Tiptap does a pretty good job of it, but really that's just using a contenteditable as an interface to update a well-defined JSON representation of the content.

Tiptap does a pretty good job of it, but really that's just using a contenteditable as an interface to update a well-defined JSON representation of the content. If you are planning to write a custom editor Tiptap is a good starting point, but it's weirdly over-complicated in places.