<html>
<head>
<style>
.clear { clear: left; }
#clear.clear { clear: none; }
</style>
</head>
<body>

<div style="float:left; width:100px;height:100px"></div>
<div id="clear" class="clear" style="width:100px;height:100px;background-color:green"></div>

</body>
</html>