请问为什么这个代码最后一个按钮么有效果？拜托了！！
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<style>
#txt{
height:200px;
width:100px;
color:red;
overflow:hidden;
}
</style>
</head>
<body>
<div id="txt">
<p>今天是2016年7月16号</p>
<p>星期六</p>
<p>天气很好</p>
</div>
<form>
<input type="button" value="绿色" onclick="mygreen()">
<input type="button" value="尺寸" onclick="mysize()">
<input type="button" value="隐藏" onclick="hidtxt()">
<input type="button" value="显示" onclick="showtxt()">
<input type="button" value="取消格式" onclick="resttxt()">
</form>
<script>
function mygreen(){
var green=document.getElementById("txt");
green.style.color="green";
}
function mysize(){
var size=document.getElementById("txt")
size.style.width="50px";
size.style.height="50px"}
function hidtxt(){
var hidden=document.getElementById("txt");
hidden.style.display="none";
}
function showtxt(){
var show=document.getElementById("txt");
show.style.display="block"
}
function resettxt(){
var a=confirm("是否取消设置");
if(a=true){
var as=document.getElementById("txt");
as.style.cssText="none";
}
else return;
}
</script>
</body>
</html>
