body {
 background-color: lightgray;
}
#topLeft {
  background-color: red;
  height: 300px;
  width: 300px;
  position: absolute;
  top: 0px;
  left: 0px;
  text-align: center;
  line-height: 300px; /* vertical alignment */
}
#bottomLeft {
  background-color: green;
  height: 300px;
  width: 300px;
  position: absolute;
  bottom: 0px;
  left: 0px;
  text-align: center;
  line-height: 300px;
}
#topRight {
  background-color: blue;
  height: 300px;
  width: 300px;
  position: absolute;
  top: 0px;
  right: 0px;
  text-align: center;
  line-height: 300px;
}
#bottomRight {
  background-color: yellow;
  height: 300px;
  width: 300px;
  position: absolute;
  bottom: 0px;
  right: 0px;
  text-align: center;
  line-height: 300px;
}