The best open source alternative to CKEditor is TinyMCE, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to CKEditor and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to CKEditor are Trumbowyg (Free, Open Source), ContentTools (Free, Open Source), Editor.js (Free, Open Source) and Aloha Editor (Free, Open Source).
Almost everyone thinks TinyMCE is a great alternative to CKEditor.
Most users think ContentTools is a great alternative to CKEditor.
Editor.js is a block-styled editor for rich media stories. It outputs clean data in JSON instead of heavy HTML-markup. And more important thing is that Editor.js is designed to be API extendable and pluggable. So there are few key features: 1. Clean data output