This is correct,the 3 div color is red.

<div class="ac">aaaa bbb<div>
<div class="a c">aaaa bbb<div>
<div class="a b">aaaa bbb<div>
<script>
$(function(){
  $('.a').filter('.b').css("color","red");
});
</script>

but i chang div sequence.

This will has a error.the 2&3 div color is red.

<div class="ac">aaaa bbb<div>
<div class="a b">aaaa bbb<div>
<div class="a c">aaaa bbb<div>

i chang div sequence again.

This will has a error.the all div color is red.

<div class="a b">aaaa bbb<div>
<div class="ac">aaaa bbb<div>
<div class="a c">aaaa bbb<div>

I use IE6.0

winxp sp2