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

<!DOCTYPE html>
<title>Custom Functions: @function inside @media</title>
<link rel="help" href="https://drafts.csswg.org/css-mixins-1">
<link rel="help" href="https://drafts.csswg.org/css-conditional-3/#at-media">
<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>
    @media (width >= 50px) {
      @function --f() { result: 50; }
    }
    @media (width >= 100px) {
      @function --f() { result: 100; }
    }
    #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(), '50', '--actual before resize');
    iframe.style.width = '100px';
    assert_equals(actualValue(), '100', '--actual after resize');
    iframe.style.width = '';
    assert_equals(actualValue(), '50', '--actual after resizing back to original');
  });
</script>