Interesting stuff - none of these seem to have that Smalltalk-like visual IDE bolted right into them.

Given how big and important a platform the modern web is - I really am starting to think - something like this should exist - where I have a neat little IDE bundled right within the browser using which I can evolve the browser...

Not really 'visual', but https://nyxt-browser.com maybe?

Otherwise the less extreme https://luakit.github.io / https://github.com/luakit/luakit comes to mind. It's not dead yet.