If you are styling calc in a GWT project, its parser might not parse calc for you as it did not for me... the solution is to wrap it in a css literal like this:
height: literal("-moz-calc(100% - (20px + 30px))");
height: literal("-webkit-calc(100% - (20px + 30px))");
height: literal("calc(100% - (20px + 30px))");