This contributes nothing more than <div contenteditable> and to present this a document editor is counter-productive to users searching for something reliable. The link below shows the many shortcommings of just calling the contenteditable browser API as an emulator. For example, Google Docs does not use contenteditable at all -- for the many reasons outlined below. Building a real doc editor that functions as a widget in blogs and forums sites requires a lot more support -- of the open source alternatives, Medium is one of the best.
You might not have looked at this closely enough, but "contenteditable" is really only one line that has been added to make the demo more useful.
The real thing this is about is the CSS. There are ~250 lines that produce the correct page sizes, margins and paged content and ensure that prints are as expected.
Why ContentEditable Is Terrible, Or: How the Medium Editor Works (2014) (medium.com) https://news.ycombinator.com/item?id=11487667