不定宽高的div垂直水平居中

１．css3的flex

 1 <style>
 2         .ss {
 3             display: flex;
 4             justify-content: center;
 5             align-items: center;
 6             height:600px;
 7             width: 1000px;
 8             background-color: #3a9;
 9         }
10         .sss {
11             width: 100px;
12             height: 100px;
13             background-color: greenyellow;
14         }
15     </style>
16 <body>
17     <div class="ss">
18         <div class="sss"></div>
19     </div>
20 </body>

2.position:absolute

 .ss {
             200px;
            height: 200px;
            background: #3a9;
            position:absolute;
            left:0;
            top: 0;
            bottom: 0;
            right: 0;
            margin: auto;
        }


<body>
    <div class="ss">
    </div>
</body>

3.position:absolute

.ss {
             40%;
            height: 200px;
            background: #3a9;
            position:absolute;
            left:50%;
            top: 50%;
            transform: translate(-50%, -50%);
        }

 <div class="ss"></div>