<style type="text/css">
body{ margin:0; padding:0;}
.Header{ height:100px; background:red;}
.Left{ float:left; 200px; height:300px; background:yellow;}
.Right{ float:right; 200px; height:300px; background:green;}
.Center{ margin:0 200px; height:300px; background:blue;}
.Content{ in-700px;_expression((document.documentelement.clientwidth||document.body.clientwidth)>700?"700px":"");}
</style>
</head>
<body>
<div class="Header"></div>
<div class="Content">
<div class="Right">右</div>
<div class="Left">左</div>
<div class="Center">中</div>
</div>
</body>