ProseMirror – A toolkit for building rich-text editors on the web

ProseMirror tries to bridge the gap between Markdown text editing and classical WYSIWYG editors. ProseMirror has built-in, ground-up, rock solid support for collaborative editing, where multiple people work on the same document in real time. Document schemas allow editing documents with a custom structure without writing your own editor from scratch.

Source: prosemirror.net