这个区别 由于什么造成的?有 class="form-control"的区别?
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>表单控件——输入框input</title>
<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
</head>
<body>
<form role="form">
<div>
<input type="email" class="form-control" placeholder="Enter email">
<input type="text" class="form-control" placeholder="userName">
<input type="text" placeholder="userName">
</div>
</form>
</body>
</html>