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chromium/third_party/blink/web_tests/external/wpt/css/css-shadow-parts/host-part-001.html 

<!doctype html>
<title>CSS Shadow Parts - :host::part()</title>
<link rel="author" title="Mozilla" href="https://mozilla.org">
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:[email protected]">
<link rel="help" href="https://drafts.csswg.org/css-shadow-parts/#part">
<link rel="help" href="https://drafts.csswg.org/css-scoping/#host-selector">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1624968">
<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=980506">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<div id="host"></div>
<script>
test(function() {
  let host = document.getElementById("host");
  host.attachShadow({ mode: "open" }).innerHTML = `
    <style>
      :host::part(mypart) {
        color: lime;
      }
      :host(.tweak)::part(mypart) {
        color: blue;
      }
    </style>
    <div part="mypart"></div>
  `;

  let part = host.shadowRoot.querySelector("[part]");

  assert_equals(
    window.getComputedStyle(part).color,
    "rgb(0, 255, 0)",
    ":host::part() works",
  );

  host.classList.add("tweak");

  assert_equals(
    window.getComputedStyle(part).color,
    "rgb(0, 0, 255)",
    ":host::part() invalidation works properly",
  );
}, ":host::part works");
</script>