[css-color-4] Add percent reference range for HSL, fix w3c#8515

Author: svgeesus

css-color-4/Overview.bs

@@ -1988,6 +1988,19 @@ HSL Colors: ''hsl()'' and ''hsla()'' functions</h2>
 	<dfn><<legacy-hsla-syntax>></dfn> = hsla( <<hue>>, <<percentage>>, <<percentage>>, <<alpha-value>>? )
 	</pre>
 
+	<table>
+		<tr>
+			<th>Percentages</th>
+			<td>Allowed for S and L</td>
+		</tr>
+		<tr>
+			<th>Percent reference range&nbsp;</th>
+			<td>
+				for S and L: 0% = 0.0, 100% = 100.0
+			</td>
+		</tr>
+	</table>
+
 	<wpt>
 		hsl-001.html
 		hsl-002.html