the link for who_bought.atom.builder is in Safari 11.0 interpreted as an atom feed and for me gave the following error.
No RSS reader is installed. Safari can’t open “feed:https://media.pragprog.com/titles/rails51/code/rails51/depot_p/app/views/products/who_bought.atom.builder” because Safari can’t display RSS feeds.
You can search the Mac App Store for an RSS app.
I was thinking that maybe seeing if there is a way to change the url so Safari does not get too smart about what it is getting via https and does what Chrome does which is display the text.