#container {
  background-color: blue;
  margin: 0 auto;
  width: 400px;
}

.square,
.circle {
  width: 100px;
  height: 100px;
  display: block;
  position: relative;
  /* float: right; */
}

.circle {
  border-radius: 50%;
}

#blockRed {
  background-color: red;
  /*position: absolute;*/
}

#blockYellow {
  /*margin-top: 400px;*/
  background-color: yellow;
}