非常简洁易懂，排名不分先后，开整！
HTML:

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

第一种：常见的margin

.main {
            position: relative;
            width: 800px;
            height: 500px;
            background-color: lightblue;
        }
        
        .content {
            position: absolute;
            width: 300px;
            height: 200px;
            top: 50%;
            left: 50%;
            margin-left: -150px;
            margin-top: -100px;
            background: lightpink;
        }

第二种：position+transform

.main {
            position: relative;
            width: 800px;
            height: 500px;
            background-color: lightblue;
        }
        
        .content {
            position: absolute;
            width: 300px;
            height: 200px;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
            background: lightpink;
        }

第三种：通过position的top、right、bottom、left

.main {
            position: relative;
            width: 800px;
            height: 500px;
            background-color: lightblue;
        }
        
        .content {
            position: absolute;
            width: 300px;
            height: 200px;
            top: 0;
            left: 0;
            right: 0;
            bottom: 0;
            background: lightpink;
            margin: auto;
        }

第四种：display:table-cell

.main {
            width: 800px;
            height: 500px;
            background: lightblue;
            display: table-cell;
            /*标配*/
            vertical-align: middle;
            /*标配*/
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
            margin: auto;
            /*标配*/
        }
        /*三个标配一起使用才能发挥作用*/

第五种：calc，不能大量使用，会影响性能

 .main {
            width: 800px;
            height: 500px;
            background-color: lightblue;
        }
        
        .content {
            width: 300px;
            height: 70px;
            background: lightpink;
            margin: auto;
            position: relative;
            top: calc((100% - 70px)/2);
        }

第六种：flex+align-items

 .main {
            display: flex;
            width: 800px;
            height: 500px;
            background-color: lightblue;
            justify-content: center;
            align-items: center;
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
        }

第七种：flex+align-self

.main {
            display: flex;
            width: 800px;
            height: 500px;
            background-color: lightblue;
            justify-content: center;
            text-align: center;
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
            align-self: center;
        }

第八种：flex+margin

.main {
            display: flex;
            width: 800px;
            height: 500px;
            background-color: lightblue;
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
            margin: auto;
        }

第九种：grid+align-content

.main {
            display: grid;
            width: 800px;
            height: 500px;
            background-color: lightblue;
            justify-content: center;
            align-content: center;
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
        }

第十种：grid+align-item

.main {
            display: grid;
            width: 800px;
            height: 500px;
            background-color: lightblue;
            justify-content: center;
            align-items: center;
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
        }

第十一种：grid+align-self

.main {
            display: grid;
            width: 800px;
            height: 500px;
            background-color: lightblue;
            justify-content: center;
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
            align-self: center;
        }

第十二种：grid+margin

.main {
            width: 800px;
            height: 500px;
            display: grid;
            background: lightblue;
        }
        
        .content {
            width: 300px;
            height: 200px;
            margin: auto;
            background: lightpink;
        }

第十三种：grid+palce-content

  .main {
            width: 800px;
            height: 500px;
            display: grid;
            place-content: center;
            /*这是justy-content与align-items的结合写法*/
            background-color: lightblue;
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
        }

第十四种：grid+place-items

 .main {
            display: grid;
            width: 800px;
            height: 500px;
            background-color: lightblue;
            place-items: center;
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
        }

第十五种：grid+template

.main {
            margin: auto;
            width: 800px;
            height: 500px;
            background-color: lightblue;
            display: grid;
            grid-template-columns: 1fr auto 1fr;
            grid-template-rows: 1fr auto 1fr;
            grid-template-areas: '. . .' '. amos .' '. . .';
        }
        
        .content {
            width: 300px;
            height: 200px;
            background: lightpink;
            grid-area: amos;
        }

好啦，十五种方法Get！