LayoutTests/fast/custom-elements/connected-callback.html

<!DOCTYPE html>
<body>
<script>
if (window.testRunner)
    testRunner.dumpAsText();
window.customElements.define('inner-element', class extends HTMLElement {
    connectedCallback() { result.textContent = 'PASS'; }
});
</script>
<p>This tests using <code>parseHTMLUnsafe</code> to create a declarative shadow DOM.<br>
connectedCallback should be called on a custom element inside the shadow DOM.</p>
<div id="result">FAIL</div>
<div id="container"></div>
<script>
container.append(...Document.parseHTMLUnsafe('<outer-element><template shadowrootmode="open"><inner-element></inner-element></template></outer-element>').childNodes);
</script>
</body>