As interactive 3D graphics has become an integral part of modern Web browsers via the low-level WebGL API, it now becomes apparent that higher-level declarative approaches integrated with HTML5 are needed in order to make 3D graphics broadly and easily accessible to Web developers. A key component of interactive 3D scenes are physics simulations. However, specifying the physics properties required major changes to the scene graph in the past. In this paper, we present a declarative and orthogonal physics annotation framework that nicely separates the generic 3D scene description from its physics properties. The approach is based on the declarative XML3D format as an extension to HTML5, has been implemented as a plug-in that runs in three browsers that support XML3D and is demonstrated with a number of examples and performance evaluations.