You probably have seen some HTML editors listing instant preview as a feature, whether they are for desktop or web platform. rsted (GitHub) is the reStructuredText to that. Here is a screenshot:

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgedTnmFCAdBae5gLVo9KM3eDLrBLVItXrZ9avnAhG0L9vKcpFsWXha1fl1W3oKNo90-QmeVAX61nZfVF3x0uVURJVavsiFS8qmMJUdb6Gyo60CaHEXYuBzctnvS_9bWUiQVUOs3lFcWTQ/s640/2014-03-26--15%253A19%253A34.png

The UI is very simple and clean, markup source code on left, live preview on the right, and a toolbar just above them. Those markup buttons are for lazy people if you don’t want to hand-coded too much. There is also a theme switch, plus a save button. That saving function is for storing your code on the server, so you could edit again or share the link, but anyone with the link can delete it.

The rendering is made by rst2html.py on the server. This editor is written by Andrey Rublev, licensed under the MIT License, built on Flask, Redis, and Docutils.