The application needs in 3D visualization culminate today, in particular in the field of geographic information systems (GIS), as evidenced by the popularity of applications like Google Earth or Google Map. Meanwhile, the popular success of mobile devices like smartphones or tablets and the explosion of cloud computing directly related to ubiquitous networks accelerates the gradual shift from the traditional desktop application development to web and specialized mobile application development. But if the latest technologies centered around HTML5 facilitate the development of rich internet applications (RIA), the gap in resources between a desktop computer and a smartphone requires still an important conceptual and algorithmic work when one aims to design web applications offering a user experience similar to desktop applications. In this paper, we propose a method of terrain simplification suitable for data compression and streaming, and therefore ideal for the GIS visualization in a web browser. Based on new parallel algorithms, this method was designed to exploit the multi-core architectures of the latest CPU and GPU, within the constraints of the latest HTML5 API (WebGL, WebSockets, WebCL). It offers the main advantage of working on irregular grids, which allows to modelize highly nonuniform terrains (containing for instance roads and buildings) that may be unprojectable (plain 3D and not only 2.5D).