css/css-nesting/nesting-layer.html

<!DOCTYPE html>
<title>Nested @layers</title>
<link rel="help" href="https://drafts.csswg.org/css-nesting/#nested-group-rules">
<link rel="help" href="https://drafts.csswg.org/css-cascade-5/#layering">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>

  .a {
    /* This should have no effect. Only at-rules containing style rules
       are vaild when nested. */
    @layer theme, base;
  }

  /* The theme layer wins over the base layer. */
  @layer base, theme;

  .a {
    @layer theme {
      & {
        z-index: 1;
      }

      .b {
        background-color: green;
      }
    }
  }

  @layer base {
    .a {
      z-index: 0;
    }
    .a .b {
      background-color: red;
    }
  }
</style>
<main>
  <div class="a">
    <div class="b">
    </div>
  </div>
</main>
<script>
  test(() => {
    let a = document.querySelector("main > .a");
    let b = document.querySelector("main > .a > .b");
    assert_equals(getComputedStyle(a).zIndex, "1");
    assert_equals(getComputedStyle(b).backgroundColor, "rgb(0, 128, 0)");
  }, '@layer can be nested');
</script>