<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style>
body {
height: 50px;
-moz-column-count: 2;
}
.container {
height: 10px;
}
.overflow {
height: 100px;
}
.bb {
border-bottom: solid 4px magenta;
}
</style>
<script>
function boom()
{
document.getElementById("x").setAttribute("class", "");
document.getElementById("y").setAttribute("class", "bb");
}
</script>
</head>
<body onload="boom();">
<div class="container"><div class="overflow" id="x"></div></div>
<div id="y"></div>
<div class="container"><div class="overflow"></div></div>
</body>
</html>