1.flex

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.father {
           display: flex;
           justify-content: center;
           align-items: center;
           width: 50px;
           height:50px;
           border:1px solid black;
       }
       .son{
           width: 30px;
           height: 30px;
           border: 1px solid red;
       }
       
 <div class="father">
     <div class="son"></div>
 </div>

2.position

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.father {
           position: relative;
           width: 50px;
           height: 50px;
           border: 1px solid black;
       }
       .son {
           position: absolute;
           margin: auto;
           top: 0;
           bottom: 0;
           left: 0;
           right: 0;
           width: 30px;
           height: 30px;
           border: 1px solid red;
       }

3.position+transform

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.father {
            position: relative;
            width: 50px;
            height: 50px;
            border: 1px solid black;
        }
        .son {
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
            width: 30px;
            height: 30px;
            border: 1px solid red;
        }

4.table-cell

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<style>
        html,
        body {
            height: 100%;
            width: 100%;
            margin: 0;
        }

        .box {
            display: table;
            height: 100%;
            width: 100%;
        }

        .content {
            display: table-cell;
            vertical-align: middle;
            text-align: center;
        }

        .inner {
            background-color: #000;
            display: inline-block;
            width: 200px;
            height: 200px;
        }
    </style>
   
     <div class="box">
        <div class="content">
            <div class="inner"></div>
        </div>
    </div>