<!DOCTYPE html>
<script>
    onload = function() {
        document.body.offsetTop; // trigger layout
        document.getElementById('elm').className = "x";
    }
</script>
<style>
    .x::before { display:block; -webkit-column-span:all; background:yellow; content:".........."; }
</style>
<p>Test adding a ::before pseudo element.</p>
<p>There should be two rectangles with identical sizes below. The first one should be yellow
    with some dots inside. The second one should be blue with nothing inside.</p>
<div style="-webkit-columns:4; line-height:100px; background:blue;">
    <span id="elm">&nbsp;</span>
</div>