The medium-editor project is pretty good. It's not a "true" clone of the medium editor, but it gets development up and going pretty quickly with a contentEditable editor. We used medium-editor for http://www.doctant.com.

As others have noted, it's really tough to compete with text editing/rich text applications (e.g., Sublime, Vim, Word, etc.) on the web. So far, the real Medium is the only app that I've used that isn't a pain to write in even though it's contentEditable.

Another promising project is draftjs from Facebook. Check it out: https://facebook.github.io/draft-js/.

I made an extensive prototype with it and definitely do not recommend it.

It's really just a UI clone of the Medium editor. It doesn't work at all the same way under the hood and instead tries to use ContentEditable as the store of a document.

Any approach which works that way is doomed to failure. ContentEditable works fine as an input but you need to have a pure mapping from all inputs to an internal structure which only your editor controls.

I've used it before - about a year ago and found it to be quite buggy in actual use.