<html>
<head>
<link id=csss rel="stylesheet" type="text/css" href="a.css">
</head>
<body>
<input type=button value="a.css" onclick="document.all.csss.href='a.css'"><br>
<input type=button value="b.css" onclick="document.all.csss.href='b.css'"><br>
</body>
</html>