Web components

Termonology

HTML templates: Fragments of HTML markup using <template> elements that won’t be rendered until they’re appended to the page with JavaScript.
Custom elements: Widely supported JavaScript APIs that let you create new DOM elements. Once you create and register a custom element using these APIs, you can use it similarly to a React component.
Shadow DOM: A smaller, encapsulated DOM that is isolated from the main DOM and rendered separately. Any styles and scripts you create for your custom components in the Shadow DOM will not affect other elements in the main DOM.
append: Accepts Node Objects/DOMStrings
appendChild: Accepts node objects only

Demo

Sample

<!--index.html-->
<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <link
      rel="stylesheet"
      href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.1/css/all.min.css"
      integrity="sha512-+4zCK9k+qNFUR5X+cKL9EIR+ZOhtIloNl9GIKS57V1MyNsYpYcUrUeQc9vNfzsWfV28IaLL3i96P9sdNyeRssA=="
      crossorigin="anonymous"
    />
    <link href="style.css" rel="stylesheet" type="text/css" />
    <script src="components/header.js" type="text/javascript" defer></script>
    <script src="components/footer.js" type="text/javascript" defer></script>
  </head>
  <body>
    <header-component></header-component>
    <main>
      <!-- Your page's content -->
    </main>
    <footer-component></footer-component>
  </body>
  <html></html>
</html>

<!--style.css-- > * {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

html,
body {
  height: 100%;
}

body {
  color: #333;
  font-family: sans-serif;
  display: flex;
  flex-direction: column;
}

main {
  flex: 1 0 auto;
}

// components/header.js
const headerTemplate = document.createElement("template");

headerTemplate.innerHTML = `
  <style>
    nav {
      height: 40px;
      display: flex;
      align-items: center;
      justify-content: center;
      background-color:  #0a0a23;
    }

    ul {
      padding: 0;
    }
    
    ul li {
      list-style: none;
      display: inline;
    }
    
    a {
      font-weight: 700;
      margin: 0 25px;
      color: #fff;
      text-decoration: none;
    }
    
    a:hover {
      padding-bottom: 5px;
      box-shadow: inset 0 -2px 0 0 #fff;
    }
  </style>
  <header>
    <nav>
      <ul>
        <li><a href="about.html">About</a></li>
        <li><a href="work.html">Work</a></li>
        <li><a href="contact.html">Contact</a></li>
      </ul>
    </nav>
  </header>
`;

class Header extends HTMLElement {
  constructor() {
    super();
  }

  connectedCallback() {
    const shadowRoot = this.attachShadow({ mode: "closed" });

    shadowRoot.appendChild(headerTemplate.content);
  }
}

customElements.define("header-component", Header);

// components/footer.js
const footerTemplate = document.createElement("template");

footerTemplate.innerHTML = `
  <style>
    footer {
      height: 60px;
      padding: 0 10px;
      list-style: none;
      display: flex;
      flex-shrink: 0;
      justify-content: space-between;
      align-items: center;
      background-color: #dfdfe2;
    }

    ul {
      padding: 0;
    }
    
    ul li {
      list-style: none;
      display: inline;
    }
    
    a {
      margin: 0 15px;
      color: inherit;
      text-decoration: none;
    }
    
    a:hover {
      padding-bottom: 5px;
      box-shadow: inset 0 -2px 0 0 #333;
    }
    
    .social-row {
      font-size: 20px;
    }
    
    .social-row li a {
      margin: 0 15px;
    }
  </style>
  <footer>
    <ul>
      <li><a href="about.html">About</a></li>
      <li><a href="work.html">Work</a></li>
      <li><a href="contact.html">Contact</a></li>
    </ul>
    <ul class="social-row">
      <li><a href="https://github.com/my-github-profile"><i class="fab fa-github"></i></a></li>
      <li><a href="https://twitter.com/my-twitter-profile"><i class="fab fa-twitter"></i></a></li>
      <li><a href="https://www.linkedin.com/in/my-linkedin-profile"><i class="fab fa-linkedin"></i></a></li>
    </ul>
  </footer>
`;

class Footer extends HTMLElement {
  constructor() {
    super();
  }

  connectedCallback() {
    const fontAwesome = document.querySelector('link[href*="font-awesome"]');
    const shadowRoot = this.attachShadow({ mode: "closed" });

    if (fontAwesome) {
      shadowRoot.appendChild(fontAwesome.cloneNode());
    }

    shadowRoot.appendChild(footerTemplate.content);
  }
}

customElements.define("footer-component", Footer);

Links

Mdn docs FreeCodeCamp article

Web components by Kris Koishigawa | FreeCodeCamp

Web components

Termonology

Demo

Sample

Links