我是css初学者，自己做了一个工字型的布局，但整个页面不居中。请指点，代码如下：
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<style type="text/css">
* {
margin: 0px;
padding: 0px;
}
body {
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
margin: 0px auto;
height: auto;
width: 760px;
border:1px #66CCFF solid;
}
.header {
height: 100px;
width: 760px;
background-image: url(header.jpg);
background-repeat: no-repeat;
margin:0px 0px 3px 0px;
border:1px #66CCFF solid;
}
.contentleft {
height: 250px;
width: 150px;
font-size: 14px;
list-style-type: none;
float:left;
border:1px #66CCFF solid;
}
.contentleft li {
float:left;
}
.contentleft li a{
color:#000000;
text-decoration:none;
padding-top:4px;
display:block;
width:97px;
height:22px;
text-align:center;
background-color: #009966;
margin-left:2px;
}
.contentleft li a:hover{
border:1px #66CCFF solid;
color:#FFFFFF;
}
.content {
height:auto;
width: 760px;
line-height: 1.5em;
padding: 10px;
border:1px #66CCFF solid;
}
.content p {
text-indent: 2em;
}
.content h3 {
font-size: 16px;
margin: 10px;
}
.footer {
height: 50px;
width: 760px;
line-height: 2em;
text-align: center;
background-color: #009966;
padding: 10px;
border:1px #66CCFF solid;
}
.contentright{
height: 250px;
width: 580px;
font-size: 14px;
list-style-type: none;
border:1px #66CCFF solid;
float:right;
}
.logo{
background-image: url(scltemp.jpg);
height: 100px;
width: 75px;
}
</style>
<TITLE> New Document </TITLE>
<META NAME="Generator" CONTENT="EditPlus">
<META NAME="Author" CONTENT="">
<META NAME="Keywords" CONTENT="">
<META NAME="Description" CONTENT="">
</HEAD>
<BODY>
<div class="header">
<div class="logo"></div>
</div>
<div class="content">
<div class="contentleft">
<li><a href="#">首 页</a></li>
<li><a href="#">文 章</a></li>
<li><a href="#">相册</a></li>
<li><a href="#">Blog</a></li>
<li><a href="#">论 坛</a></li>
<li><a href="#">帮助</a></li>
</div>
<div class="contentright">
<h3>前言</h3>
<p>第一段内容</p>
<h3>理解CSS盒子模式</h3>
<p>第二段内容</p>
</div>
</div>
<div class="footer">
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Guohui. All Rights Reserved</p>
</div>
</BODY>
</HTML>
另外，还有很多不完美的地方，请指点一下!