Since early last week Safari [12.1.1 on my 2014 5K iMac running Mac OS X 10.12.6] opens Youtube in the mobile version (m.youtube) instead of the ‘regular’ desktop one. Checking, changing and or reverting User Agent settings in Safari with the Develop menu doesn’t help. Worse, the listed clips will only open (but fail to play) in an old RealPlayer version I rarely use but keep around for certain purposes. I haven’t yet found a way to change that either, so for now I download the clips and view them with the local viewer I prefer. I would like to know how to revert to Youtube’s desktop version in Safari, assuming the mobile version hasn’t been made the default beyond any tinkering I might have done. FWIW, the Installations screen of the System report (About This Mac/Apple menu) doesn’t list any updates on or around the date of this change.

I was having a similar problem with Paypal opening in a non-domestic site. What worked for me was to go to Safari>Preferences>Privacy>Manage Website Data… and remove everything associated with the offending site. Worth a try.

Funny you’d suggest that. I have that Privacy window open all the time to delete certain crud on the go, while leaving other stuff intact, but I missed this one. Sure enough, there were several Youtube related items and deleting the lot restored Youtube’s desktop version. I didn’t retain the names of them, but I suspect the culprit had something to do with ad-free Youtube viewing…

Thanks, Ira!