<div> tag is a static tag and as such can't perform the dynamic task of validating a users browser. you would need to use a scripting language for that bit.
i don't know if it is possible to use percentages to position a dhtml layer on the screen or not. if not, then it is not possible to do so and have it look the same on everyones screens, as the center will vary based on both the browser and resolution of the users picking.