Difference between auto, 0, and no z-index?

Question

What is the difference between:

1. z-index: auto
2. z-index: 0
3. no z-index at all

All the above sc

Answer 1

z-index: auto

Sets the stack order equal to its parents. This is default.

z-index:0

does nothing

z-index:not

Sets the stack order equal to its parents same as auto.

z-index:inherit

Specifies that the z-index should be inherited from the parent element

Reference for further reading and testing:

Link