If this is such a great idea, I would recommend the author of this proposal to work instead on a transpiler that converts the proposed javascript-only websites into a mix of HTML+CSS+js. If the idea catches on and, at some point in the future, people start using the transpiler almost exclusively, then the web browser developers could add direct support, and even eventually phase out support for legacy individual HTML/CSS/js files. Another benefit of implementing a transpiler first is that it would make the proposed javascript-only approach usable today.