Revit Family Web Viewer is a Three.js-based project viewer. Revit projects / families must be exported using RvtVa3c exporter
Revit Family Web Viewer is licensed under GPLv3 license, which is a viral license, therefore you can use it applications distributed under the SAME (GPL) or compatible licenses. In most scenarios Revit Family Web Viewer CAN'T be distributed as part of a proprietary applications, even if you do not change it or charge money for you application. Please contact CodeCave, PUE for further information.
Revit Family Web Viewer is inspired by Va3c viewer, so part of credits go to it developers.