#a {
  position: relative;
  width: 300px;
  height: 300px;
  background: firebrick;
}

#b {
  position: absolute;
  top: 50px;
  left: 50px;
  width: 100px;
  height: 100px;
  background: steelblue;
}

.label {
  position: absolute;
  bottom: 4px;
  right: 6px;
  font-size: 24px;
  font-weight: bold;
  color: white;
  user-select: none;
}