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有個問題不明白、為何不設置padding值，背景圖案就顯示不出來、

<!DOCTYPE?html?PUBLIC?"-//W3C//DTD?XHTML?1.0?Transitional//EN"?"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html?xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta?http-equiv="Content-Type"?content="text/html;?charset=utf-8"?/>
<title>編程挑戰(zhàn)3-3</title>
<style?type="text/css">
body{?margin:0;}
div{background:#460E29;width:700px;padding:20px}
img{background:url(http://img1.sycdn.imooc.com//5385ac820001905201440133.jpg)?no-repeat?;
margin:0?10px?0?0;?padding:?0;}
</style>
</head>
<body>
<div>
????<img?src="http://img1.sycdn.imooc.com//5385acb000013b0d00950095.jpg"/>
????<img?src="http://img1.sycdn.imooc.com//5385acb000013b0d00950095.jpg"?/>
????<img?src="http://img1.sycdn.imooc.com//5385acb000013b0d00950095.jpg"?/>
????<img?src="http://img1.sycdn.imooc.com//5385acb000013b0d00950095.jpg"?/>?

</div>
</body>
</html>

只要給padding設幾個值，背景圖案就慢慢出來了，為何設margin值不行，