Position

#parent {
  position: relative;
  background-color: teal;
  width: 200px;
  height: 200px;
}
#absolute {
  /* No width & height */
  position: absolute;
  background-color: orange;
  top: 20px;
  right: 0;
  bottom: 20px;
  left: 0;
}

#parent {
  position: relative;
  background-color: teal;
  width: 200px;
  height: 200px;
}
#absolute {
  /* Width & height specified */
  position: absolute;
  background-color: orange;
  width: 100px;
  height: 100px;
  top: 20px;
  right: 0; /* right ignored */
  bottom: 20px; /* bottom ignored */
  left: 0;
}