[Reinmar]

Still, that does not mean it cannot output HTML. CKEditor 5 implements a custom data model, works in the way you described (cancel native actions or extracts stuff from it, apply those changes to its data model and re-renders the DOM if needed), but it still outputs HTML. Thanks to that you don't need a processor to use that content on your page. Plus, there are cases where converting a custom data model to HTML is actually a tricky job (e.g. inline formatting) and even Editor.js chose HTML for those.

[redmorphium]

JSON is quite nice compared to HTML, in my view. It's less verbose. Also, I like the flatness of Editor.js's data model.

Being JSON makes it easier to render into other formats, too.