After doing some fiddling I was reminded that absolute positioning removes the element from the document flow. You cannot depend on an absolute positioned element to affect the other elements because it will not. Because you do not know the height of the content then using margin-top is clearly not option.
So I came up with this: basically do a normal layout with floats then use position relative to move the items where you want them. This way the elements still affect the document flow, however, now you have total control over the position. This is precisely what relative positioning is for: You want total control over the position of an element but you still want they element to affect the layout normally.
<html>
<head>
<style>
body {
text-align:center;
}
#container {
position:relative;
margin:0 auto;
width: 1000px;
text-align:left;
}
#header {
position:relative;
top:0px;
left:0px;
width:1000px;
height: 100px;
border:solid 1px #000;
}
#sidebar {
position:relative;
top:10px;
left:0px;
width:300px;
height: 500px; /* for demo */
float:left;
margin-bottom: 20px;
border:solid 1px #000;
}
#main {
position:relative;
top:10px;
left:310px;
width:690px;
height: 200px; /* for demo */
margin-bottom:20px;
border:solid 1px #000;
}
#footer {
margin:0 auto;
top:20px;
width: 1000px;
text-align:left;
height: 100px;
clear:both;
border:solid 1px #000;
}
</style>
</head>
<body>
<div id="container"> <!-- Holds all the content except the footer -->
<div id="header">Header content here</div>
<div id="sidebar">Sidebar content here</div>
<div id="main">Main content here</div>
</div>
<div id="footer">Footer content here</div>
</body>
</html>