<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style>
#a {
-moz-column-count: 2;
-moz-column-width: 100px;
float: left;
border: 2px solid magenta;
height: 200px;
}
#b {
-moz-column-count: 2;
-moz-column-width: 100px;
float: left;
border: 2px solid green;
height: 300px;
}
</style>
</head>
<body onload="document.getElementById('span').style.display = '-moz-inline-grid';">
<div id="a"><div id="b"></div><optgroup label="foo"><span id="span"></span></optgroup></div>
</body>
</html>