I have a main content DIV with a width of 960px, and two floated DIV elements inside. Here's a simplified version of it:
<div id='content'>
<div id='main_data'>
....Data....
</div>
<div id='sidebar'>
....data....
</div>
</div>
And the CSS:
div#content { width: 960px; position: relative }
div#main_data { float: right; width: 755px; }
div#sidebar { float: left; width: 190px; padding: 4px }
The DIV #main_data appears to be centered within #content and #sidebar appears floated left, but below #main_data. #content does not have defined margins or padding, and even including the 4px padding in #sidebar, the widths of the floats do not surpass 960px.
This appears fine in Firefox, IE8, Chrome, Safari and Opera - but not in IE7 and IE6.
I should add that if I switch the position in the DOM, they appear correctly - I'm attempting to make this site more SEO accessible by putting the more pertinent information in #main_data closer to the top of the document (Wikipedia does the same thing where the left sidebar comes later in the source).