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

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

        this.root.innerHTML = `
            <style>
                span {
                    background-color: coral;
                    color: white;
                }
            </style>
            <span>Hello <slot></slot>!</span>
        `;
    }
}

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