<article contenteditable="true">
  <p>Lorem ipsum dolor sit amet consectetur, adipisicing elit. Animi voluptatem consequatur veritatis architecto id illo itaque eos doloremque deleniti facilis esse, hic iusto aperiam possimus libero ex blanditiis. Ut, illo?</p>
  <p is="word-count"></p>
</article>

body {
  padding: 2rem;
}

article {
  padding: 0.5rem 1rem;
  border: 1px solid #ccc;
  border-radius: 5px;
}

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

    // count words in element's parent element
    var wcParent = this.parentNode;

    function countWords(node) {
      var text = node.innerText || node.textContent;
      return text.trim().split(/\s+/g).length;
    }

    var count = "Words: " + countWords(wcParent);

    // Create a shadow root
    var shadow = this.attachShadow({ mode: "closed" });

    // Create text node and add word count to it
    var text = document.createElement("span");
    text.textContent = count;

    // Append it to the shadow root
    shadow.appendChild(text);

    // Update count when element content changes
    setInterval(function () {
      var count = "Words: " + countWords(wcParent);
      text.textContent = count;
    }, 200);
  }
}

// Define the new element
customElements.define("word-count", WordCount, { extends: "p" });