HTML
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>文本框 - 含帮助文本</title>
<link rel="stylesheet" href="https://cdn.w3cbus.com/library/mdui/1.0.2/css/mdui.min.css"/>
</head>
<body>
<div class="mdui-container">
<div class="mdui-textfield">
<input type="email" class="mdui-textfield-input" placeholder="邮箱"/>
<div class="mdui-textfield-helper">Helper Text</div>
</div>
</div>
<script src="https://cdn.w3cbus.com/library/mdui/1.0.2/js/mdui.min.js"></script>
</body>
</html>