A significant amount of research project funding is spent creating customized annotation systems, re-inventing the wheel once and again, developing the same common features. In this paper, we present WACline, a Software Product Line to facilitate customization of browser extension Web annotation clients. WACline reduces the development effort by reusing common features (e.g., highlighting and commenting) while putting the main focus on customization. To this end, WACline provides already implemented 111 features that can be extended with new ones. In this way, researchers can reduce the development and maintenance costs of annotation clients.