<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    <script src="Scripts/jquery-1.4.1.js" type="text/javascript"></script>
    <style type="text/css">
    .bc{ color:gray }
    </style>
    <script type="text/javascript">
        $(function () {
            $("input").val("请输入关键词").addClass("bc")
            .blur(function () {//失去焦点时
                if ($("input").val() == "")
                $("input").val("请输入关键词").addClass("bc")
            })
            .focus(function () {//得到焦点时
                if ($("input").val() == "请输入关键词")
                $("input").val("").removeClass("bc")
            });
        });
    </script>
</head>
<body>
<input type="text" />
</body>
</html>