main.js

// Create a class for the element
class PopUpInfo extends HTMLElement {
  constructor() {
    // Always call super first in constructor
    super();

    // Create a shadow root
    const shadow = this.attachShadow({mode: 'open'});

    // Create spans
    const wrapper = document.createElement('span');
    wrapper.setAttribute('class', 'wrapper');

    const icon = document.createElement('span');
    icon.setAttribute('class', 'icon');
    icon.setAttribute('tabindex', 0);

    const info = document.createElement('span');
    info.setAttribute('class', 'info');

    // Take attribute content and put it inside the info span
    const text = this.getAttribute('data-text');
    info.textContent = text;

    // Insert icon
    let imgUrl;
    if(this.hasAttribute('img')) {
      imgUrl = this.getAttribute('img');
    } else {
      imgUrl = 'img/default.png';
    }

    const img = document.createElement('img');
    img.src = imgUrl;
    icon.appendChild(img);

    // Apply external styles to the shadow dom
    const linkElem = document.createElement('link');
    linkElem.setAttribute('rel', 'stylesheet');
    linkElem.setAttribute('href', 'style.css');

    // Attach the created elements to the shadow dom
    shadow.appendChild(linkElem);
    shadow.appendChild(wrapper);
    wrapper.appendChild(icon);
    wrapper.appendChild(info);
  }
}

// Define the new element
customElements.define('popup-info', PopUpInfo);