为什么加block却不变成块状
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>流动模式下的内联元素</title>
<style type="text/css">
div{
display:block;
}
</style>
</head>
<body>
<div>
<a href="http://www.imooc.com">www.imooc.com</a><span>强调</span><em>重点</em>
<strong>强调</strong>
</div>
</body>
</html>