--- WebIDL.py
+++ WebIDL.py
@@ -1987,6 +1987,7 @@ class IDLInterface(IDLInterfaceOrNamespace):
                 or identifier == "RunConstructorInCallerCompartment"
                 or identifier == "WantsEventListenerHooks"
                 or identifier == "Serializable"
+                or identifier == "Abstract"
             ):
                 # Known extended attributes that do not take values
                 if not attr.noArguments():