class HelloSayerComponent extends HTMLElement {
    static observedAttributes = ["name"];

    constructor() {
        super();

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

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

    attributeChangedCallback(name, oldValue, newValue) {
        this.root.querySelector("div").innerHTML = `Hello ${newValue}!`;
    }
}

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

const names = ["Joe", "Allie"];
let counter = 0;

setInterval(() => document.querySelector("hello-sayer").setAttribute("name", names[counter++ % 2]), 2000);