Instead of creating a fullblown NetBeans Platform application for doing WYSIWYG editing for EPUB, similar to Sigil, I decided to focus purely on the very narrow scope of EPUB reading. The scope is narrower and, since the application will be a lot less ambitious and smaller, a pure JavaFX implementation makes sense.
When you somehow get, e.g., buy, an EPUB file, you typically read it on a tablet or mobile device. However, some people in the world, e.g., me, still have laptops. Therefore, I'm creating a small JavaFX application that unzips EPUB files, into a temp directory, and then loads them into a JavaFX WebView.
For an application like this, simplicity is the most important thing. Very few buttons, very few options, preferably no configuration of anything. Just let the user open the EPUB file and read it, that's it, nothing fancy.
CSS stylesheets and images are correctly read. It's exactly what it looks like, a reader for EPUB files. The back and forward buttons are working and you can also switch to the table of contents.
When it is complete, which it pretty much is right now, publishers of EPUB files can make this small app available from their site, to simplify life for their readers, since it will run easily and well on all operating systems.