当type特性的值为password时<iinput>元素会创建一个用起来和单行文本框，唯一的不同之处在于其中的字符被掩盖了。
<!DOCTYPE html>
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<html>
<head>
<title>TODO supply a title</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div>TODO write content</div>
<fomr action="http://www.example.com/login.php">
<p>账号：
<input type="text" name="username" size="15" maxlength="30"/>
</p>
<p>密码:
<input type="password" name="password" size="15" maxlength="30" />
</p>
</fomr>
</body>
</html>