[css-color-5] Add missing mixing color spaces for serialization. Fix w3c#6859

Author: Chris Lilley

css-color-5/Overview.bs

@@ -2350,13 +2350,17 @@ is the same as that specified in
 
 <table class="data">
 	<tr>
-		<th>color space</th>
+		<th>mixing color space</th>
 		<th>form</th>
 	</tr>
 	<tr>
 		<td>srgb</td>
 		<td>color(srgb r g b)</td>
 	</tr>
+	<tr>
+		<td>srgb-linear</td>
+		<td>color(srgb r g b)</td>
+	</tr>
 	<tr>
 		<td>hsl</td>
 		<td>hsl(h s l)</td>
@@ -2365,9 +2369,17 @@ is the same as that specified in
 		<td>hwb</td>
 		<td>hwb(h w b)</td>
 	</tr>
+	<tr>
+		<td>xyz-d65</td>
+		<td>color(xyz-d65 x y z)</td>
+	</tr>
+	<tr>
+		<td>xyz-d50</td>
+		<td>color(xyz-d50 x y z)</td>
+	</tr>
 	<tr>
 		<td>xyz</td>
-		<td>color(xyz x y z)</td>
+		<td>color(xyz-d65 x y z)</td>
 	</tr>
 	<tr>
 		<td>lab</td>