It is GWT, so it is transpiled to JS. I remember when I first saw this project years ago, I was impressed. Back then GWT was the best choice for statically-typed, large JS project development IMO.
We just chopped GWT out of our project a month or so ago; I'll fully admit that I didn't know much about it, but every step of the way I felt like I was battling it - it ended up being easier, faster and more testable to write our own JS for the front-end.
I do a lot of geometry related programming and the ability to add some graphic illustrations directly into the source code comes in handy every now and then.
Also, everybody needs a small ascii graphic in the output from time to time.
It's a clever tool for creating Ascii art. Not for everyone, but if you've ever tried to create boxes and lines in ascii, this is a tool that makes that so much easier .
This is something I might use in source code to describe / draw the relationship between elements.