如何让一个div水平和垂直居中对齐

以下方法来自百度知道：https://zhidao.baidu.com/question/558984366971173044.html

方法1：

.parent {
width: 800px;
height: 500px;
border: 2px solid #000;
position: relative;
}

.child {
width: 200px;
height: 200px;
margin: auto;
position: absolute;
top: 0;
left: 0;
bottom: 0;
right: 0;
background-color: red;
}

方法2：

.parent {
width: 800px;
height: 500px;
border: 2px solid #000;
display: table-cell;
vertical-align: middle;
text-align: center;
}

.child {
width: 200px;
height: 200px;
display: inline-block;
background-color: red;
}

方法3：

.parent {
width: 800px;
height: 500px;
border: 2px solid #000;
display: flex;
justify-content: center;
align-items: center;
}

.child {
width: 200px;
height: 200px;
background-color: red;
}

方法4：

.parent {
width: 800px;
height: 500px;
border: 2px solid #000;
position: relative;
}

.child {
width: 300px;
height: 200px;
margin: auto;
position: absolute; /*设定水平和垂直偏移父元素的50%，再根据实际长度将子元素上左挪回一半大小*/
left: 50%;
top: 50%;
margin-left: -150px;
margin-top: -100px;
background-color: red;
}