css/css-properties-values-api/animation/custom-property-transition-inherited-used-by-standard-property.html

<!DOCTYPE html>
<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="../resources/utils.js"></script>
<div id="container">
    <div id="target"></div>
</div>
<script>

test(() => {
  const customProperty = "--my-length";

  CSS.registerProperty({
    name: customProperty,
    syntax: "<length>",
    inherits: true,
    initialValue: "100px"
  });

  target.style.marginLeft = `var(${customProperty})`;
  assert_equals(getComputedStyle(target).marginLeft, "100px");
  assert_equals(getComputedStyle(target).getPropertyValue(customProperty), "100px");

  container.style.transition = `${customProperty} 1s -500ms linear`;
  container.style.setProperty(customProperty, "200px");

  assert_equals(getComputedStyle(target).marginLeft, "150px");
}, "Running a transition an inherited CSS variable is reflected on a standard property using that variable as a value");

</script>