Dynamically update the elemnts in the box (even change the number of elements), change the background and font colour of each individual element (but not the font face and size unfortunately unless you want to change the font of all the elements in the dropdown at once), change which item is currently selected.
If you can't change the border it will be because drop down boxes are rendered as part of the OS widget set (at least in IE). This means they are inherently less flexible when it comes to styling.
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