<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>