This is coming close to WWW's original vision because the very first web browser was also an editor. Tim Berners-Lee's application on the NeXT was basically a wrapper for the operating system's built-in rich text editing class named TextView. (It later became NSTextView on Apple's Mac OS X and still powers the TextEdit app on Mac.)

We lost editing for two reasons:

1) The HTTP PUT method didn't exist yet, so edited HTML files could only be saved locally.

2) Mosaic built a cross-platform web browser that defined what the WWW was for 99% of users, and they didn't include editing because that would have been too complex to build from scratch in their multi-platform code base.

I don't say this often or lightly: my mind is blown.

Why is this not catching fire? Especially in the day of vibe coding this is a far better and far more effective route to app building

This reminds me of some of the better experiments in mid-aughts Web dev, and is exactly the kind of project that helps push standards & user expectations forward.