[css-conditional-3] Revert text about CSS.supports(prop,val) checking…

… the grammar of registered custom properties, so we can support @Property.

css-conditional-3/Overview.bs

@@ -627,23 +627,13 @@ partial namespace CSS {
         for any defined CSS property that the UA supports,
         and |value| successfully [=CSS/parses=] according to that property's grammar,
         return <code>true</code>.
-      2. Otherwise, if |property| is an exact match for the property name of any item
-        in the document's {{[[registeredPropertySet]]}} slot,
-        and the syntax of that item is not "*":
-
-        1. If |value| successfully [=CSS/parses=] according to the syntax of that item,
-          return <code>true</code>.
-        2. Otherwise, return <code>false</code>.
-      3. Otherwise, if |property| is a [=custom property name string=],
+      2. Otherwise, if |property| is a [=custom property name string=],
         return <code>true</code>.
-      4. Otherwise, return <code>false</code>.
+      3. Otherwise, return <code>false</code>.
 
       Note: No CSS escape or whitespace processing is performed on the property name,
       so <code>CSS.supports(" width", "5px")</code> will return <code>false</code>,
       as " width" isn't the name of any property due to the leading space.
-
-      Note: This version of the method will verify the grammar of registered custom properties,
-      but the other version will not.
     </div>
 
 		When the {{supports(conditionText)}} method is invoked