For HTML to Markdown this works in Pandoc, because HTML elements are valid Markdown. Haven't tried with other target formats. One option if you can run Haskell

For HTML to Markdown this works in Pandoc, because HTML elements are valid Markdown. Haven't tried with other target formats. One option if you can run Haskell code somewhere is to use the Pandoc library, which will produce an in-memory structure that you can manipulate as the user makes edits.