firefox/testing/web-platform/tests/css/css-paint-api/one-custom-property-animation.https.html

<!DOCTYPE html>
<html class="reftest-wait">
<link rel="help" href="https://drafts.css-houdini.org/css-paint-api/">
<link rel="match" href="one-custom-property-animation-ref.html">
<style>
.container {
  width: 100px;
  height: 100px;
}
@keyframes expand {
  0% { --foo: 0; }
  100% { --foo: 100; }
}
.animate {
  background-image: paint(geometry);
  /* Use a long animation that start at 50% progress where the slope of the
     selected timing function is zero. By setting up the animation in this way,
     we accommodate lengthy delays in running the test without a potential drift
     in the animated property value. This is important for avoiding flakes,
     especially on debug builds. The screenshots are taken as soon as the
     animation is ready, thus the long animation duration has no bearing on
     the actual duration of the test. */
  animation: expand 1000000s cubic-bezier(0,1,1,0) -500000s;
}

#canvas-geometry {
  background-color: blue;
}
</style>

<script src="/common/reftest-wait.js"></script>
<script src="/web-animations/testcommon.js"></script>

<body>
<div id="canvas-geometry" class="container"></div>
<div id="log"></div>

<script id="code" type="text/worklet">
registerPaint('geometry', class {
  static get inputProperties() { return ['--foo']; }
  paint(ctx, geom, properties) {
    let fooValue = parseFloat(properties.get('--foo').toString());
    ctx.fillStyle = 'green';
    ctx.fillRect(0, 0, fooValue, fooValue);
  }
});
</script>

<script>
CSS.registerProperty({
  name: '--foo',
  syntax: '<number>',
  initialValue: '0',
  inherits: false
});

function logAssert(condition, message) {
  if(!condition) {
    document.getElementById("log").innerHTML = "<p>" + message + "</p>";
  }
}

window.onload = async() => {
  await waitForCompositorReady();

  const blob = new Blob([document.getElementById('code').textContent],
                         {type: 'text/javascript'});
  // Load the module and verify it was loaded.
  try {
    await CSS.paintWorklet.addModule(URL.createObjectURL(blob));
  } catch (error) {
    logAssert(false, 'The module was not loaded: ' + error.message)
  }

  // Check that custom property is registered.
  const style = getComputedStyle(document.documentElement);
  logAssert(style.getPropertyValue('--foo'),
            'Custom property is not registered');

  // Trigger CSS-animation.
  document.getElementById('canvas-geometry').classList.add('animate');
  const animations = document.getAnimations();
  // Verify we have an animation.
  logAssert(animations.length == 1, 'Animation was not added');

  // Force failure as a result of image mismatch instead of Timeout if
  // animation is not created
  if(animations.length == 1){
    await animations[0].ready;
  }
  await waitForAnimationFrames(3);
  takeScreenshot();
};


</script>
</body>
</html>