In this work, we propose a framework to simplify the creation of Augmented Reality (AR) extensions for web applications, without modifying the original web applications. We implemented the framework in an open source package called Alpaca. AR extensions developed using Alpaca appear as a web-browser extension, and automatically bridge the Document Object Model (DOM) of the web with the SceneGraph model of AR. To transform the web application into a multi-device, mixed-space web application, we designed a restrictive and minimized interface for cross-device event handling. We demonstrate our approach to develop mixed-space applications using three examples. These applications are, respectively, for exploring Google Books, exploring biodiversity distribution hosted by the National Park Service of the United States, and exploring YouTube’s recommendation engine. The first two cases show how a 3rd-party developer can create AR extensions without making any modifications to the original web applications. The last case serves as an example of how to create AR extensions when a developer creates a web application from scratch. Alpaca works on the iPhone X, the Google Pixel, and the Microsoft HoloLens.