Editor.js is a so called “block style editor” like the one Medium (and recently WordPress) sport.
The Editor.js workspace consists of separate Blocks: paragraphs, headings, images, lists, quotes, etc. Each of them is an independent contenteditable element (or more complex structure) provided by Plugin and united by Editor’s Core.
The output is not HTML but a JSON object which describes each Block.
The resulting JSON reminds me of Jasonette, a tool to describe Mobile Apps using JSON.