特殊情况下如何获取span里面的值

<div class="warpper">
<span class="content">
A
<span>123</span>
</span>
</div>

<script type=text/javascript src=./jquery-1.7.2.min.js></script>
<script type=text/javascript>
var a = $(".content")[0].firstChild.data;
function al(){
alert(a);
}
window.onload=al;
</script>