问一下<span> text-align:cender; </span>为何下面文字不居中？谢谢
<!DOCTYPE HTML" >
<html>
<head>
<meta charset="utf-8"/>
<title>开始学习标签，添加段落</title>
<style type="text/css">
span{
color:red;
font-size: 30px;
text-align:center;
}
</style>
</head>
<body><h1>标题</h1>
<p><span>123</span></p>
</body>
</html>
