At it's core, a browser is an HTML/JS rendering engine with some chrome to allow users to manage what pages they're looking at – and in most cases, that chrome is actually pretty minimal. It seems like a natural evolution to play with the idea of implementing that chrome in the natural UI language of the browser engine too. Yeah, the tools aren't there yet – but experiments like this will give us some scope to play around with what might be possible and identify the pain points that must be overcome to make it a reality.
Great stuff.