I've got a really annoying problem with Safari that was imported from my last machine onto my new one. Unless I set my user agent to "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_0) AppleWebKit/600.6.3 (KHTML, like Gecko) Version/7.1.6 Safari/537.85.15", Youtube will refuse to play videos-- offering me a choice of 240p, 360p, or 720p. If I set my user agent to the above-- i get access to 1080p, and even 4k, and the video plays... I think this has to do with HTML5. and Flash. By making sure that a custom user agent is used, it saves me some annoyance, as I don't need to manually select the user agent each time I restart Safari. If I try to use Vimeo, the above trick doesn't work, suggesting that Vimeo hates flash. I usually end up watching the video using Firefox, or chrome. None of this happens when I use my Guest user option-- videos play fine, suggesting that at some point in time, my Safari preferences picked up some cruft. I have the Develop and Debug menus turned on, (as probably was evident from my discussion of UserAgent strings.) How can I either: resolve my issue directly or reset my Safari installation, including any preferences I may have changed, to factory specs, in hopes that this will resolve the issue? ?