getElementById 有关问题

getElementById 问题

HTML code

<li class="nav_pbdn" id="apbdn"><a href="http://hmu061192.chinaw3.com/page-%E5%B9%B3%E6%9D%BF%E7%94%B5%E8%84%91.html?css=pb"></a></li><script>var pb=document.getElementById("apbdn");alert(pb);</script>