Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
CKEditor - Real-time collaborative future-ready rich text editor
ClassicPress - The WordPress fork. No Gutenberg. Great future!
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more
Froala Editor - Froala Editor is a WYSIWYG HTML editor that enables rich text editing capabilities for the applications.