how to overlay elements css code example

Example 1: css div overlay div

#container { //goes to the bottom
  width: 100px;
  height: 100px;
  position: relative;
}
#div1{ //comes to the top
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  z-index: 10;
}

Example 2: overlay a box in css

html, body {
  min-height: 100%;
}

body {
  position: relative; /* needed if you position the pseudo-element absolutely */
}

body:after {
  content: "";
  display: block;
  position: fixed; /* could also be absolute */ 
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  z-index: 10;
  background-color: rgba(0,0,0,0.2);
}