body {
  margin: 0px;
  padding: 0px;
}
span {
  width: 100%;
  height: 100%;
  position: fixed;
  background-color: rgba(0, 0, 0, 0.1);
}
div {
  margin: auto;
  padding: 50px;
  width: 800px;
  text-align: center;
  vertical-align: middle;
  background-color: rgba(200, 200, 200, 0.1);
}