It uses our own HTML5 engine. Rendering is based on a simple canvas tag and 2d context draw calls. The code is written in CoffeeScript. The server runs on node. I just used SublimeText as text editor. We ship the game on Steam via Electron and the greenworks Steam integration.
It has sold over 130,000 units so far. The regular price is around 15$ on Steam. Generally we're obviously very happy with the success considering that 95% of the content was created by two persons.
The project started as a hobby project in 2012, so there are some technical decisions which I would approach differently now: - Using modern JS instead of Coffeescript - Using WebGL instead of Canvas
For our next project we're considering switching to Unity since 90% of our distribution happens through Steam and 3d engines like Unity and Unreal are imho way more mature than HTML5 engines at the moment. The biggest obstacle for us in using HTML5 has been the player perception of web games being for free games that are built around grinding.