1. Stick to raw DOM, no framework
2. Index.html can embed pre-baked DOM, CSS and JS so it appears instant, would it work best?
3. The rest would be an app using: 3.1. Milkdown 3.2. Ag-Grid (probably) 3.3. XLS parser (maybe?)

One part of the build would need to process some "core" part of the code and inject it into HTML.

Another part of the build would build the whole app into JS+CSS files.

GET http://google.com

Preview this:

[{ red: 1, green: 2, blue: 3},
{apple: 10, pear: 20, google: 30}]

previewThis.map(x => <div>{JSON.stringify(x, null, 2)</div>)

/http://*** (+ headers) --> code block with http go! language prefixed with GET

POST http://** (+headers, body) --> code block with http go! language

*** or txt:*** --> Markdown but show unicode formatting controls

fmt:*** --> Markdown with normal Markdown formatting

js:** --> code block with js go! language

file:/path --> File manager

file:????? --> File manager use some funky compressed payload format