I have been using REPL.it for codebooks for some time now (to replace text books). Up until now I have been having students put comments in the code. Then I started adding a README.md to the codebook for the notes to remove the comment clutter from the code. Students are much more engaged as they compose their own text book with working code.
However, we currently have no way to preview the CommonMark rendering. I think if students saw it rendered, perhaps by changing the preview pain to show the rendered HTML whenever a CommonMark file is in focus, that it would really promote student's desire to write this as well as teaching one of the most important languages of web content in the world (CommonMark/Markdown).
Please, please, please do not implement anything but CommonMark, however. Markdown is seriously plagued by a lack of standards. (I even have a BaseML specification that his a Discord-like simplification of all the common syntax elements.)
Thanks for your consideration.
You might be able to rip the code directly from VSCode's preview pane that is built into the editor.
I even noticed that these posts accept CommonMark (Markdown) but there is really no place to learn/test/use it on REPL.it itself.