css/cssom/getComputedStyle-root.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSSOM: :root selector parsed in getComputedStyle()</title>
<link rel="help" href="https://drafts.csswg.org/selectors/#root-pseudo">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">

<style>
  :root {
      --a: red;
  }
</style>

<script>
"use strict";

test(() => {
  const a = getComputedStyle(document.documentElement).getPropertyValue("--a");
  assert_equals(a, "red");

}, "Accessing variables in :root pseudo-selector");

</script>