Latest Chrome or Firefox needed
CSS pseudo-elements are used for UI elements within the contentEditable and these are given the appearance of clickable widgets by tracking the mouse position and adjusting the CSS cursor accordingly
Source: Doctored.js @ Github.
About a decade ago I saw the Conglomerate XML editor and I thought the interface was brilliant but it wasn't web-based and, later, the project was abandoned
I always wanted to make an editor that wasn't yet-another-WYSIWYG and when I got the idea of using pseudo-elements within a contentEditable that was what started the process about a year ago