Yes, tool can work offline to some extent. Basically user can design the model, make changes to the model. and this model will be saved in browser cache as draft. But when user gets online, user need to click save so it is saved in the server.
But we wont recommend this practice, because there are chances to lose the browser cache and user can lose all the changes.
I would suggest to use the tool only in online mode.
Team - SqlDBM
Customer support service by UserEcho