为什么我下面这样调用里面的同id div 不行？？

Question

<div id=a1>
<div>ss
<div id='a' class="sss1"></div>
</div>
</div><div id=a2>
<div>ss
<div id='a' class="sss2"></div>
</div>
</div><script>
var a1=document.getElementById("a1")
var a2=document.getElementById("a2")var b1=a1.getElementById('a')
alert(b1.className)
var b2=a2.getElementById('a')
alert(b2.className)
</script>

Answer 1

a1.getElementById('a')该成a1.childNodes('a')能用不

Answer 2

getElementById是document的方法getElementById Method  SyntaxoElement = document.getElementById(sIDValue)Applies Todocument