IE is a browser that instead of supporting standards, tries to define its own defacto standards by the idea that the grand majority of users use IE.
Mozilla and its derivatives support all current standards at the time of their release.
In other words, designing a site for IE will usually cause it to not work at all for anything else, but designing a site that uses standards properly will work in everything, with IE misfiring fairly often.
In the case of CSS, its sometimes useful to get the client's browser by some means (like
phpsniff if you are doing php) and have different outputs for IE, and real browsers.