<html xmlns="http://www.w3.org/1999/xhtml"> <head> <style> body { height: 50px; 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>