When using the calc function with vh units the browser doesn't recognise that value and ignores it. I have tried a couple of options to confirm whether it was a problem with the unit itself or the calc function and percentages, but it seems like the problem is only with vh units + calc Here is an example: http://jsfiddle.net/yaili/ZsFey/2/
*** This bug has been marked as a duplicate of bug 94158 ***