Apparently all the major browsers tried to provide a zoom facility to improve the overall accessibility for the web users. Sadly that often breaks horribly your layout, if you are developing pixel precise interaction you might get a flood of strange bug reports you might not reproduce.
We got bitten by it while developing Glossom, severely…
Our way to figure out it’s value is quite simple once you discover it: Firefox scales proportionally the borders and makes the block dimensions constant, Webkit seems to do the opposite. It’s enough to check if a element with known dimensions and border width has it’s value reported as different and your can find our which is the factor.
This obviously is quite browser dependent and nobody grants that in different version it might get changed, anyway so far it seems to serve us well.