Here you have 2 rectangles on top of each other. Lower (red) is set with conventional CSS as width:100, height:100, upper (blue) is set with CSS transform as width:1, height: 1, scale 100. Upper has opacity set to 0.5.
You shall see one rectangle in lilac color at every Browser Zoom setting. Firefox and IE get that right. Webkit based browsers mostly not (sometimes some gets it right at 100 multiple)