position之absolute居中定位

.parent {
    position: relative;
    width: 200px;
    height: 150px;
    background: blue;    
}
.child {
    position: absolute;
    width: 80px;
    height: 80px;
    background: red;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    margin: auto auto;
}

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