水平垂直居中（固定宽不固定宽）

<div class="box">
    <div class="center-middle"></div>
</div>

.box{
    position:relative;
    width:300px;
    height:300px;
    background-color:yellow;
    
}
.center-middle {
    width:100px;
    height:100px;
    position:absolute;
    left:0;
    right:0;
    top:0;
    bottom:0;
    margin:auto;
    background-color:#ccc;
}