css/css-scoping/host-dom-001.html

<!doctype html>
<title>CSS Test: :host in DOM APIs</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="help" href="https://drafts.csswg.org/css-scoping/#host-selector">
<div id="host"></div>
<script>
test(function() {
  let root = host.attachShadow({ mode: "open" });
  root.innerHTML = `<div></div>`;
  assert_false(host.matches(":host"), ":host shouldn't match from CSSOM from outside the shadow tree");
  assert_true(root.firstElementChild.matches(":host div"), ":host should match from within the shadow tree");
  assert_equals(root.querySelector(":host div"), root.firstElementChild, ":host should match from within the shadow tree");
})
</script>