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chromium/third_party/blink/web_tests/external/wpt/html/dom/elements/elements-in-the-dom/unknown-element.html 

<!DOCTYPE html>
<title>HTMLUnknownElement</title>
<link rel="author" title="Ms2ger" href="mailto:[email protected]">
<link rel="help" href="https://html.spec.whatwg.org/multipage/#htmlunknownelement">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<div id="log"></div>
<script>
[
  "applet",
  "attachment",
  "layer",
  "nolayer",
  "xxx"
].forEach(name => {
  test(() => {
    const elt = document.createElement("xxx");
    assert_true(window.HTMLUnknownElement && elt instanceof HTMLUnknownElement, "not an instance of HTMLUnknownElement");
    assert_true(window.HTMLSpanElement && !(elt instanceof HTMLSpanElement), "an instance of HTMLSpanElement");
  }, `<${name}> is an HTMLUnknownElement`);
});
</script>