I took the CSS from bettermotherfuckingwebsite.com and turned it into a bookmarklet. Works alright at least on this page: https://jsfiddle.net/49uzxkeu/
You are doing God's work.
Just don't fullscreen the browser ;)
Perhaps browsers should have p { max-width: 72ex; } by default. And a user setting for this, just like default font size.
I don't like my freedom of screen width being encroached on.
You might be sitting too close to your monitor if you can't read across it comfortably.