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Test Info:

<!DOCTYPE html>
<title>Custom Elements: the HTML fragment parsing algorithm must not create a custom element synchronously</title>
<meta name="author" title="Rob Buis" href="">
<meta name="assert" content="The HTML fragment parsing algorithm must enqueue a custom element upgrade reaction instead of synchronously invoking its constructor">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/custom-elements-helpers.js"></script>
<div id="log"></div>
let iteration = 0;
document_types().forEach(function (entry) {
var documentName =;
var getDocument = entry.create;
let calls = 0;
promise_test(function () {
class Parenter extends HTMLElement {
connectedCallback() {
const child = this.firstChild;
class Child extends HTMLElement {
connectedCallback() { calls++; }
let parenter = 'x-parenter' + iteration;
let child = 'x-child' + iteration;
customElements.define(parenter, Parenter);
customElements.define(child, Child);
return getDocument().then(function (doc) {
document.documentElement.innerHTML = `<${parenter}><${child}></${child}></${parenter}>`;
assert_equals(calls, 1);
}, `Inserting a custom element into ${documentName} using HTML fragment parsing must enqueue a custom element upgrade reaction, not synchronously invoke its constructor`);