The Web of Things (WoT) aims to extend the Web into the physical world by promoting the adoption of Web protocols by situated services and smart objects (ambient artifacts). However, real-world ambient artifacts often adopt proprietary and/or non-Web protocols, making them invisible to Web search engines and inaccessible to conventional Web agents. Smart Gateways have been proposed as a way to “Web-enable” proprietary ambient artifacts through intermediary proxy nodes; however, the requisite infrastructure is difficult to deploy at Web scale. To address such challenges, we are developing Ambient Dynamix (Dynamix): a plug-and-play context framework for mobile devices, which enables Web agents to interoperate with non-Web ambient artifacts – directly from the browser. In this paper, we describe how Dynamix can be used to transform the user’s device into an ad-hoc Smart Gateway in-situ, enabling Web applications (in the device’s browser) to seamlessly interact with non-Web ambient artifacts in the physical environment. We describe an operational prototype implementation, which enables Web apps to discover and control nearby UPnP and AirPlay media devices uniformly. We also present a performance evaluation that indicates the prototype imposes low processing and memory overhead, and is suitable for deployment on many commodity mobile devices.