Uh-oh, sorry Artie, I read your message too fast! Now I see what you are saying about them being HTMLs. Hurray! Not sure why I am getting that error message though.
Also, I saw this in Apple Discussions which verifies what you said.
Note: The new default behaviour for Safari is not to enable flash on web sites. You have to explicitly enable it per website in Safari Preferences. Since most of YouTube content is available in HTML5 you do not need to enable Flash for YouTube, you would need to enable Flash for BBC iPlayer though as one example.I already have Allow..... checked. The videos that played already are ads or something. I didn't even see them. I could hear them as I was on another website.
I just went to YouTube and found out I cannot play any videos. I get the message:
Your browser does not currently recognize any of the video formats available.. I used to get that on my old MBP but I thought it was because I hadn't upgraded my OS. But I could still play flash which I choose not to do and HTML. Now I have no choice. Whatever video I click on starts with the loading circle in the middle of the screen. It doesn't stop no matter if I put it on pause or play and it never loads up. I may have to call Apple tomorrow. But, hey, I have AppleCare!
Or maybe I"m a little dense.
The videos that are playing without permission are HTML5; in the Sierra version of Safari you get an "Allow?" pop-up when you hit a Flash video. The controlling pref is Safari > Prefs > Security > Internet plug-ins, where I've got "Allow..." and Plug-in Settings > Adobe Flash Player (and SharePoint Browser Plug-in) checked, which results in that pop-up popping up.
I uninstalled ClickToFlash long ago, because its functionality was duplicated in ClickTo Plugin, but CTP's functionality has become unnecessary in the Sierra version of Safari, and I've now uninstalled it, too.
