没有宽高的情况下实现水平垂直居中

一、

.main {
    width: 300px;
    height: 500px;
    background-color: #50ba8b;
    

    position: relative;
   }
 
.content {
    width: 100px;
    height: 100px;
    background-color: #5b4d4e;
 

    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    margin: auto;
}

二、

.main {
    width: 300px;
    height: 300px;
    background-color: #50ba8b;
    

    position: relative;
}
 
.content {
    width: 150px;
    height: 150px;
    background-color: #5b4d4e;
 

    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

三、

.main {
    width: 300px;
    height: 300px;
    background-color: #50ba8b;
 

    display: flex;
  }
 
  .content {
    width: 150px;
    height: 150px;
    background-color: #5b4d4e;
 

    margin: auto;
 }

四、

.content {
     width: 150px;
     height: 150px;
     background-color: #5b4d4e;
 
     /*关键代码*/
     display: flex;
     align-items: center;
     justify-content: center;
     margin: auto;
    }
<div class="content">
    <div>12313</div>   
</div>