CSS relative zoom

Right now, the "zoom in" graphic is linked to an alternate stylesheet with the following css:

body {
zoom: 1.2; -moz-transform: scale(1.2); -moz-transform-origin: 0 0}
} 

This works fine, but ideally I'd like to have the link trigger a relative zoom rather than an absolute zoom value -- so i would need to establish a variable that determined the user's current zoom level, and then increase that zoom by 120%. This way the same link could be clicked multiple times to increase the zoom incrementally.

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