example-web-component/wc1.html

<!DOCTYPE html>
<html dir="ltr" lang="en">
<head>
  <meta charset="utf-8">
  <title>Web components example</title>
  <meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=5.0">

  <script>
    // Step 1: Create a class that defined the behavior of our new element
    class ExampleElement extends HTMLElement {
      constructor() {
        super();
        this.name = 'Dolphin';
      }

      connectedCallback() {
        console.log('ExampleElement#connectedCallback - means the element was attached to the DOM');
        const newHeading = document.createElement('h2');
        newHeading.innerHTML = 'Hello ' + this.name;
        this.appendChild(newHeading)
      }

      disconnectedCallback() {
        console.log('ExampleElement#disconnectedCallback - means the element was removed from the DOM :(');
      }
    }
    // Step 2: Tell the browser to register that element with a specific tag name.
    window.customElements.define('hello-world', ExampleElement);
  </script>
</head>
<body>
  <hello-world></hello-world>
</body>
</html>