In order to run this test you should enable the debugging options that indicate what type of compositing layer is being used.
defaults write com.apple.Safari WebCoreLayerRepaintCounter -bool yes defaults write com.apple.Safari WebCoreLayerBorders -bool yes
Directly composited image layers will have a yellow border and no repaint counter.
     Basic image - no style - can be directly composited
    Basic image - no style - can be directly composited
  
     5px blue border - can NOT be directly composited
    5px blue border - can NOT be directly composited
  
     margin - can NOT be directly composited
    margin - can NOT be directly composited
  
     solid background - can be directly composited
    solid background - can be directly composited
  
     background image - can NOT be directly composited
    background image - can NOT be directly composited
  
     rotated but otherwise no style - can be directly composited
    rotated but otherwise no style - can be directly composited