I just use a simple bookmarklet that I label 'Make Narrow': javascript:(function(){var%20b=document.getElementsByTagName("body")[0];b.style.maxWidth='50em';b.s

I just use a simple bookmarklet that I label 'Make Narrow': javascript:(function(){var%20b=document.getElementsByTagName("body")[0];b.style.maxWidth='50em';b.style.marginLeft='auto';b.style.marginRight='auto';})() Just manually create a new bookmark and put that as the URL, then when you click it, it runs that snippet and make the body have a max-width of 50em.