Custom elements works for Safari but not for Firefox and Chrome

I'm sure I'm missing something fundamental from the spec, but after building a large set of custom elements on the Mac running on Safari, I find they don't work on Firefox and Chrome. What am I missing?

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>NoCE</title>
        <script>
            class NoCE extends HTMLElement {
                constructor(args) {
                    super();
                }

                connectedCallback() {
                    this.innerHTML = "<p>It Works</p>";
                }

                attributeChangedCallback(name, oldValue, newValue, namespaceURI) {}

                disconnectedCallback() {}

                adoptedCallback() {}

                static get observedAttributes() { return []; }
            }

            customElements.define("no-ce", NoCE, { extends: "div" });
        </script>
    </head>
    <body>
        <no-ce>
            No custom elements
        </no-ce>
    </body>
</html>

In Safari the page shows "It Works". In Firefox and Chrome it shows "No Custom Elements" (running on Mac OS X.)

Safari 12.0.2 Firefox 64.0.2 Chrome 71.0.3578.98