css/css-animations/animation-css-variable-dependent-property.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS Animations: Dependent property updates correctly when animating a declared custom property</title>
<link rel="help" href="https://drafts.csswg.org/css-animations/">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="support/testcommon.js"></script>
<style>
@property --c {
  syntax: "<color>";
  inherits: true;
  initial-value: black;
}
@keyframes color-shift {
    0% {
        --c: black;
    }
    100% {
        --c: white;
    }
}
#target {
    color: var(--c);
    animation: color-shift 1s linear 1 forwards paused
}
</style>
<div id=target></div>
<div id="log"></div>
<script>

test(t => {
  const animation = target.getAnimations()[0];

  assert_equals(
    getComputedStyle(target).color,
    'rgb(0, 0, 0)'
  );

  animation.currentTime = 500;

  assert_equals(
    getComputedStyle(target).color,
    'rgb(128, 128, 128)'
  );

  animation.currentTime = 1000;

  assert_equals(
    getComputedStyle(target).color,
    'rgb(255, 255, 255)'
  );

}, 'Dependent property updates correctly');

</script>