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

ProseMirror is a well-behaved rich semantic content editor based on contentEditable, with support for collaborative editing and custom document schemas. Rich content editors should, ideally, produce clean, semantically meaningful documents while still being easy for users to understand. ProseMirror tries to bridge the gap between editing explicit, unambiguous content like Markdown or XML, 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. It is modular architecture makes sure you only load the code you need, and can replace parts of the system as needed.