Merge pull request w3c#217 from jonathanyc/master

Fix register/unregisterProperty on CSS interface to be static.

Author: tabatkins

css-properties-values-api/Overview.bs

@@ -69,8 +69,8 @@ dictionary PropertyDescriptor {
 };
 
 partial interface CSS {
-  void registerProperty(PropertyDescriptor descriptor);
-  void unregisterProperty(DOMString name);
+  static void registerProperty(PropertyDescriptor descriptor);
+  static void unregisterProperty(DOMString name);
 };
 </pre>