class StyledElement extends HTMLElement {
    constructor() {
        super();

        const shadow = this.attachShadow({ mode: "open" });

        const style = document.createElement("style");

        style.appendChild(document.createTextNode(`* { color: red; }`));

        shadow.appendChild(style);

        const div = document.createElement("div");

        shadow.appendChild(div);

        this.childNodes.forEach((childNode) => div.appendChild(childNode));
    }
}

customElements.define("styled-element", StyledElement);