The Web Excelsior Editor imports McNeel’s Rhinoceros 3DM files.

The Web Excelsior Viewer uses our own proprietary format, which can only be authored from the Web Excelsior Editor and can only be explored from the Web Excelsior Viewer.

This is quite an important feature:

Because of this, your published interactive designs are no longer in 3DM or any other readable format that a malicious third-party could steal.