为什么在文本前面设置span没有反应？
<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>定宽块状元素水平居中</title>
<style>
div{
border:1px solid red;
margin:20px;
}
span{
text-align:center;
}
.imgCenter{text-align:center;}
</style>
</head>
<body>
<div class="txtCenter"><span>我想要在父容器中水平居中显示。</span></div>
<!--下面是任务部分-->
<div class="imgCenter"><img src="http://img1.sycdn.imooc.com//52da54ed0001ecfa04120172.jpg" /></div>
</body>
</html>
