[css-color-5] Separate out the conceptual aspects of RCS from the syn…

…tactic details. Clarify which color space the channel keywords relate to, #10191

css-color-5/Overview.bs

@@ -796,6 +796,10 @@ which is rgb(52.446% 45.821% 62.953%)
  Relative Color Syntax
 </h2>
 
+<h3 id="rcs-intro">
+ Introduction to Relative Color Syntax
+</h3>
+
 In previous levels of this specification,
 the color functions could only specify colors in an absolute manner,
 by directly specifying all of the color channels.
@@ -811,6 +815,15 @@ can <em>either</em> be directly specified,
 or taken from the origin color
 (and possibly modified with [=math functions=]).
 
+The origin color and the relative color need not use the same color function.
+
+All operations take part in the 
+[=color space=] of the [=relative color=] function;
+the [=origin color=] is converted to that color space,
+and [=channel keywords=] 
+refer to the color space of the relative color,
+<em>not</em> the [=origin color=].
+
 
 <wpt>
  relative-currentcolor-a98rgb-01.html
@@ -840,7 +853,6 @@ but they all follow a common structure:
 
 * An [=origin color=] can be specified with a ''from <<color>>'' value at the start of the function.
  This includes the optional alpha channel, if specified.
-* The origin color and the relative color need not use the same color function.
 * If an [=origin color=] is specified,
  the remaining arguments can either be specified directly, as normal,
  or be specified as a <dfn>channel keyword</dfn>