Given this markup,
<div id="header"></div>
<div id="content"></div>
<div id="sidebar"></div>
#header {
no styles at all
}
#content {
float: left;
}
#sidebar {
float: right;
}
How do I position the divs using CSS floats without changing the markup?

Do you need smth like this?
http://jsfiddle.net/JVJ9Q/2/