Can I use Ellipsus offline?

Ellipsus has offline support in the mobile apps. 

In the browser, it is not currently possible to open or edit documents or drafts offline, as doing so will prevent you from syncing or saving, but offline functionality is a high priority for us. 

Ellipsus began as a collaborative web app because real-time collaboration is much harder to add later than it is to build from the beginning (this is why a lot of offline tools do not include collaboration retroactively). Offline mode is a big technical initiative, especially for a tool built around syncing and collaboration.