http://stackoverflow.com/questions/1041344/how-can-i-select-an-element-with-multiple-classes?answertab=active#tab-top
例如：$('.a.b') 或 $('#a.b.c')
css多个class时的选择器用法：
https://api.jquery.com/multiple-selector/
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>multiple demo</title>
<style>
div, span, p {
width: 126px;
height: 60px;
float: left;
padding: 3px;
margin: 2px;
background-color: #eee;
font-size: 14px;
}
</style>
<script src="https://code.jquery.com/jquery-1.10.2.js"></script>
</head>
<body>
<div>div</div>
<p class="myClass">p class="myClass"</p>
<p class="notMyClass">p class="notMyClass"</p>
<span>span</span>
<script>
$( "div, span, p.myClass" ).css( "border", "3px solid red" );
</script>
</body>
</html>
