It's always nice to learn how tools implement this. Especially interesting to me, because I am currently working on a similar tool, although more focused on the

It's always nice to learn how tools implement this. Especially interesting to me, because I am currently working on a similar tool, although more focused on the plain text aspect (think Markdown + widgets). I am facing slightly different challenges because widgets have to live in a plain text context. For instance, having a widget (say interactive diagram) as part of a Markdown blockquote. On the other hand I don't have to deal with nested blocks (yet), because regular Markdown elements are just text (with fancy styling though). Another challenge is something like $a+b=c$ where it is shown as rendered LaTeX when the user's caret is not touching that part of the document. Also, contenteditable is hard.