未知父元素子元素宽高的居中

既然说到居中，那就总结一下：

.parent {
  width: 300px;
  height: 300px;
  background-color: #222;
  
  overflow: hidden;
}
.child {
  width: 100px;
  height: 100px;
  background-color: red;
  
  margin: 100px auto;
}

.parent {
  width: 300px;
  height: 300px;
  background-color: #222;
  
  position: relative;
}
.child {
  width: 100px;
  height: 100px;
  background-color: red;
  
  position: absolute;
  
  top:50%;
  left: 50%;
  
  margin-left: -50px;
  margin-top: -50px;
}

.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;
}