Web Components sample

error-message.js

let template = document.getElementById("error-message-template"); 
// template is any HTML based content. Elements, styles, etc.

class ErrorMessage extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({ mode: "open" });
    this.shadowRoot.appendChild(template.content);
  }
}

window.customElements.define('error-message', ErrorMessage);