Disable HTML5 video auto-play in OS X Safari? I want to disable videos in Safari, as I have to pay for each byte downloaded.  Some videos I may want to watch, but I want to decide whether I use my precious bandwidth, rather than have it automatically play.  Is there any way to accomplish this?
This seems to work for me (macOS Sierra 10.12.2, Safari 10.0.2), courtesy of jeffreyv in an Apple Support thread:

In the Terminal, enter:

defaults write com.apple.Safari IncludeInternalDebugMenu 1


That enables the Debug menu in Safari. Then tick this menu item on:  

Debug -&gt; Media Flags -&gt; Video Needs User Action