What's the different between WebID and IDL(XPIDL)?

Question

WebIDL : Is the interface description language used to describe the data types, interfaces, methods, properties, and other components which make up a Web application programming interface (API).

IDL(XPIDL) : It is an Interface Description Language used to specify XPCOM interface classes. (XPIDL is expected to converge towards WebIDL in the future.)

Those two interfaces using XPCOM to implement the cross-language IPC mechanism.

So, my questions are :

1. What's the different between WebIDL and XPIDL?
2. Why IDL will converge towards WebIDL in the feature?

Thank you.