PWA demo site for YOW! Connected 2016 - you can also view the slides for the accompanying talk.
The javascript in here uses ES6 arrow functions and a couple of other things that Safari 9 doesn't support. Most of the other recent browsers do, and they also support service workers. If you use Safari 10 (download the Safari Technology Preview if you're on OS X 10.11 or less), then the site will render - but there is no version of Safari that uses service workers, unfortunately.