1.通过左上的外边距设定：
box{
position: absolute;
top: 50%;
background-color: gold;
height: 40px;
margin-top: -40px; /* negative half of the height */
width: 100px;
left: 50%;
margin-left: -50px;
text-align: center;
line-height: 40px;
}
2.通过margin:auto设定：
.box{
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
margin: auto;
height: 40px;
width: 20%;
font-size: 16px;
}
3.HTML：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>高考加油</title>
<style type="text/css">
*{
margin: 0;
padding: 0;
}
/*.box{
position: absolute;
top: 50%;
background-color: gold;
height: 40px;
margin-top: -40px; /* negative half of the height
width: 100px;
left: 50%;
margin-left: -50px;
text-align: center;
line-height: 40px;
}*/
.box{
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
margin: auto;
height: 40px;
width: 20%;
font-size: 16px;
}
</style>
</head>
<body>
<div class="box">高考加油</div>
</body>
</html>
4.实现的效果：
