Slate

Slate is a completely customizable framework for building rich text editors. Slate lets you build rich, intuitive editors like those in Medium, Dropbox Paper or Google Docs—which are becoming table stakes for applications on the web—without your codebase getting mired in complexity.

It can do this because all of its logic is implemented with a series of plugins, so you aren't ever constrained by what is or isn't in "core". You can think of it like a pluggable implementation of contenteditable built on top of React. It was inspired by libraries like Draft.js, Prosemirror and Quill.