css/css-mixins/function-container-dynamic.html

<!DOCTYPE html>
<title>Custom Functions: @container responds to changes</title>
<link rel="help" href="https://drafts.csswg.org/css-mixins-1/#conditional-rules">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  #container {
    container-type: size;
    width: 50px;
    height: 50px;
  }
  @function --f() {
    result: A;
    @container (width = 100px) {
      result: B;
    }
    @container ((width >= 110px) and (width <= 140px)) {
      result: C;
    }
    @container (width = 150px) {
      result: D;
    }
  }
  #target {
    --actual: --f();
  }
</style>

<div id=container>
  <div id=target>
  </div>
</div>

<script>
  test(() => {
    let actualValue = () => getComputedStyle(target).getPropertyValue('--actual');

    assert_equals(actualValue(), 'A', '--actual before resize');

    // [<width of container>, <expected function result>]
    let data = [
      ['100px', 'B'],
      ['105px', 'A'],
      ['110px', 'C'],
      ['125px', 'C'],
      ['140px', 'C'],
      ['145px', 'A'],
      ['150px', 'D'],
      ['155px', 'A'],
    ];

    for (let d of data) {
      container.style.width = d[0];
      let expected = d[1];
      assert_equals(actualValue(), expected, `--actual after resize to ${d[0]}`);
    }
  });
</script>