I've been looking for a modern WYSIWYG/contenteditable editor for the web for a couple of years. There are some projects that meet some good criteria (Quill, Trix). But the day I found CKEditor 5 is the day I stopped looking.
When you look through the source code that makes up CK5 you immediately understand how powerful this editor actually is. By bolting on different plugins you can go from a basic input field to a complete rich document editor.
Then you find out that it has a collaboration service which takes less than an hour to setup where you'll find yourself editing the same document with co-workers (whilst you're still working on it in dev) in awe of how easy and how solid it is.
I've been a very vocal supporter of CK5 since it was in beta. From that early on I knew that this was going to be the one to stick with. The CK team has been fantastic and receptive to questions and resolving issues. They're absolutely killing it. Go check out the project and see for yourself, its all there to see on GitHub.
Modern architecture. Completely modular. Easy to extend with plugins. A WYSIWYG editor built for modern browsers. Always updating, improving
The modern ECMAScript will not work legacy browsers (for now) -- tiny fraction of users or restrictive/legacy corporate environments.