chromium/src/third_party/blink/web_tests/external/wpt/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/custom-element.window.js

// The document open steps have:
//
// 2. If document's throw-on-dynamic-markup-insertion counter is greater than
//    0, then throw an "InvalidStateError" DOMException.
//
// The throw-on-dynamic-markup-insertion counter is only incremented when the
// parser creates a custom element, not when createElement is called. Test for
// this.
//
// See: https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#document-open-steps

const noError = Symbol("no error");
let err = noError;

class CustomElement extends HTMLElement {
  constructor() {
    super();
    try {
      assert_equals(document.open(), document);
    } catch (e) {
      err = e;
    }
  }
}
customElements.define("custom-element", CustomElement);

test(t => {
  err = noError;
  document.createElement("custom-element");
  assert_equals(err, noError);
}, "document.open() works in custom element constructor for createElement()");

test(t => {
  err = noError;
  document.write("<custom-element></custom-element>");
  assert_throws_dom("InvalidStateError", () => {
    throw err;
  });
}, "document.open() is forbidden in custom element constructor when creating element from parser");