> I don't want to step on your toes by linking my own project

Not at all, it's in good spirit! Happy to talk through any technical details from my side :)

> though if you try to listen for those changes with a mutation observer, it won't fire

It doesn't have to fire an immediate mutation imo. When you click on an element to edit, you could inject a data-attribute into it (not sure if the Chrome Devtool let you do this but an extension might). At write-time, you can query the elements that were marked, get the delta and write it to code.

You might've thought of this longer than I have so I think that might be a naive solution.

> can then be transpiled into any framework the user wants

You might be interested in Mitosis https://github.com/BuilderIO/mitosis