The child element should margin-top why it affects the parent element "Go"

<!DOCTYPE HTML Public "-//W3C//DTD XHTML 1.0 transitional//en" "http://www.w3.org/TR/xhtml1/DTD/ Xhtml1-transitional.dtd "><HTMLxmlns= "http://www.w3.org/1999/xhtml"><Head><Metahttp-equiv= "Content-type"content= "text/html; charset=utf-8" /><title>Why is the Margin-top property of the sub-footer_top wrong?</title><styletype= "Text/css">#footer{width:1314px;Height:432px;Background-color:#666666;Border:1px solid #ffffff;}#footer_top{width:1200px;Height:328px;Margin-top:24px;Margin-left:32px;Background-color:#06C;}#footer_btm{Height:90px;Background-color:#3399fe;}</style></Head><Body>Why does the Margin-top property of a child footer_top cause a change in the parent footer Margin-top attribute? <DivID= "Footer">        <DivID= "Footer_top">T</Div>        <DivID= "Footer_btm">B</Div>    </Div></Body></HTML>

The child element should margin-top why it affects the parent element "Go"