                        課程
                    
                        /前端開發(fā)
                        
                            /JavaScript
                        
                        /展開與收起效果

為什么點擊之后不收起呢？坐等大神

<!doctype?html>
<html>
?<head>
??<meta?charset="UTF-8">
??<meta?name="Generator"?content="EditPlus?">
??<meta?name="Author"?content="">
??<meta?name="Keywords"?content="">
??<meta?name="Description"?content="">
??<title>Document</title>
??<script?>
function??down(){
document.getElementById("td").style.display='block';
document.getElementById("yg").href="javascript:top()";
document.getElementById("yg").style.background="url('images/top.gif')?no-repeat??60px?1px";
}

function??top(){
document.getElementById("td").style.display='none';
document.getElementById("yg").href="javascript:top()";
document.getElementById("yg").style.background="url('images/top.gif')?no-repeat??60px?1px";
}
????</script>
?</head>
?<style>
*{margin:0;padding:0;}
?????a{text-decoration:none;}
ul,li{list-style:none;}
#td{border:1px?solid?#ccc;padding:5px;width:100px;}
#yg{background:url('images/down.gif')?no-repeat?60px?1px?;width:80px;display:inline-block;}
?</style>
<body>
<a?href="javascript:down()"?id="yg">員工通道</a>
<ul?id="td"?style="display:none;">
<li><a?href="">OA系統(tǒng)登陸1</a></li>
<li><a?href="">OA系統(tǒng)登陸2</a></li>
<li><a?href="">OA系統(tǒng)登陸3</a></li>
</ul>
</body>
</html>