+ 100px * sibling-index());
    }
    #test::before {
      content: "";
      z-index: calc(sibling-index() * 2);
      widows: calc(sibling-count() * 2);
    }
  </style>
</head>
<body>
  <div>
    <div></div>
    <div id="test"></div>
    <div></div>
    <div></div>
    <ul></ul>
  </div>
  <script>
test(() => {
  let style = getComputedStyle(document.getElementById('test'));
  assert_equals(style.zIndex, '2');
}, 'basic sibling-index() test');

test(() => {
  let style = getComputedStyle(document.getEle