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because IE for the Mac and IE for windows utilize two seperate rendering engines, there were significant differences in the CSS capabilities of these two browsers. Because of this, different stylesheets are served to Mac users. Now that a Mac also has Gecko (Netscape) and KHTML (Safari) based webbrowsers this situation is further muddled. IMO the user agent detection needs some fine tuning, to deal with newer browsers, or altogether removal.
If the current theme includes a Mac specific stylesheet, this sheet will always be sent to a Mac computer. If not, the default theme stylesheet (styles.css) will be served to the user.