Thank you Jeremy.
At the end I ve generated an obj from Blender.
I passed from Rhino to Blender.
Finally the file .mtl (created in this software) has been read by Webgl.
I am still waiting for the best solution to reduce the size of the obj directly from Rhino, during the save time. Not plugin for Mac available at the moment!
I will wait for that!
Thank you for the help.