用多个class选择元素

注意下面两个的区别：
$(".role-developer.role-designer").css("color","red");
$(".role-developer .role-designer").css("background","#ff0");

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script src="http://code.jquery.com/jquery-1.7.2.min.js"></script>
<script type="text/javascript">
$(function(){
    $("li").click(function(){
        $(".role-developer.role-designer").css("color","red");
        $(".role-developer .role-designer").css("background","#ff0");
    });
});
</script>
</head>
<body>
<h3>用多个class选择元素</h3>
<ul>
<li class="role-developer role-designer">Mark
    <ul>
    <li class="role-developer">mmmm</li>
    <li class="role-developer">mmmmm2</li>
    </ul>
</li>
<li class="role-developer ">Simon
    <ul>
    <li class="role-developer">mmmm</li>
    <li class="role-designer">mmmmm2</li>
    </ul>
</li>
<li class="role-developer role-manager">Kevin
    <ul>
    <li class="role-designer">mmmm</li>
    <li class="role-designer">mmmmm2</li>
    </ul>
</li>
</ul>
</body>
</html>