The implication of offsetting the outer edges of a positioned element is that everything about an element -- margins, borders, padding, and content -- is moved in the process of positioning the element. In other words, it is possible to set margins, borders, and padding for a positioned element. These will be preserved and kept with the positioned element, and will be contained within the area defined by the side-offset properties. Opera 3.6.
Now let us consider the cases where two of these three properties areset to auto. If both the margins are set toauto, then they are set to equal lengths, thuscentering the element within its parent, as you can see from Figure 8-14:width cannot drop. The value of this minimum can vary between browsers, as it is not defined in the specification.