<div class="page">
  <div class="backTop">position:relative</div>
  <div class="normal">position:normal</div>
</div>

.page {
  width: 600px;
  height: 600px;
  background-color: #ccc;
  margin: 100px auto;
}
.backTop {
  width: 300px;
  height: 300px;
  background-color: #f34;
  position: relative;
  left: 100px;
  top: 100px;
}
.normal {
  width: 200px;
  height: 200px;
  background-color: #3f2;
}