This script is a handler for X3D, an open standard to represent 3D scenes/applications in an XML dialect. By simply dropping unityx3d.cs into your Unity assets, a new menu entry Assets/X3D should appear.
Simply select parts of the scene or everything with CTRL+A and click Assets/X3D/Export to X3D. Currently, there is an option to use the CommonSurfaceShader instead of the standard X3D material to enhance some of the material visuals.
Material: The material mapping from the physically based standard shader in Unity 5 cannot be fully done with the currently available X3D nodes. The standard needs better material representation. Other material representations apart from the standard shader are currently ignored.
Environment: Currently, there is no working code to export environment maps or skydomes. Procedurally generated backgrounds are ignored.
Textures: Generated textures are ignored.
Camera: Camera placement is currently wrong due to coordinate system issues.
Shaders: Custom shaders are currently ignored. Need to find code to trigger GLSL conversion first, then need to find a way to map shader inputs (textures, shader constants etc.) from Unity to X3D and vice versa.
Mesh: Animated meshes with rigs aren't exported. HAnim would make sense here.
Import: The X3D importer is incomplete.