What is the difference between:
z-index is the same as
z-index: auto; that is its initial value.
0 mean the same thing if your element doesn't create its own stacking context; e.g. it is not positioned as relative, absolute or fixed.
If your enclosing
div isn't positioned, then whatever you set its
z-index to doesn't matter; it and all its contents will participate in the stacking context of
html, and its descendants will always be positioned in front of it.