css/css-values/ch-empty-pseudo-recalc-on-font-load.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>Length unit 'ch' used in pseudo elements without text should be recalculated after loading a web font</title>
<link rel="help" href="https://www.w3.org/TR/css-values-4/#font-relative-lengths">
<link rel="help" href="https://drafts.csswg.org/css-pseudo-4#generated-content">
<link rel="author" href="xiaochengh@chromium.org">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
.before::before,
.after::after,
.backdrop::backdrop {
  font: 25px/1 "custom font", monospace;
  background: linear-gradient(45deg, red, blue);
  background-size: 1ch 1ch;
}
</style>

<div class="container before"></div>
<div class="container after"></div>
<dialog class="container backdrop"></dialog>

<script>
function parseBackgroundSizeInPx(element, pseudoElement) {
  const x = getComputedStyle(element, pseudoElement).backgroundSize.split(' ')[0];
  if (!x.endsWith('px'))
    return NaN;
  return parseFloat(x);
}

const testCases = ['before', 'after', 'backdrop'];
const elements = testCases.map(testCase => document.querySelector('.' + testCase));
const asyncTests = testCases.map(
    testCase => async_test(`ch in pseudo-element ::${testCase} should be recalculated after loading a web font`));

// Before loading custom font, tests should be rendered with monospace
// fallback and have a '1ch' measurement much shorter than 25px.
for (let i = 0; i < testCases.length; ++i) {
  asyncTests[i].step(() => {
    const backgroundSizePx = parseBackgroundSizeInPx(elements[i], '::' + testCases[i]);
    assert_less_than(backgroundSizePx, 24);
  });
}

// Insert custom font into style sheet and load it
const customFont = new FontFace('custom font', 'url(/fonts/Ahem.ttf)');
document.fonts.add(customFont);

// After loading custom font, tests should be rendered with the custom font,
// which is Ahem, and have a '1ch' measurement that equals 25px.
customFont.load().then(
    () => {
      for (let i = 0; i < testCases.length; ++i) {
        asyncTests[i].step(() => {
          const backgroundSizePx = parseBackgroundSizeInPx(elements[i], '::' + testCases[i]);
          assert_equals(backgroundSizePx, 25);
          asyncTests[i].done();
        });
      }
    },
    () => {
      for (let i = 0; i < testCases.length; ++i) {
        asyncTests[i].step(() => {
          assert_unreached('Failed to load font');
        });
      }
    }
);
</script>