<!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. The DOM element is preceded by content.</p>
<p>There should be three rectangles with identical sizes below. The first and third one
    should be blue with nothing inside. The second one should be yellow with some dots
    inside.</p>
<div style="-webkit-columns:4; line-height:100px; background:blue;">
    <span>&nbsp;</span>
    <span id="elm">&nbsp;</span>
</div>