css/css-multicol/multicol-rule-nested-balancing-004.html

<!DOCTYPE html>
<html>
  <meta charset="utf-8">
  <title>CSS Multi-column Layout Test: Test the column rules' block-size with nested balancing multicol container</title>
  <link rel="author" title="Ting-Yu Lin" href="mailto:tlin@mozilla.com">
  <link rel="author" title="Mozilla" href="https://www.mozilla.org/">
  <link rel="help" href="https://drafts.csswg.org/css-multicol-1/#cf">
  <link rel="help" href="https://drafts.csswg.org/css-multicol-1/#column-gaps-and-rules">
  <link rel="help" href="https://github.com/w3c/csswg-drafts/issues/2309">
  <link rel="match" href="multicol-rule-nested-balancing-004-ref.html">
  <meta name="assert" content="This test verifies that the column-rules are extended to the content block-end edges of their corresponding inner and outer multicol container.">

  <style>
  .outer {
    column-count: 1;
    column-rule: 6px solid black;
    background-color: rgba(0, 0, 255, 0.3);
    width: 200px;
    height: 300px;
  }
  .inner {
    column-count: 1;
    column-rule: 3px solid gray;
    background-color: rgba(255, 0, 255, 0.3);
    height: 500px;
  }
  .inner-block {
    background-color: rgba(0, 255, 0, 0.3);
    height: 600px;
  }
  </style>

  <!-- It is deliberate that the inner column's block-size is greater than
       outer column's, and the inner-block's block-size is also greater than
       inner column's. -->
  <article class="outer">
    <article class="inner">
      <div class="inner-block"></div>
    </article>
  </article>
</html>