I have always used ClicktoFlash and Clicktoplugin but these are not working on Sierra. Maybe they are HTML videos, I don't know, but it's annoying. I read where there was a terminal command to get rid of this, which I did, but it's not working either.

Terminal showed the Debug menu for me. Then I'm supposed to click media flags (which I can't find) and then "disallow inline video" or "Video needs user action."
I did the latter but it didn't work and now I can't even find media flags under the debug section.

Any ideas? Something simples, step by step.

I'm not exactly sure what your problem is, but I posted a while back that I had to uninstall ClickToPlugin to get videos playing. (ClickTo Flash probably also needs to be uninstalled.)

Before you uninstall, try unchecking, i.e. disabling, them to see what happens.

Maybe I wasn't clear. Clicktoplugin and Clicktoflash work perfectly on my old MBP. But now with Sierra they don't. I only installed them on my new MBP because I was going to websites that were playing videos without my permission like in the past before I installed those apps. So, I thought they might help but they don't.

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.

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!

OK, I just tried something. I disabled clicktoflash and clicktoplugin and now I can get Youtube Videos but I don't know whether they are playing in flash or HTML. Since I heard flash allows people to hack your computer, I prefer HTML. How do I tell which is playing and if it is flash, how can I change it to the HTML format like I used to do?

I'm not sure if it's universal or selective, but at least some videos put a little volume control at the right end of my address field (next to the reload arrow), and clicking on it turns the sound off.

More: The error message is somehow prompted by the "ClickTo-s"; I got the same message until I uninstalled ClickTo Plugin.

You're right. After I got rid of the plugins that message stopped. Thanks.

Safari > Preferences > Websites > Auto-Play then you can allow or disallow automatic media play on a website-by-website basis.

I went to Safari - Prefs but there is no "website" word. Maybe you skipped a step?

That pref doesn't exist in the current version of Safari, i.e. 10.1.1; I guess it's a beta or technology preview thing? (See my post #45214.)

Thanks. I thought I was losing it.

Oops!
That, along with a lot of other site by site options is in the Safari Technology Preview Release 33 (Safari 11.0, WebKit 12604.1.25.0.2) and the version of Safari that will ship with High Sierra. Consider my comment a preview of coming attractions.

Will do..

I did, and I like what's coming...in this instance, anyhow.