It sounds like you are trying to say that the equal top and bottom padding is what is giving you the centering, simply by way of the fact that there is no empty

It sounds like you are trying to say that the equal top and bottom padding is what is giving you the centering, simply by way of the fact that there is no empty space remaining to fill. Fair enough. Regardless, the fact that you used "margin: auto" as opposed to "margin: 0 auto" in this particular example is somewhat misleading, as well as the fact that in the description you refer to this as the "margin auto trick". As noted previously, "margin: auto" does nothing in the vertical direction, and the sample would behave identically in every browser, past and present, even if you had used the more well-defined "margin: 0 auto" like in the other examples.