css z index

                                    #nav {
  position: fixed;
  ...
  z-index: 2;
}
#main {
  position: absolute;
  ...
  z-index: 1;
}

                                    /* The z-index CSS property sets the z-order of a positioned element and its
descendants or flex items. Overlapping elements with a larger z-index cover
those with a smaller one. */

/* Keyword value */
z-index: auto;

/* <integer> values */
z-index: 0;
z-index: 3;
z-index: 289;
z-index: -1; /* Negative values to lower the priority */

/* Global values */
z-index: inherit;
z-index: initial;
z-index: unset;

                                    Definition and Usage. The z-index property specifies the stack order of an element. An element with greater stack order is always in front of an element with a lower stack order. Note: z-index only works on positioned elements (position: absolute, position: relative, position: fixed, or position: sticky).
Default value: auto

                                    the default z-index is 0. 

                                    img {
  position: absolute; 
  /*position: relative;
  //position: fixed;*/
  left: 0px;
  top: 0px;
  z-index: -1;
}

The z-index property specifies the stack order of an element.