It is an interesting little piece of kit, and The Royal Wedding example does a nice job of showing off all the various things it is capable of.
There are a few features that I would like to see added, though. My biggest wish would be for graphical tile support, followed closely by 100% deterministic mode where all randomness determined is by initial seed + player input (which allows for easily creating a replay system as well as shareable "challenge seeds").
Github for Rot.js:
I might fork it and give adding some features a shot sometime.