>
+	must define the how to <a>resolve color values</a> for the new extensions.
 
 <h2 id='numeric-rgb'>
 RGB Colors</h2>
@@ -639,17 +699,10 @@ The ''transparent'' keyword</h3>
 
 
 <h3 id='currentcolor-color'>
-The ''currentcolor'' keyword</h3>
-
-	The keyword <dfn value for="<color>">currentcolor</dfn> takes its value from the value of the 'color' property on the same element.
-	This happens at <strong>used-value time</strong>,
-	which means that if the value is inherited,
-	it's inherited as ''currentcolor'',
-	not as the value of the 'color' property,
-	so descendants will use their <em>own</em> 'color' property to resolve it.
+The ''<color>/currentcolor'' keyword</h3>
 
-	If ''currentcolor'' is used as the value of the 'color' property,
-	it instead takes its value from the inherited value of the 'color' property.
+	The keyword <dfn value for="<color>">currentcolor</dfn> represents value of the 'color' property on the same element.
+	Its <a>computed value</a> and <a>used values</a> are determined by <a>resolving color values</a>.
 
 	<div class='example'>
 		Here's a simple example showing how to use the ''currentcolor'' keyword: