The live, production project can be found here.
The editor is minimalist and responsive, employing modern HTML5 and Javascript ES6 technologies now supported across all major browsers. Crucially, all work is done client-side, including EPUB packaging and ZIP file creation, as well as downloading (via HTML5 Blobs and CORS), eliminating unnecessary server calls entirely.
Warning: this downloader makes it all too easy to get addicted to fanfiction :).
Want to look under the hood? Need to report a bug? Want a new feature implemented? Check out the project on Github or open an issue. I'm completely open to contributions in the form of pull requests and will review them as soon as possible.