<style>h1 { color: red; }</style>
Hello, <slot>World</slot>!
const { content } = document.currentScript.previousElementSibling;
class MyComponent extends HTMLElement {
super().attachShadow({ mode: 'open' }).append(content.cloneNode(true));
customElements.define('my-component', MyComponent);
<my-component>Web components</my-component>