css/cssom/getComputedStyle-live.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>getComputedStyle() returns live values</title>
<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<!-- regression test for https://github.com/jsdom/jsdom/issues/3562 -->

<div id="div"></div>

<script>
"use strict";

test(() => {
  const div = document.getElementById("div");
  const computedStyles = window.getComputedStyle(div);
  assert_equals(computedStyles.backgroundColor, "rgba(0, 0, 0, 0)", "before");

  div.style.setProperty("background-color", "red");
  assert_equals(computedStyles.backgroundColor, "rgb(255, 0, 0)", "after");
});
</script>