CSS（九）：设置盒子水平垂直居中

通过设置下面的样式可以使盒子水平垂直居中：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style type="text/css">
      div{
          height: 200px;
          width: 200px;
          background-color: red;
          position: absolute;
          top: 50%;
          left: 50%;
          margin-left: -100px;/*负数，数值是宽度的一半*/
          margin-top: -100px;/*负数，数值是高度的一半*/
      }
    </style>
</head>
<body>
    <div>

    </div>
</body>
</html>

效果：

示例：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>设置盒子水平垂直居中</title>
    <style type="text/css">
      *{
          margin: 0px;
          padding: 0px;
      }
      .bg{
          width: 100%;
          height: 100%;
          background: #000000;
          background: rgba(0, 0, 0, 0.5);
          background: #000000;
          filter: alpha(opacity=30);
          opacity: 0.3;
          position: absolute;
          left: 0px;
          top: 0px;
      }
      .opinion{
          width: 500px;
          height: 330px;
          border: 1px solid #666666;
          /* margin: 0px auto;设置DIV水平居中 */
          position: absolute;
          left: 50%;
          top: 50%;
          margin-left: -250px;
          margin-top: -165px;
          background: white;
      }
      .opinion h3{
          margin:15px;
          font-size: 16px;
      }
      .opinion span{
         color: green;
      }
      .opinion hr{
          margin: 15px;
      }
      .opinion em{
          float: right; /*向右浮动*/
          width: 21px;
          height: 20px;
          background: url(images/tb.gif) no-repeat;
      }
      .opinion p{
          margin: 15px;
          line-height: 25px;
          font-size: 15px;
      }
      .email,.content,.btnSubmit{
        display: block;
        margin: 15px;
      }
      .email,.content{
          width: 460px;
      }
      .email{
          height: 20px;
          line-height: 20px;
          padding-left: 5px;
      }
      .content{
          height: 100px;
          padding-left: 5px;
      }
      .btnSubmit{
          width: 60px;
          height: 25px;
          background: orangered;
          color: white;
          border: 0px;
          cursor: pointer;
      }
    </style>
</head>
<body>
    <div class="bg">

    </div>
    <div class="opinion">
        <h3>
             <span>意见反馈</span>
             <em class="cllse"></em>
        </h3>
        <hr />
        <p>
            亲爱的用户，欢迎来到gwiff，我们非常希望看到您对gwiff的看法，
            以便将来更好的服务到您。
        </p>
        <div>
            <form action="" method="POST" id="myForm">
               <input type="text" class="email" name="email" />
               <textarea class="content"></textarea>
               <input type="button" value="提   交" class="btnSubmit"/>
            </form>
        </div>
    </div>

</body>
</html>

效果：

CSS（九）：设置盒子水平垂直居中

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