Current:

internal static Event Unspecialized<T>(Event<T> ev) where T : Element => ev;
internal static MouseEvent Unspecialized<T>(MouseEvent<T> ev) where T : Element => ev;
internal static KeyboardEvent Unspecialized<T>(KeyboardEvent<T> ev) where T : Element => ev;

'Ideal':

internal static U Unspecialized<T, U>(U<T> ev) where T : Element where U : Event<?>, Event => ev;