用来创建多种不同的表单控件，其type特性的值决定了它将要创建哪种控件。
<!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>
<form action="http://www.example.com/login.php">
<p>用户名：
<input type="text" name="username" size="15" maxlength="30" />
</p>
</form>
</body>
</html>