SOURCE

console 命令行工具 X clear

                    
>
console
<div class="parent">
    <div class="child"></div>
</div>
.parent {
    position: relative;
    width: 200px;
    height: 200px;
    border: 1px solid;
}

.child {
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    width: 50%;
    height: 50%;
    margin: auto;
    background-color: #699;
}