It's just that you write it once and it can run anywhere without "installing" many dependencies other than a browser. No fugly GUIs, no inconsistencies, etc.

By web-based I didn't mean it to be on the Internet.

So basically, "web-based" is the new portable GUI library.

Absolutely — you just open a high port on localhost and open the URL in a browser. I've used this technique before on consulting projects with great success.

That's not really web-based then, more HTML-based.

I've actually thought about making something like this.

My reasons: collaborative coding + debugging, and IDEs without needing an environment aside from your browser (coding + compiling + debugging code on the device it's made for).

We actually tried to make something like that with a friend a few years ago. It didn't end well but it was an interesting experience.

What's left of it today: http://www.gravytea.com/labs/metacoding/