如何让一个div水平和垂直居中对齐

.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;
}

.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;
}

.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;
}

.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;
}