Smartphone sensors can be leveraged by malicious apps for a plethora of different attacks, which can also be deployed by malicious websites through the HTML5 WebAPI. In this paper we provide a comprehensive evaluation of the multifaceted threat that mobile web browsing poses to users, by conducting a large-scale study of mobile-specific HTML5 WebAPI calls used in the wild. We build a novel testing infrastructure consisting of actual smartphones on top of a dynamic Android app analysis framework, allowing us to conduct an end-to-end exploration. Our study reveals the extent to which websites are actively leveraging the WebAPI for collecting sensor data, with 2.89% of websites accessing at least one mobile sensor. To provide a comprehensive assessment of the potential risks of this emerging practice, we create a taxonomy of sensor-based attacks from prior studies, and present an in-depth analysis by framing our collected data within that taxonomy. We find that 1.63% of websites could carry out at least one of those attacks. Our findings emphasize the need for a standardized policy across browsers and the ability for users to control what sensor data each website can access.