Why does this CSS margin-top style not work?

Question

I try to add margin values on a div inside another div. All works fine except the top value, it seems to be ignored. But why?

What I expected:
<

Answer 1

I guess setting the position property of the #inner div to relative may also help achieve the effect. But anyways I tried the original code pasted in the Question on IE9 and latest Google Chrome and they already give the desirable effect without any modifications.