CSS

css z index

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.

css z-index

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

css z-index

/* 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;

CSS z-index Property

img {
  position: absolute;
  left: 0px;
  top: 0px;
  z-index: -1;
}