Toolkit for building rich-text editors on the web. Developed by The Guardian and others.
An ideal content editor produces structured, semantically meaningful documents, but does so in a way that is easy for users to understand. ProseMirror tries to bridge the gap between Markdown text editing and classical WYSIWYG editors.
If you are using ProseMirror to make profit, there is a social expectation that you help fund its maintenance. Start here
ProseMirror is maintained by Marijn Haverbeke, whose page the Start here links to.
The Guardian article on moving from their previous editor Scribe to ProseMirror is also a good read.