[css-color-4] Clarify serialization of "none", see w3c#6959

Author: svgeesus

css-color-4/Overview.bs

@@ -962,7 +962,7 @@ In this section, the strings used in the specification and the corresponding cha
 <table class="data">
 	<tr>
 		<th>String</th>
-		<th>Character</th>
+		<th>Character(s)</th>
 	</tr>
 	<tr>
 		<td>" "</td>
@@ -984,12 +984,26 @@ In this section, the strings used in the specification and the corresponding cha
 		<td>"/"</td>
 		<td>U+002F SOLIDUS</td>
 	</tr>
+	<tr>
+		<td>"none"</td>
+		<td>U+006E LATIN SMALL LETTER N
+			U+006F LATIN SMALL LETTER O
+			U+006E LATIN SMALL LETTER N
+			U+0065 LATIN SMALL LETTER E
+		</td>
+	</tr>
+
 </table>
 
 The string "." shall be used as a decimal separator,
 regardless of locale,
 and there shall be no thousands separator.
 
+For syntactic forms which support [=missing values=],
+the value ''none'' (equivalently NONE, nOnE, etc),
+shall be serialized in all-lower case
+as the string "none".
+
 
 <h4 id="serializing-alpha-values">Serializing alpha values</h4>