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HTML

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About the <head>

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CSS

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CSS Base

About CSS Base

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JavaScript

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JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                
<body>
	  <!-- Summer is approaching, and the June #CodePenChallenge has just kicked off! So why not celebrate with a
  (smoothly) scrollable rainbow smoothie? The recipe is refreshingly easy: vibrant colors, healthy ingredients, positive vibes, and, of course, smooth scrolling! Plus, it's a great opportunity to experiment with Scrollspy :)
  | CodePen Challenge, June 2024, "Smooth Scrolling" -->
  <header id="header">
    <div class="page-title">
      <h1>Rainbow Smoothie</h1>
    </div>
  </header>
  <main>
    <nav id="navbar">
      <ul class="nav">
        <li class="nav-item">
          <a class="nav-link emoji" href="#purple" aria-label="blueberries">🫐</a>
        </li>
        <li class="nav-item">
          <a class="nav-link emoji" href="#green" aria-label="kiwi">🥝</a>
        </li>
        <li class="nav-item">
          <a class="nav-link emoji" href="#yellow" aria-label="banana">🍌</a>
        </li>
        <li class="nav-item">
          <a class="nav-link emoji" href="#orange" aria-label="orange">🍊</a>
        </li>
        <li class="nav-item">
          <a class="nav-link emoji" href="#red" aria-label="watermelon">🍉</a>
        </li>
      </ul>
    </nav>
    <div class="smoothie">
      <div id="straw"></div>
      <div data-bs-spy="scroll" data-bs-target="#navbar" tabindex="0" class="scrollspy glass">
        <div id="purple" class="layer">
          <div class="info">
            <h4>Blueberries</h4>
            <p>(rich in antioxidants)</p>
          </div>
        </div>
        <div id="green" class="layer">
          <div class="info">
            <h4>Kiwi</h4>
            <p>(source of fiber)</p>
          </div>
        </div>
        <div id="yellow" class="layer">
          <div class="info">
            <h4>Banana</h4>
            <p>(high in potassium)</p>
          </div>
        </div>
        <div id="orange" class="layer">
          <div class="info">
            <h4>Orange</h4>
            <p>(for Vitamin C)</p>
          </div>
        </div>
        <div id="red" class="layer">
          <div class="info">
            <h4>Watermelon</h4>
            <p>(high water content)</p>
          </div>
        </div>
      </div>
    </div>
    <div id="backtop">
      <a class="backtop-link emoji" href="#header" aria-label="backtop">👆</a>
    </div>
  </main>
  <footer id="footer">
    <div class="page-bottom">
      <p>Thanks for scrolling!</p>
      <p class="grey">#cpc-smooth-scrolling</p>
    </div>
  </footer>
</body>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css2?family=Bricolage+Grotesque:opsz,wght@12..96,200..800&display=swap');

*,
*::before,
*::after {
  box-sizing: border-box;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
}

:root {
  --item-width: 500px;
  --purple: #4a00f0;
  --green: #24b760;
  --yellow: #ecd200;
  --orange: #ff9900;
  --red: #f10000;
  --grey: #888;
  --title: 2.5rem;
  --text: 1rem;
  --emoji: 2rem;
}

@media only screen and (max-width: 600px) {
  :root {
    --item-width: 400px;
    --title: 2rem;
  }
}

@media only screen and (max-width: 500px) {
  :root {
    --item-width: 280px;
    --title: 1.5rem;
    --emoji: 1.5rem;
  }
}

html {
  width: 100%;
  height: 100%;
  font-family: "Bricolage Grotesque", sans-serif;
  font-optical-sizing: auto;
  font-style: normal;
}

@media (prefers-reduced-motion: no-preference) {
  html {
    scroll-behavior: smooth;
  }
}

body {
  background-color: #000;
  height: 100%;
  width: 100%;
  margin: 0;
  padding: 0;
  overflow-x: hidden;
  color: #fff;
  position: relative;
}

h1,
h2,
h3,
h4,
h5,
h6,
p {
  margin: 0;
}

h1 {
  font-size: var(--title);
}

p {
  font-size: var(--text);
}

a {
  color: #fff;
  font-size: var(--emoji);
  text-decoration: none;
  transition: all .15s ease;
}

a.emoji {
  font-size: var(--emoji);
  text-decoration: none;
}

a:hover,
a.active,
a:active,
a:focus {
  text-shadow: 0px 0px 10px #fff, 0px 0px 10px #fff;
}

.grey {
  color: var(--grey);
}

.page-title,
nav {
  width: var(--item-width);
  margin: auto;
  background: rgba(255, 255, 255, 0.2);
  backdrop-filter: blur(5px);
  -webkit-backdrop-filter: blur(5px);
  overflow: hidden;
  text-align: center;
}

