css/css-fonts/font-display/font-display-change.html

<!DOCTYPE html>
<html class="reftest-wait">
<title>Updating font-display value while loading</title>
<link rel="help" href="https://drafts.csswg.org/css-fonts-4/#font-display-desc">
<link rel="match" href="font-display-change-ref.html">
<script>

window.onload = () => {
    const displayValues = [ 'auto', 'block', 'swap', 'fallback', 'optional' ];

    let updateFuncs = [];

    for (let initialValue of displayValues) {
        for (let targetValue of displayValues) {
            let face = new FontFace(initialValue + '-' + targetValue,
                                    'url("/fonts/Ahem.ttf?pipe=trickle(d5)")',
                                    {display: initialValue});
            document.fonts.add(face);
            face.load();
            updateFuncs.push(() => {
                face.display = targetValue;
            });
        }
    }

    setTimeout(() => {
        for (let update of updateFuncs) {
            update();
        }
        document.documentElement.classList.remove("reftest-wait");
    }, 200);
};

</script>
<table>
  <tr>
    <th>auto</th>
    <th>block</th>
    <th>swap</th>
    <th>fallback</th>
    <th>optional</th>
  </tr>
  <tr>
    <td>from auto</td>
    <td style="font-family: auto-auto">a</td>
    <td style="font-family: auto-block">a</td>
    <td style="font-family: auto-swap">a</td>
    <td style="font-family: auto-fallback">a</td>
    <td style="font-family: auto-optional">a</td>
  </tr>
  <tr>
    <td>from block</td>
    <td style="font-family: block-auto">a</td>
    <td style="font-family: block-block">a</td>
    <td style="font-family: block-swap">a</td>
    <td style="font-family: block-fallback">a</td>
    <td style="font-family: block-optional">a</td>
  </tr>
  <tr>
    <td>from swap</td>
    <td style="font-family: swap-auto">a</td>
    <td style="font-family: swap-block">a</td>
    <td style="font-family: swap-swap">a</td>
    <td style="font-family: swap-fallback">a</td>
    <td style="font-family: swap-optional">a</td>
  </tr>
  <tr>
    <td>from fallback</td>
    <td style="font-family: fallback-auto">a</td>
    <td style="font-family: fallback-block">a</td>
    <td style="font-family: fallback-swap">a</td>
    <td style="font-family: fallback-fallback">a</td>
    <td style="font-family: fallback-optional">a</td>
  </tr>
  <tr>
    <td>from optional</td>
    <td style="font-family: optional-auto">a</td>
    <td style="font-family: optional-block">a</td>
    <td style="font-family: optional-swap">a</td>
    <td style="font-family: optional-fallback">a</td>
    <td style="font-family: optional-optional">a</td>
  </tr>
</table>
</html>