jquery网页字体变大小

<!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>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
<link href="css/style.css" rel="stylesheet" type="text/css" />
 <!--   引入jQuery -->
 <script src="../scripts/jquery-1.3.1.js" type="text/javascript"></script>
 <script type="text/javascript">
    $(function(){
        $("span").click(function(){
            var thisEle = $("#para").css("font-size"); 
            var textFontSize = parseFloat(thisEle , 10);
　　　　　　　　　　//parseFloat(第一个参数，10)中的10表示转为10进制  
                       http://www.w3school.com.cn/jsref/jsref_parseFloat.asp

　　　　　　 var unit = thisEle.slice(-2); //获取单位

                        //arrayObject.slice(start,end)方法可从已有的数组中返回选定的元素。
                            //start    必需。规定从何处开始选取。如果是负数，那么它规定从数组尾部开始算起的位置。也就是说，-1 指最后一个元素，-2 指倒数第二个元素，以此类推。
                            //end    可选。规定从何处结束选取。该参数是数组片断结束处的数组下标。如果没有指定该参数，那么切分的数组包含从 start 到数组结束的所有元素。如果                                              //这个参数是负数，那么它规定的是从数组尾部开始算起的元素。 http://www.w3school.com.cn/jsref/jsref_slice_array.asp
            var cName = $(this).attr("class");
            console.info("thisEle:" + thisEle + "   textFontSize:" + textFontSize + "   unit:" + unit + "   cName:" + cName);
             //thisEle:16px   textFontSize:16   unit:px   cName:bigger 
            if(cName == "bigger"){
                   if( textFontSize <= 22 ){
                        textFontSize += 2;
                    }
            }else if(cName == "smaller"){
                   if( textFontSize >= 12  ){
                        textFontSize -= 2;
                    }
            }
            $("#para").css("font-size",  textFontSize + unit);
        });
    });
  </script>
</head>
<body>

<div class="msg">
    <div class="msg_caption">
        <span class="bigger" >放大</span>
        <span class="smaller" >缩小</span>
    </div>
    <div>
        <p id="para">
        This is some text. This is some text. This is some text. This is some text. This
        is some text. This is some text. This is some text. This is some text. This is some
        text. This is some text. This is some text. This is some text. This is some text.
        This is some text. This is some text. This is some text. This is some text. This
        is some text. This is some text.
        </p>
    </div>
</div>

</body>
</html>

css

* { margin:0; padding:0; }
.msg {300px; margin:100px; }
.msg_caption { 100%; overflow:hidden; margin-bottom:1px;}
.msg_caption span { display:block; float:left; margin:0 2px; padding:4px 10px; background:#898989; cursor:pointer;font-size:12px;color:white; }
.msg textarea{ 300px; height:80px;height:100px;border:1px solid #000;}