JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://getfirebug.com/firebug-lite-debug.js"></script>
<script>
window.customElements.define('elmt-container', class extends HTMLElement {
	constructor() {
      super();
      const observer = new MutationObserver(changes => {
        	console.log(changes);
      })
    	observer.observe(this, {childList: true});
      console.log("elmt-container created");
	}

	connectedCallback() {
    console.log("elmt-container connected");
  }
});

window.customElements.define('elmt-test', class extends HTMLElement {

	constructor() {
      super();
      console.log("elmt-test created");
    }
   connectedCallback() {
    		console.log("elmt-test connected");
    }
});

</script>

<elmt-container>
        <div>1</div>
        <elmt-test>2</elmt-test>
</elmt-container>