[css-color-4] Add computed/used entry for system colors, linking to #3847.

Author: tabatkins

css-color-4/Overview.bs

@@ -231,6 +231,10 @@ Resolving Color values</h2>
 		it must be converted
 		as specified in [[#cmyk-colors]].
 
+	: [=system colors=]
+	::
+		Issue(3847): Make system color keywords compute to themselves.
+
 	: ''currentcolor''
 	::
 		The ''currentcolor'' keyword computes to itself.
@@ -725,8 +729,8 @@ Named Colors</h3>
 For historical reasons, this is also referred to as the X11 color set.
 
 
-<h3 id="css-system-colors">
-CSS System Colors</h3>
+<h3 id="css-system-colors" dfn lt="system colors">
+System Colors</h3>
 
 	In [=forced colors mode=],
 	most colors on the page are forced into a restricted, user-chosen palette.