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：
