如何解决CSS水平垂直居中的问题？

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

.parent {
 width:400px;
 height:400px;
 background: red;
 position: relative;
}
.child {
 position: absolute;
 left:50%;
 top:50%;
 background: yellow;
 width:50px;
 height:50px;
 margin-left:-25px;
 margin-top:-25px;
}

.parent {
 width:400px;
 height:400px;
 background: red;
 position: relative;
}
.child {
 position: absolute;
 left:50%;
 top:50%;
 transform: translate(-50%,-50%);
}

.parent {
 position:relative;
 width:200px;
 height:200px;
 background: red;
}
.child {
 width:80px;
 height:40px;
 background: yellow;
 position: absolute;
 left:0;
 top:0;
 right:0 ;
 bottom:0;
 margin:auto;
}

.parent {
  height: 300px;
  width:400px;
  border: 1px solid red;
  display: table-cell;
  vertical-align: middle;
  text-align: center;
}
.child {
 display: inline-block;
 width:50px;
 height:50px;
 background: blue;
}
/* 或者 */
.parent {
  width: 400px;
  height: 300px;
  display: table-cell;
  vertical-align: middle;
  border: 1px solid red;
  text-align: center;
}
.child {
  display: inline-block;
  vertical-align: middle;
  background: blue;
}

.parent {
 width:400px;
 height:200px;
 background:red;
 display: flex;
 justify-content:center;
 align-items:center;
}
.child {
 height:100px;
 width:100px;
 background:green;
}

.parent {
 width:200px;
 height:200px;
 background:red;
 text-align: center;
}
.child {
 height:100px;
 width:100px;
 background:yellow;
 display: inline-block;
 vertical-align: middle;
}
.parent:before {
 content:"";
 height:100%;
 vertical-align: middle;
 display: inline-block;
}