Nowadays, Web browser extensions are the more convenient way to modify existing Web applications and the browser itself to satisfy non-contemplated requirements. Extensions play an important role because of the relevance that the Web and the Web browser have in our daily life. The functionality offered by Web extensions varies. Many Web extensions are tools that improve the interaction with existing Web sites based on techniques such as mashups and Web augmentation. Accessing and manipulating Web sites’ content is a challenge for the development and maintenance of extensions. In this paper, we present a data service layer that allows extension developers to create APIs based on Web annotations to access existing Web content. The data service layer simplifies the retrieval of structured information from Web pages, without the need of DOM manipulation in the extension code. We provide visual programming tools to create these APIs so there is no need for advanced programming skills, making extension development and maintenance easier.