<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
<title> </title>
<style>
.b{
color: blue;
}
.a{
color: red;
}
</style>
</head>
<body>
<div class="a b">
<div class="child">DEMO</div>
</div>
</body>
</html>按照style里面的顺序执行