弹窗居中，为什么标题一定要10个字？@慕课网原创_慕课网

<!doctype html>
<html>
     <head>
             <meta charset="utf-8">
            <meta name="viewport" content="width=device-width,initial-scale=1">
             <title>弹窗居中</title>
             <style type="text/css">
               .outbox:before{
                        content:"";
                        width:0;
                         height:100%;
                         display:inline-block;
                        vertical-align:middle;
                     }
                .outbox{
                        position:fixed;
                        top:0;
                        right:0;
                        bottom:0;
                        left:0;
                        text-align:center;
                }
               .content{
                        width:200px;
                        height:200px;
                        background-color:#ccc;
                        display:inline-block;
                       vertical-align:middle;
               }
             </style>
     </head>
     <body>

     <div class="outbox">
             <div class="content">
拉拉
             </div>
         </div>
     </body>
</html>