请问，为什么我最后一个函数老是写不好？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" Content="text/html; charset=utf-8" />
<title>javascript</title>
<style type="text/css">
body{font-size:12px;}
#txt{
height:400px;
width:600px;
border:#333 solid 1px;
padding:5px;}
p{
line-height:18px;
text-indent:2em;}
</style>
</head>
<body>
<h2 id="con">JavaScript课程</H2>
<div id="txt">
<h5>JavaScript为网页添加动态效果并实现与用户交互的功能。</h5>
<p>1. JavaScript入门篇，让不懂JS的你，快速了解JS。</p>
<p>2. JavaScript进阶篇，让你掌握JS的基础语法、函数、数组、事件、内置对象、BOM浏览器、DOM操作。</p>
<p>3. 学完以上两门基础课后，在深入学习JavaScript的变量作用域、事件、对象、运动、cookie、正则表达式、ajax等课程。</p>
</div>
<form>
<!--当点击相应按钮，执行相应操作，为按钮添加相应事件-->
<input type="button" value="改变颜色" onclick = "changeColor()">
<input type="button" value="改变宽高" onclick = "changeWH()" >
<input type="button" value="隐藏内容" onclick = "changeDis()" >
<input type="button" value="显示内容" onclick = "changeDis1()" >
<input type="button" value="取消设置" onclick = "reset()">
</form>
<script type="text/javascript">
//定义"改变颜色"的函数
function changeColor(){
var txt = document.getElementById("txt");
var con = document.getElementById("con");
txt.style.color = "red";
con.style.color = "red";
}
//定义"改变宽高"的函数
function changeWH(){
var txt = document.getElementById("txt");
var con = document.getElementById("con");
txt.style.width = "100px";
con.style.width = "100px";
txt.style.height = "100px";
con.style.height = "100px";
}
//定义"隐藏内容"的函数
function changeDis(){
var txt = document.getElementById("txt");
var con = document.getElementById("con");
txt.style.display = "none";
con.style.display = "none";
}
//定义"显示内容"的函数
function changeDis1(){
var txt = document.getElementById("txt");
var con = document.getElementById("con");
txt.style.display = "block";
con.style.display = "block";
}
//定义"取消设置"的函数
function reset(){
var message=confirm("你确定要重置所有设置么？");
if(message == true){
var txt=document.getElementById("txt");
txt.style.cssText="null";
}
}
</script>
</body>
</html>
