[css-mixins-1] Put a type on @env, in anticipation of it being testable/etc.

Author: tabatkins

css-mixins-1/Overview.bs

@@ -1421,16 +1421,26 @@ in any other context it's invalid and ignored.
 Its grammar is:
 
 <pre class=prod>
-	<<@env>> = @env <<custom-property-name>> : <<declaration-value>>? ;
+	<<@env>> = @env <<custom-property-name>> <<css-type>>? : <<declaration-value>>? ;
 </pre>
 
 This defines a [=scoped environment variable=]
 with a name given by the <<custom-property-name>>,
+optionally a type given by <<css-type>>,
 and a value given by the <css><<declaration-value>>?</css>.
 Its scope is the rule it's nested in,
 or the stylesheet it's defined in
 if it's not nested.
 
+If a <<css-type>> is given,
+then when an ''env()'' referencing this [=scoped environment variable=]
+is [=replace an arbitrary substitution function|replaced=],
+the value must [=CSS/parse=] according to the <<css-type>>,
+or else the ''env()'' is replaced by the [=guaranteed-invalid value=].
+If it successfully parses,
+the value has the specified type.
+
+
 <h4 id=using-scoped-env>
 Using Scoped Environment Variables</h4>