How to block auto-play videos in Safari
This is a personal bug bear of mine. I have too many tabs, so hunting that rogue which is playing something is an annoyance.
For Safari, there used to be an excellent plugin called ClickToPlugin, but it is no longer being updated. So to turn off autoplay videos, you need to first work in Terminal. Quit Safari, then open Terminal (it’s in /Applications/Utilities). Paste this command into the window: defaults write com.apple.Safari IncludeInternalDebugMenu 1
Press Return, and wait a few seconds.
Many thanks to Kirkville