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

<!DOCTYPE html>
<title>Custom Functions: @media 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>
  iframe {
    width: 50px;
    height: 50px;
  }
</style>
<iframe id="iframe" srcdoc="
  <div id=target></div>
  <style>
    @function --f() {
      result: A;
      @media (width = 100px) {
        result: B;
      }
      @media ((width >= 110px) and (width <= 140px)) {
        result: C;
      }
      @media (width = 150px) {
        result: D;
      }
    }
    #target {
      --actual: --f();
    }
  </style>
"></iframe>

<script>
  function waitForLoad(w) {
    return new Promise(resolve => w.addEventListener('load', resolve));
  }

  promise_test(async () => {
    await waitForLoad(window);
    const target = iframe.contentDocument.querySelector('#target');
    let actualValue = () => getComputedStyle(target).getPropertyValue('--actual');

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

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