<html> <head> <style> #colset { -moz-column-count: 3; } #a { height: 0; } #x { height: 3px; } #b { height: 0; } #c { height: 2px; } #d { height: 2px; } /* Following style is just for visualization -- doesn't affect assertion */ #colset { -moz-column-gap: 0px; width: 18px; } div { width: 5px; } #a { background: purple; } #x { background: orange; } #b { background: blue; } #c { background: black; } #d { background: lime; } </style> <script> function boom() { document.getElementById("a").style.height = "auto"; document.getElementById("c").style.height = "0"; } </script> </head> <body onload="boom()" id="colset" ><div id="a"><div id="x"></div></div ><div id="b"><div id="c"></div><div id="d"></div></div ></body> </html>