tests/wpt/web-platform-tests/css/css-multicol/multicol-breaking-000.html

<!DOCTYPE HTML>
<title>CSS Test: breaking of a multicolumn</title>
<meta charset="utf-8">
<link rel="author" title="L. David Baron" href="https://dbaron.org/">
<link rel="author" title="Mozilla" href="https://mozilla.org/">
<link rel="help" href="https://drafts.csswg.org/css-multicol/#column-gaps-and-rules">
<link rel="help" href="https://drafts.csswg.org/css-multicol/#cf">
<link rel="match" href="multicol-breaking-000-ref.html">
<style>

.outer {
  height: 100px;
  column-fill: auto;
  width: 800px;
  column-count: 4;
  column-rule: 4px solid blue;
  column-gap: 16px;
  background: rgba(0, 0, 255, 0.3);
}

.inner {
  column-count: 2;
  column-rule: 2px solid fuchsia;
  column-gap: 16px;
  background: rgba(255, 0, 255, 0.3);
  font: 16px/1.25 sans-serif;
}

</style>

<div class="outer">
  <div class="inner" style="height: 300px">
    AAAAA<br>
    BBBBB<br>
    CCCCC<br>
    DDDDD<br>
    EEEEE<br>
  </div>
</div>