Ну вот пример:
css
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body{ text-align:center; }
#header {width:100%;min-width:700px;background:red;}
#main {width:100%;min-width:800px;background:yellow;}
#main1 {width:400px;background:cyan;float:left;overflow:hidden;margin:0 auto}
#main2 {width:400px;background:blue;float:right;overflow:hidden;margin:0 auto}
#footer {width:1000px;min-width:700px;background:green;}
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<html>
<head>
<title>test</title>
<link rel="stylesheet" type="text/css" href="style.css">
</head>
<body>
<div id="header">
header header header header header header header header header headerheader header header header header header header header header header
header header header header header header header header header headerheader header header header header header header header header header
header header header header header header header header header headerheader header header header header header header header header header
header header header header header header header header header headerheader header header header header header header header header header
header header header header header header header header header headerheader header header header header header header header header header
header header header header header header header header header headerheader header header header header header header header header header
</div>
<div id="main">
<div id="main1">
main1main1main1main1main1main1main1main1main1
main1main1main1main1main1main1main1main1main1
main1main1main1main1main1main1main1main1main1
main1main1main1main1main1main1main1main1main1
main1main1main1main1main1main1main1main1main1
main1main1main1main1main1main1main1main1main1
main1main1main1main1main1main1main1main1main1
main1main1main1main1main1main1main1main1main1
</div>
<div id="main2">
main2main2main2main2main2main2main2main2main2
main2main2main2main2main2main2main2main2main2
main2main2main2main2main2main2main2main2main2
main2main2main2main2main2main2main2main2main2
main2main2main2main2main2main2main2main2main2
main2main2main2main2main2main2main2main2main2
main2main2main2main2main2main2main2main2main2
main2main2main2main2main2main2main2main2main2
</div>
</div>
<div id="footer">
footerfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooter
footerfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooter
footerfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooterfooter
</div>
</body>
</html>
