The footer is anchored to the bottom of the page because it's absolutely positioned 100% from the top of the container/body, which has a calc() min-height on it. This also means the footer just gets pushed down by the content as expected, as the body grows in size or the viewport gets smaller. And the footer can be bigger than the amount you choose to show (100px here), it just scrolls below the page fold, and you're never required to calc the actual/current height the footer to keep it in place. You just choose to set a min-height on the footer/body-calc to reveal as much as you wish.

In non calc() capable browsers (eg, IE8), I've added a simple fallback to block it below the content with a large min-height to stop it showing white screen below it. (an ugly hack - but could fallback to a javascript height calc solution if you prefer)

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