It is server site rendering, use whatever you want to generate an html file and send it to the browser, most websites can do this (blogs,news, forums) then the small interactive interactions you can make them in JS using GPL/BSD whatever.
With JS enabled any site could potentially run non-free code so this has no benefits. As far as I can tell the proposed solution wasn't to develop some tech that ensures (somehow) that all functionality JS provides is transferred to something "free-only" but rather to give up that functionality entirely. That's not a solution, that's just running away from the problem.
There's no fix for criminal activity on the internet (objectively worse than non-free code) and yet I don't think anyone (RMS included) is proposing to sever the connections just to make sure they don't accidentally encounter such activity. The more radical comparison would be "bad stuff" in society and completely isolating yourself from it.