class HelloSayerComponent extends HTMLElement {
    connectedCallback() {
        const shadowRoot = this.attachShadow({mode: "open"});

        shadowRoot.innerHTML = `
            <style>
                div {
                    background-color: coral;
                    color: white;
                }
            </style>
            <div>Hello!</div>
        `;
    }
}

window.customElements.define("hello-sayer", HelloSayerComponent);