Kirk McElhearn, Stop Auto-Play Videos from Annoying You in Your Browser on macOS, on March 13, 2017:

• Here are the direct commands to activate and deactivate this feature in Safari from Terminal, without displaying the Debug menu:

defaults write com.apple.Safari WebKitMediaPlaybackAllowsInline -bool false

defaults write com.apple.SafariTechnologyPreview WebKitMediaPlaybackAllowsInline -bool false

defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2AllowsInlineMediaPlayback -bool false

defaults write com.apple.SafariTechnologyPreview com.apple.Safari.ContentPageGroupIdentifier.WebKit2AllowsInlineMediaPlayback -bool false

Amen.