[css-env-1] Per WG resolution, the set of supported UA-defined variables is *not* open-ended, and they should all be listed in the spec.

Author: tabatkins

css-env-1/Overview.bs

@@ -72,40 +72,31 @@ and via CSS.
 Note that mixing CSS rules and JS-defined stuff can easily get messy,
 as demonstrated by CSSFontFaceRule vs FontFace...
 
-The following UA-defined [=environment variables=] are officially defined and must be supported,
-tho user agents may define additional undocumented [=environment variables=]:
+The following UA-defined [=environment variables=] are officially defined and must be supported.
+Additional UA-defined [=environment variables=] *must not* be supported
+unless/until they are added to this list.
 
-* Issue: define list of predefined vars
+<index type=value for=env()></index>
 
 Safe area inset variables {#safe-area-insets}
 ------------------------------------------------------------------
 
-<table>
+<table dfn-type=value dfn-for="env()">
 	<tr>
-		<th>Name</th>
-		<th>Value</th>
-		<th>Media</th>
-	</tr>
+		<th>Name
+		<th>Value
 	<tr>
-		<td>safe-area-inset-top</td>
-		<td><<length>></td>
-		<td>visual</td>
-	</tr>
+		<td><dfn>safe-area-inset-top</dfn>
+		<td><<length>>
 	<tr>
-		<td>safe-area-inset-right</td>
-		<td><<length>></td>
-		<td>visual</td>
-	</tr>
+		<td><dfn>safe-area-inset-right</dfn>
+		<td><<length>>
 	<tr>
-		<td>safe-area-inset-bottom</td>
-		<td><<length>></td>
-		<td>visual</td>
-	</tr>
+		<td><dfn>safe-area-inset-bottom</dfn>
+		<td><<length>>
 	<tr>
-		<td>safe-area-inset-left</td>
-		<td><<length>></td>
-		<td>visual</td>
-	</tr>
+		<td><dfn>safe-area-inset-left</dfn>
+		<td><<length>>
 </table>
 
 The safe area insets are four [=environment variables=] that define a rectangle by