To appreciate what the Histrion is, first, read the very first paper on DNAP over here: https://www.scribd.com/document/463360971/DNAP-JWL
There is more recent literature about DNAP and the Histrion that that resource, however, it is much recommended to start with that paper.
The Original DNAP Web Histrion is based off of a SPA hand-coded on CodePen some years ago: http://codepen.io/mcnemesis/pen/XjNbqN.
The purpose? Building a Histrion implementation for the modern web.
There is an older document about DNAP and the histrion, though it might not be online at the moment (since the Nuscribes project is still offline due to lack of funding): https://nuscribes.com/scribes_app/book/40/read/?#chapter-500