X3D is defined here http://www.web3d.org/x3d/specifications/ there is an English description of the standard and a formal definition of the syntax in both the DTD and alternatively the schema.
Since the DTD and schema are designed to be machine readable, they are not especially easy to understand, so I have been working on the following diagrams to help me implement.
I have put some ideas for extending these standards to support simulation and games here.
Proto Declare Nodes
These diagrams have been drawn on ARGO - here is a file containing the diagram