Retina 5K Display is one of the wonderful engineering developed by Apple but general web content reading is sometimes especially to some people really painful and annoying because of the lettering.
Page zoom (⌘+) on Safari comes into play to avoid such situations. But how to set the default page zoom on Safari rather than zooming every page separately? Here is the way.
Safari → Preferences → Websites → Page Zoom → Set the preferred zone factor.