text-align: center;

margin: auto;

        .parent {width: 400px;height: 400px;background-color: aquamarine;text-align: center;}.child {width: 100px;height: 100px;background-color: #43ff26;display: inline-block;}

.parent {position:relative;}
.child {position:absolute;left:50%;transform:translateX(-50%);}

.parent {width: 400px;height: 400px;background-color: aquamarine;display: flex;justify-content: center;}

  .parent {display:table-cell;vertical-align:middle;}

.parent {position:relative;}             .child{position:absolute;top:50%;transform:translateY(-50%);}

.parent {display:flex;align-items:center;}

 .parent {text-align:center;display:table-cell;vertical-align:middle;}.child {display:inline-block;}

.parent {position:relative;}
.child{position:absolute;left:50%;top:50%;transform:translate(-50%,-50%);}

 .parent {display:flex;justify-content:center;align-items:center;}

<div class="absolute_p1"><div class="absolute_c1"></div>
</div>

.absolute_p1 {position: relative;width: 200px; height: 200px;
}
.absolute_p1 .absolute_c1 {position: absolute; /* fixed 同理 */left: 50%;          top: 50%;width: 100px;       height: 100px;margin-left: -50px; margin-top: -50px;  /* 需根据宽高计算偏移量 */
}

<div class="absolute_p2"><div class="absolute_c2"></div>
</div>

.absolute_p2 {position: relative;width: 200px; height: 200px;
}
.absolute_p2 .absolute_c2 {position: absolute; /* fixed 同理 */left: 0; top: 0; bottom: 0; right: 0; /* 定位为 0 */width: 100px; height: 100px;margin: auto; /* 不用计算偏移量 */
}