Mach ich doch glatt!
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<body style="margin-top:20px; margin-bottom:20px;">
<div style="background-color:#FF0000; min-height: 500px; width:500px; margin-left:auto; margin-right:auto; height:100%;">Test</div>
</body>
</html>
oder XHTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<body style="margin-top:20px; margin-bottom:20px;">
<div style="background-color:#FF0000; min-height: 500px; width:500px; margin-left:auto; margin-right:auto; height:100%;">Test</div>
</body>
</html>
Geht auch mit Transitional.
Merkst Du was?