css z index

1img {
2  position: absolute; 
3  /*position: relative;
4  //position: fixed;*/
5  left: 0px;
6  top: 0px;
7  z-index: -1;
8}
9
10The z-index property specifies the stack order of an element.

1/* The z-index CSS property sets the z-order of a positioned element and its
2descendants or flex items. Overlapping elements with a larger z-index cover
3those with a smaller one. */
4
5/* Keyword value */
6z-index: auto;
7
8/* <integer> values */
9z-index: 0;
10z-index: 3;
11z-index: 289;
12z-index: -1; /* Negative values to lower the priority */
13
14/* Global values */
15z-index: inherit;
16z-index: initial;
17z-index: unset;

1the default z-index is 0. 

1#nav {
2  position: fixed;
3  ...
4  z-index: 2;
5}
6#main {
7  position: absolute;
8  ...
9  z-index: 1;
10}

1z-index