Firefox OS was a really cool thing that was probably the closest we've seen to that. It implemented full HTML5 web technology in the DE, but I think it got discontinued. A lot of DEs use XML too, including Android, so it's a popular idea. As far as something for Windows/Linux/Mac, electron and others based on Nodejs and Chromium are fascinating, but you have to think long-term with platforms, such a GUI toolkit.
Think about this. Nodejs is also a huge mess. (lackluster event system and chaotic ecosystem.) That is all because, in my belief, it was hyped to boot and pushed out the door too quickly, and once a platform is established, developers have to live with the ecosystem and build off of it, which perpetuates the chaos. Similar situation with the browser, which is due to mainly vendor compatibility, legacy codebase, and unexpected rapidly growing specifications outside of the original platform model. Now you combine those two, and yes, it's revolutionary the power you can get out of it if you're careful enough to wade through all that. You can create some awesome apps and get them out the door too.
All that being said, it's not going to be nearly the power and stability you can get out of a platform that's designed specifically to handle the paradigm, ecosystem, and scale. Rust in fact holds to that discipline, integrating the paradigm completely first as a foundation, which is why entire kernels and OS and even GUI have been built from scratch with it! That's the power no other modern language is achieving!
Servo is a much needed initiative to modernize web technology, and break free of the legacy code that's holding it back. In order to do that, they needed a whole new language! So we have Rust. I'm sure we could hook up Servo to Nodejs, but that may take a step backwards, and we may find ourselves where we were before. Of course experimentation is fun and valuable too.
Platforms like GUI toolkits should ultimately empower the developer, not the application. Food for thought.