css/css-values/lh-unit-003.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS Values and Units Test: lh depending on @font-face</title>
<link rel="help" href="https://drafts.csswg.org/css-values-4/#font-relative-lengths">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  #lh {
    font-family: customfont;
    font-size: 20px;
    width: 10lh;
  }
</style>
<div id="lh">
  X<br>
  X<br>
  X<br>
  X<br>
  X<br>
  X<br>
  X<br>
  X<br>
  X<br>
  X
</div>
<script>
  test(() => {
    assert_equals(lh.offsetWidth, lh.offsetHeight);
  }, "Line-height and lh before @font-face loads");

  let original_size = lh.offsetHeight;

  promise_test(async (t) => {
    let custom_font = new FontFace("customfont", "url(/css/css-fonts/support/fonts/Rochester.otf)");
    document.fonts.add(custom_font);
    await document.fonts.load("20px customfont");
    assert_not_equals(lh.offsetHeight, original_size,
                      "Test is useless if normal line-height is the same for both the web font and the fallback font");
    assert_equals(lh.offsetWidth, lh.offsetHeight,
                  "lh and normal line-height both affected");
  }, "Line-height and lh after @font-face loaded");
</script>