[css-color-4] General note about legacy color syntax, w3c#6911

Author: svgeesus

css-color-4/Overview.bs

@@ -522,6 +522,32 @@ Representing Colors: the <<color>> type</h2>
 	but fully transparent--
 	i.e. <nobr>''rgb(0 0 0 / 0%)''</nobr>.
 
+	All of the <<color>> syntactic forms
+	defined in this specification
+	use space, not comma,
+	to separate the color components.
+	They also use a solidus ("/")
+	to separate an optional alpha term
+	from the color components.
+
+	For Web compatibility,
+	<<color>> syntactic forms
+	defined in earlier levels of this specification
+	also support
+	<dfn export>legacy color syntax</dfn>
+	which has the following differences:
+
+	* color components are separated by comma-whitespace
+	* non-opaque forms use a separate notation
+	    (for example ''hsla()'' rather than ''hsl()'')
+	* minimum required precision is lower, 8 bits per component
+
+	For the syntactic forms introduced
+	in this or subsequent levels,
+	where there is no Web compatibility issue,
+	<a>legacy color syntax</a> is not supported,
+	and attempting to use it is an error.
+
 <h3 id="hue-syntax">The <<hue>> syntax</h3>
 
 Hue is represented as an angle of the color circle