核心提示：!DOCTYPE html PUBLIC -//W3C//DTD XHTML 1.0 Transitional//EN https://www.w3.org/TR/xhtml1/DTD/xhtml1-...
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "https://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="https://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>已知宽高度DIV垂直水平居</title>
<style type="text/css">
<!--
.main{ position:absolute; top:50%; left:50%; width:821px; height:392px; margin:-196px 0 0 -410.5px; background:#C8EDFB}
-->
</style>
</head>
<body>
<p class="main">已知宽高度DIV垂直水平居</p>
</p>
</body>
</html>
说明：
width:821px; height:392px
margin的取值为高和宽的一半
