方法一 position/margin

.wrap {
    width: 200px;
    height: 200px;
    background: yellow;
    position: relative;
}
.wrap .center {
    width: 100px;
    height: 100px;
    background: green;
    margin: auto;
    position: absolute;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
}

方法二 display:table-cell

.wrap{
    width: 200px;
    height: 200px;
    background: yellow;
    display: table-cell;
    vertical-align: middle;
    text-align: center;
}
.center{
    display: inline-block;
    vertical-align: middle;
    width: 100px;
    height: 100px;
    background: green;
}

方法三 ：position加 transform

.wrap {
    position: relative;
    background: yellow;
    width: 200px;
    height: 200px;}
 
.center {
    position: absolute;
    background: green;
    top:50%;
    left:50%;
    -webkit-transform:translate(-50%,-50%);
    transform:translate(-50%,-50%);
    width: 100px;
    height: 100px;
}

方法四：flex;align-items: center;justify-content: center

.wrap {
   background: yellow;
   width: 200px;
   height: 200px;
   display: flex; 
   align-items: center; 
   justify-content: center;
}

.center {
   background: green;
   width: 100px;
   height: 100px;
}

方法五：display:flex;margin:auto

.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    display: flex; 
}

.center {
    background: green;
    width: 100px;
    height: 100px;
    margin: auto;
}