<html>
<head>
<style>
.a { background:#F00; }
.b { background:#0F0; }
</style>
<script>
var i = 0;
function change()
{
if (i) {
document.getElementById("btn").className="a";
i=0;
}
else{
document.getElementById("btn").className="b";
i=1;
}
}
</script>
</head>
<body>
<input id="btn" type="button" class="a" value="click me" onClick="change()">
</body>
</html>
