[css-values-4] Clarify allowed/required units in bracket-range notation. w3c#7572

Author: tabatkins

css-values-4/Overview.bs

@@ -1096,7 +1096,9 @@ Range Restrictions and Range Definition Notation</h3>
 	within the angle brackets, after the identifying keyword,
 	indicating a closed range
 	between (and including) <var>min</var> and <var>max</var>.
-	For example, <<integer [0,10]>> indicates an integer between ''0'' and ''10'', inclusive.
+	For example, <<integer [0,10]>> indicates an integer between ''0'' and ''10'', inclusive,
+	while <<length [0, 100px]>> indicates a length between ''0px'' and ''100px''
+	(expressed in any unit).
 
 	Note: CSS values generally do not allow open ranges;
 	thus only square-bracket notation is used.
@@ -1113,6 +1115,12 @@ Range Restrictions and Range Definition Notation</h3>
 	or in the property description,
 	then <code>[&minus;&infin;,&infin;]</code> is assumed.
 
+	Values of &minus;&infin; or &infin; must be written without units,
+	even if the value type uses units.
+	Values of ''0'' <em>can</em> be written without units,
+	even if the value type doesn't allow "unitless zeroes"
+	(such as <<time>>).
+
 	Note: At the time of writing,
 	the <a>bracketed range notation</a> is new;
 	thus in most CSS specifications