hello-world.html

<template id="hello-world">
    <p>Hello World</p>
</template>
<script>
    const templateHelloWorld = (document.currentScript || document._currentScript)
      .ownerDocument.querySelector('#hello-world').content;
    
    class HelloWorld extends HTMLElement {
        connectedCallback() {
            let shadowRoot = this.attachShadow({mode: 'open'});
            const content = document.importNode(templateHelloWorld, true);
            this.shadowRoot.appendChild(content);
        }
    };
    window.customElements.define('hello-world', HelloWorld); 
</script>