为什么js没用？？哪里错了？
<!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=gb2312" /><style type="text/css"> #a_div { line-height:25px;font-weight:600;color:blue;} #a_div ul { display:none;text-decoration:none;margin:0px;} #a_div .ch_div { color:red;} #a_div ul.ch_ul { display:block;}</style><script type="text/javascript">start=function() { var a=document.getElementById("a_div"); var b=a.getElementsByTagName("ul"); a.onmouseover=function(){ this.className="ch_div";b.className="ch_ul";} a.onmouseout=function(){ this.className="";b.className="";} }window.onload=start;</script></head><body><div id="a_div" >导航<ul><li>内容1</li><li>内容2</li></ul></div></body></html>// var b = a.getElementsByTagName("ul"); var b = a.getElementsByTagName("ul")[0];