Elements can be scaled with transform: scale(sx), where sx is the (unit-less) scale factor. Is there some way to scale an element down by a certain number of pixel (without knowing its width).
Edit
I would like to resize elements of different sizes, like buttons, down when they are pressed. However, applying the same scale factor, say scale(.95), on all buttons leads to the bigger (smaller) ones to be scaled too much (little). So, what I would like to do is resize all elements when pressed absolutely, i.e. by a certain number of pixel or em or so.