overlay one element on another css code example

Example: 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;
}