mozilla-release/testing/web-platform/tests/custom-elements/parser/parser-constructs-custom-element-synchronously.html

<!DOCTYPE html>
<html>
<head>
<title>Custom Elements: Changes to the HTML parser</title>
<meta name="author" title="Ryosuke Niwa" href="mailto:rniwa@webkit.org">
<meta name="assert" content="HTML parser must construct a custom element synchronously">
<link rel="help" href="https://html.spec.whatwg.org/#create-an-element-for-the-token">
<link rel="help" href="https://dom.spec.whatwg.org/#concept-create-element">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
</head>
<body>
<div id="log"></div>
<script>

var childElementCountInConstructor;
var containerChildNodesInConstructor = [];
var containerNextSiblingInConstructor;
class MyCustomElement extends HTMLElement {
    constructor() {
        super();
        var container = document.getElementById('custom-element-container');
        for (var i = 0; i < container.childNodes.length; i++)
            containerChildNodesInConstructor.push(container.childNodes[i]);
        containerNextSiblingInConstructor = container.nextSibling;
    }
};
customElements.define('my-custom-element', MyCustomElement);

</script>
<div id="custom-element-container">
    <span id="custom-element-previous-element"></span>
    <my-custom-element></my-custom-element>
    <div id="custom-element-next-element"></div>
</div>
<script>

test(function () {
    var instance = document.querySelector('my-custom-element');

    assert_equals(containerChildNodesInConstructor.length, 3);
    assert_equals(containerChildNodesInConstructor[0], instance.parentNode.firstChild);
    assert_equals(containerChildNodesInConstructor[1], document.getElementById('custom-element-previous-element'));
    assert_equals(containerChildNodesInConstructor[2], instance.previousSibling);
    assert_equals(containerNextSiblingInConstructor, null);

}, 'HTML parser must only append nodes that appear before a custom element before instantiating the custom element');

</script>
</body>
</html>