css/cssom/getComputedStyle-pseudo-shadow.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSSOM: getComputedStyle() returns empty declarations for Shadow DOM pseudo-elements</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>
<link rel="help" href="https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle">

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

<script>
"use strict";

for (const pseudo of ["::part(foo)", "::slotted(foo)"]) {
  test(() => {
    assert_equals(getComputedStyle(document.getElementById("test"), pseudo).length, 0);
  }, `getComputedStyle() returns an empty declaration for ${pseudo}`);
}
</script>