既然说到居中，那就总结一下：

<div class="parent">
  <div class="child"></div>
 </div>

父元素子元素高度已知

.parent {
  width: 300px;
  height: 300px;
  background-color: #222;
  /*防止外边距合并*/
  overflow: hidden;
}
.child {
  width: 100px;
  height: 100px;
  background-color: red;
  /*(300-100)/2*/
  margin: 100px auto;
}

子元素高度已知

.parent {
  width: 300px;
  height: 300px;
  background-color: #222;
  /*父元素定位*/
  position: relative;
}
.child {
  width: 100px;
  height: 100px;
  background-color: red;
  /*子元素绝对定位*/
  position: absolute;
  /*先推出50%*/
  top:50%;
  left: 50%;
  /*再找回子元素的一半*/
  margin-left: -50px;
  margin-top: -50px;
}

未知父元素子元素宽高

.parent {
  width: 300px;
  height: 300px;
  background-color: #222;
  position: relative;
}
.child {
  width: 100px;
  height: 100px;
  background-color: red;
  position: absolute;
  top:0;
  left: 0;
  bottom: 0;
  right: 0;
  margin: auto;
}