<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>居中的问题</title>
<style>
.dv1 {
300px;
height:300px;
background-color: red;
position: fixed;
left:50%; // 百分之五十
top:50%; // 百分之五十
margin-left:-150px; // 负一半
margin-top:-150px; // 负一半
}
</style>
</head>
<body>
<div class="dv1"></div>
</body>
</html>
