<div class="main">
   <div class="center"></div>
  </div>

 .main{
    width: 300px;
    height: 300px;
    background-color: red;
    position: relative;
   }
   .center{
    width: 100px;
    height: 100px;
    background-color: skyblue;
    position: absolute;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
    margin: auto;
   } 

 .main{
    width: 300px;
    height: 300px;
    background-color: red;
    display: flex;
    justify-content: center;
    align-items: center;
   }
   .center{
    width: 100px;
    height: 100px;
    background-color: greenyellow;
   } 

 .main{
     width: 300px;
     height: 300px;
     background-color: red;
     position: relative;
    }
    .center{
     width: 100px;
     height: 100px;
     background-color: pink;
     position: absolute;
     left: 50%;
     top: 50%;
     transform: translateX(-50%) translateY(-50%);
    } 

 .main{
     width: 300px;
     height: 300px;
     background-color: red;
     position: relative;
    }
    .center{
     width: 100px;
     height: 100px;
     background-color: pink;
     position: absolute;
     left: 50%;
     top: 50%;
     margin-left: -50px;
     margin-top: -50px;
    } 

.main{
     width: 300px;
     height: 300px;
     background-color: red;
     display: table-cell;
     vertical-align: middle;
    }
    .center{
     width: 100px;
     height: 100px;
     background-color: #000;
     margin: auto;
    }