Best free and open source wysiwyg html editor

Summernote WYSIWYG HTML editor uses jquery and bootstrap, it allows customization using modules or plugins. Its an Open Source editor licensed under MIT and maintained by the community.

Features

• Supports Bootstrap 3.x.x to 4.x.x
• Lightweight (js+css: 100Kb)
• Smart User Interaction
• Works in all Major Browsers:
• Safari, Chrome, Firefox, Opera, Edge and Internet Explorer 9+
• Works in all Major Operating Systems: Windows, macOS, Linux

TinyMCE is one of the most used free HTML editor. It is an open-source project licensed under LGPL. They also have a commercially licensed version that comes with support. The free version doesn't provide any support. TinyMCE editor is easy to extend and customize through plugins.

CKEditor is ready to use rich text editors created with a powerful framework, offers free HTML editor for 1 site and 5 active users. If you need it for more than one site need to buy a licensed version

A beautiful small content editor and is free and open-source. ContentTools was created by Anthony Blackshaw - a founder of and developer at Getme. The project is maintained by the crew at Getme along with contributors. The code is licensed under MIT.

Editor.js is a block-styled editor for rich media stories. It outputs clean data in JSON instead of heavy HTML-markup.

Key Features:

1. Clean data output
2. API pluggable
3. Open source

Quill is a free, open-source WYSIWYG editor built for the modern web. With its modular architecture and expressive API, it is completely customizable to fit any need.  QuillJs Supports all modern browsers on desktops, tablets, and phones.

Slate JS is a completely customizable framework for building rich text editors. Slate lets you build rich, intuitive editors for your web applications. The most crucial part of Slate is that plugins are first-class entities. That means you can completely customize the editing experience, to build complex editors.