$ strings /Applications/Safari.app/Contents/MacOS/Safari | grep DebugSafari4
DebugSafari4TabBarIsOnTop
DebugSafari4IncludeToolbarRedesign
DebugSafari4IncludeFancyURLCompletionList
DebugSafari4IncludeGoogleSuggest
DebugSafari4LoadProgressStyle
DebugSafari4IncludeFlowViewInBookmarksView
DebugSafari4TopSitesZoomToPageAnimationDimsSnapshot
DebugSafari4IncludeTopSites
How to use example
defaults write com.apple.Safari DebugSafari4TabBarIsOnTop -bool NO
defaults delete com.apple.Safari DebugSafari4TabBarIsOnTop
defaults write com.apple.Safari DebugSafari4LoadProgressStyle -bool NO