The menu appears below the content on Microsoft Internet Explorer 8 if the site's directory contains an Unicode non-ASCII character.
Steps to reproduce the issue
Make a very simple site using the default main menu, naming its directory with only ASCII characters. Let's say sites/zen . In Configuration → Performance, choose to aggregate the CSS files and save your choice.
Copy the site, i.e. the database and the files, naming its directory with at least one ASCII character, let's say sites/zèn , the database name still including only ASCII characters, like "zen1". Change the database name in the new directory's settings.php accordingly.
In the ASCII site, the menu appears above the content, as expected, but in the non-ASCII site, it appears below the content on Microsoft Internet Explorer 8, even if you access through a all-ASCII alias defined in sites/sites.php . In Firefox, both sites' menu appear above the content.