required 属性可以在 input、select 和 textarea 元素上使用。
示例
<!DOCTYPE html>
<html>
<head>
<title>HTML placeholder attribute</title>
</head>
<body>
<h2>Register</h2>
<form action = "/new.PHP">
<input type = "text" placeholder = "Student UserName" name = "name" required/><br>
<input type = "submit" value = "Submit">
</form>
</body>
</html>
