First of all this is a volunteer experiment done in one day to discover the features of Graphene.
Graphene is a build of B2G used in Browser.html and for Firefox OS testing.
The interesting feature of this project that is like a Firefox without the UI, only the Web rendered engine and that support the Firefox OS APIs.
My idea when I started that project was a “Firefox OS is not dead to give us that chance” to fight the monopoly of WebKit in the web.
So after the discover of that build I tried to experiment to create an alternative to Electron/nw.js that are webkit based.
In the repo you can find all the information to replicate and try yourself and the list of api supported on that build and that works.
After the experiments I see that this build is not enough to create an alternative because the interesting APIs like Service Worker, Video/Audio capture and Notification not works.
That is ok because is an internal build used for specific use and my was only an experiment to search an alternative.
So if you want an HTML5 app that run on a browser without the UI (like a packaged app) Graphene is ok but remember is an internal build.
The rest of infos are in the Readme, if you have other questions I am here.