see:
the div is relative with z-index 5, and the :after thing is absolute with z-index 2.
So shouldn't :after be behind the div?
You would have to give pseudo elements a negative z-index to get it to go behind it's parent, plus remove the z-index on the parent. http://jsfiddle.net/jklm313/Kq2PY/4/
来源：https://stackoverflow.com/questions/12463061/z-index-not-working-on-pseudo-elements