[css-color-6] Make contrast-color() candidates optional

Also move grammar above description of syntax, and some minor prose cleanup

Author: LeaVerou

css-color-6/Overview.bs

@@ -52,27 +52,40 @@ Introduction {#intro}
 	https://caniuse.com/mdn-css_types_color_color-contrast
 -->
 
-Selecting the most contrasting color: the ''contrast-color()'' function {#colorcontrast}
+Computing a sufficiently contrasting color: the ''contrast-color()'' function {#colorcontrast}
 ========================================================================================
 
-	This function takes, firstly, a single color
-	(typically a background, but not necessarily),
-	secondly, a list of two or more colors,
-	and thirdly, an optional target luminance contrast [[!WCAG21]].
+	The purpose of this function is to generate sufficiently contrasting colors from other colors,
+	without authors having to compute them manually.
 
-	It returns the first color in the list
-	to meet or exceed the specified target contrast
-	or, if no target is given,
-	the color in the list with the greatest contrast.
+	Its syntax is:
+
+	
+		contrast-color() = contrast-color( <> [ vs <>#{2,} ]? [ to [<> | AA | AA-large | AAA | AAA-large]]? )
+	
+
+	The only mandatory argument is the first color, which is the color to be contrasted,
+	typically (but not necessarily) a background color.
+
+	An optional list of color candidates is provided after a ''vs'' keyword.
+	If no candidates are provided, the default candidates are used: ''white, black''.
+
+	An optional target luminance contrast [[!WCAG21]]
+	is provided after a ''contrast-color()/to'' keyword.
+
+	If a target contrast is specified,
+	the returned color is the first color candidate that meets or exceeds the target contrast.
+	If no target is given,
+	the color candidate with the greatest contrast is returned.
+	If multiple candidates have the same contrast,
+	the one that comes earliest in the list is returned.
 
 	The single color is separated from the list
 	with the keyword vs
 	and the target contrast, if present, is separated from the list
 	with the keyword to.
 
-	
-		contrast-color() = contrast-color( <> vs <>#{2,}  [ to [<> | AA | AA-large | AAA | AAA-large]]? )
-	
+
 
 	The keyword AA is equivalent to ''4.5'',
 	 AA-large is equivalent to ''3'',
@@ -173,10 +186,10 @@ Selecting the most contrasting color: the ''contrast-color()'' function {#colorc
 
 	
 
-	The colors in the list (after the keyword vs) are tested sequentially,
+	The candidate colors are tested sequentially,
 	from left to right;
 	a color is the temporary winner
-	if it has  the highest contrast of all those tested so far.
+	if it has the highest contrast of all those tested so far.
 
 	List traversal is terminated once the target contrast has been met or exceeded.
 
@@ -260,7 +273,7 @@ Selecting the most contrasting color: the ''contrast-color()'' function {#colorc
 
 			the contrast with cadetblue is 1.97
 			while the contrast with hsl(200 83% 23%) is 6.09
-			which exceeds 4.5, the value for 'AA';
+			which exceeds 4.5, the value for ''AA'';
 			so it has the computed value of the resolved hsl function: