Disable HTML5 video auto-play in OS X Safari?
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