css/css-scoping/host-descendant-invalidation.html

<!doctype html>
<title>CSS Test: element style is correctly updated for rule with :host</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 id="slotted"></div></div>
<script>
test(function() {
  let root = host.attachShadow({ mode: "open" });
  root.innerHTML = `
    <style>
      :host ::slotted(div) { width: 100px; height: 100px; background: red; }
      :host ::slotted(.foo) { background: green; }
    </style>
    <slot></slot>
  `;
  assert_equals(getComputedStyle(slotted).backgroundColor, "rgb(255, 0, 0)");
  host.firstElementChild.classList.add('foo');
  assert_equals(getComputedStyle(slotted).backgroundColor, "rgb(0, 128, 0)");
});
</script>