引入<buttton>元素的目的是让用户更好地控制按钮的显示方式，
<!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/add.php">
<button><img src="images/add.gif" alt="add" width="10" height="10" />Add</button>
<input type="hidden" name="bookmark" value="lyrics" />
</form>
</body>
</html>