CSS - Height of 100% minus #px - Header and Footer

Question

The webpage in question looks like this:

// The Header //
/*            */
/*  CONTENT   */
/*            */
// The footer //

Both the head

Answer 1

#header /* hypothetical name */
{
    height:100%;
}

#header p /* or any other tag */
{
    margin:20px 0 20px 0;
}

Just make sure to not place margin and height in the same tag. You will experience some crazy results.