This looks like a very promising project, I have been looking for exactly this.

One feature I would love to see a declarative diagramming solution would support is a hover pop-up with more information or nested diagrams.

Thanks! I think that sounds interesting, to make sure I'm understanding your use case would these pop ups be for your own use or for other people? For example would you want to send a link to someone else on your team and then the link shows the diagram with popups and nesting? Or would it be sufficient to send over the .mmd files and then the other person can use the cli to open the web interface which supports the popups and nesting. I imagine I could add the latter one quickly but for the former I would either add an easy way for users to self serve like with ngrok or some cloud solution. Or alternatively I could add some way to export the diagram just as a standalone HTML file in which case that could be sent and support the popups and hovering without the person you're sending it to having to have the CLI installed.