I have an iMac (Late 2013), running High Sierra. When I launch Safari, the User Agent defaults to a PPC Safari version. That agent changes how sites load. The user opens Safari to Yahoo Mail, which then displays an error version (unsupported browser) Easy enough to fix by changing the User Agent in the Develop menu. I change it to the Safari agent for the version of Safari that I am running, and the Yahoo page reloads immediately to the correct page. Problem: This is a work computer, goes to sleep each day. The next morning, I launch Safari to check in Yahoo mail for, well, new mail - and the same thing "Unsupported browser". Fixed again by a manual User Agent change. I SHOULD be able to reset Safari to defaults, but I don't want to simply delete the existing com.Apple.Safari.plist There doesn't seem to be any setting in that file for the user agent (or I simply can't find it (?) ) I have a couple of people who use this iMac (they switch off on different days), and it's a bit annoying to always click on the User Agent menu to change Safari's settings each and every day. Yes, it's only once a day (or each time that Safari is inadvertantly closed/quit, then relaunched) but this should be an easy fix.... I found steps to change a line in the plist, but the tip was from 10 years ago, and doesn't work on newer OS X systems. So, short story is that Safari always defaults to a PPC Safari User agent. I want it to default to whatever the real default should be, so I don't have to change the user agent every time I launch Safari. Any clues that might help this weird little annoyance?