%div
  Posicionamiento vertical y horizontal con CSS3 calc();

div {
  width: 200px;
  height: 200px;
  background-color: #e74c3c;
  position: absolute;
  left: calc(50% - 100px);
  top: calc(50% - 100px);
}

body {
  font-family: "HelveticaNeue-Light", "Helvetica Neue Light", "Helvetica Neue", Helvetica, Arial, "Lucida Grande", sans-serif; 
  font-weight: 300;
  color: white;
  background-color: #2980b9;
}