Discovering ins and outs of CSS
1) HTM elements like p and h1 have default margins. Most probably the default margins are specified in em. When margin is specified in em, it bases it's value in it's font-size. i.e If font-size of p is 20px and margin is 1 em, then it is computed to to 20px margin. 2 When margin is specified in %, it computes it's value based on width of its parent element.