.page-title {
  padding-top: 10px;
  border: .5px solid rgba(255, 255, 255, 0.3);
  border-top: 0;
  border-bottom: 0;
}

nav {
  position: -webkit-sticky;
  position: sticky;
  top: 0px;
  padding: 10px 0 15px;
  border: .5px solid rgba(255, 255, 255, 0.3);
  border-top: 0;
  border-radius: 0 0 100px 100px;
  z-index: 999;
}

nav ul {
  padding: 0;
  margin: 0;
  list-style: none;
  display: flex;
  justify-content: center;
  gap: 2rem;
}

footer {
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  align-items: center;
}

#backtop {
  display: none;
  opacity: 0;
  position: -webkit-sticky;
  position: sticky;
  bottom: 0px;
  width: var(--item-width);
  padding: 15px 0 10px;
  margin: auto;
  background: rgba(255, 255, 255, 0.2);
  border-radius: 100px 100px 0 0;
  backdrop-filter: blur(5px);
  -webkit-backdrop-filter: blur(5px);
  border: .5px solid rgba(255, 255, 255, 0.3);
  border-bottom: 0;
  text-align: center;
  z-index: 999;
}

#backtop.show {
  display: block;
  opacity: 1;
}

.page-bottom {
  text-align: center;
  width: var(--item-width);
  margin: 0 auto;
  padding-bottom: 15px;
  background: rgba(255, 255, 255, 0.2);
  border-radius: 0;
  backdrop-filter: blur(5px);
  -webkit-backdrop-filter: blur(5px);
  border: .5px solid rgba(255, 255, 255, 0.3);
  border-top: 0;
  border-bottom: 0;
}

.page-bottom p {
  margin: 0;
}

.smoothie {
  margin-top: 170px;
  margin-bottom: 200px;
}

@media only screen and (max-height: 500px) {
  .smoothie {
    margin-top: 40px;
  }
}

.smoothie,
.glass {
  position: relative;
}

.glass {
  width: calc(var(--item-width) + 20px);
  height: 100%;
  padding: 200px 10px 10px;
  margin: auto;
  z-index: 1;
}

.glass:after {
  content: "";
  position: absolute;
  width: 100%;
  height: 100%;
  left: 0;
  top: 0;
  background: linear-gradient(90deg,
      rgba(255, 255, 255, 0.5) 0%,
      rgba(255, 255, 255, 0.9) 10%,
      rgba(255, 255, 255, 0.5) 25%,
      rgba(255, 255, 255, 0.2) 40%,
      rgba(255, 255, 255, 0.2) 60%,
      rgba(255, 255, 255, 0.5) 75%,
      rgba(255, 255, 255, 0.9) 90%,
      rgba(255, 255, 255, 0.5) 100%);
  opacity: .5;
  border-radius: 0 0 100px 100px;
  border: .5px solid #fff;
}

.glass:before {
  content: "";
  position: absolute;
  width: calc(100% - 20px);
  height: calc(100% - 210px);
  left: 10px;
  bottom: 10px;
  filter: contrast(170%) brightness(100%);
  background-image: url('data:image/svg+xml,<svg viewBox="0 0 400 400" xmlns="http://www.w3.org/2000/svg"><filter id="noiseFilter"><feTurbulence type="fractalNoise" baseFrequency="4.02" numOctaves="4" stitchTiles="stitch"/></filter><rect width="100%" height="100%" filter="url(%23noiseFilter)"/></svg>');
  z-index: 1;
  opacity: .3;
  border-radius: 0 0 95px 95px;
  background-position: 0 0;
  transform: translateZ(0);
  animation: texture 10s linear infinite;
}

@media (prefers-reduced-motion: no-preference) {
  @keyframes texture {
    to {
      background-position: 0 -10%;
    }
  }
}

#straw {
  width: 24px;
  height: 400px;
  margin: auto;
  margin-bottom: -180px;
  transform: rotate(7deg) translate(150px, 5px);
  z-index: -2;
  background: linear-gradient(0deg, var(--red), var(--purple), var(--green), var(--yellow), var(--orange), var(--red));
  background-size: 200% 200%;
  background-position: 0% 0%;
  animation: rainbow 3s linear infinite;
}

@media (prefers-reduced-motion: no-preference) {
  @keyframes rainbow {
    to {
      background-position: 200% 200%;
    }
  }
}

#straw::after {
  content: "";
  position: absolute;
  width: 100%;
  height: 100%;
  left: 0;
  top: 0;
  background: linear-gradient(90deg, transparent, rgba(255, 255, 255, 0.5) 10%, white 30%, rgba(255, 255, 255, 0.5) 50%, transparent);
  opacity: .5;
}

@media only screen and (max-width: 600px) {
  #straw {
    transform: rotate(7deg) translate(100px, 15px);
  }
}

@media only screen and (max-width: 500px) {
  nav ul {
    gap: 1rem;
  }

  #straw {
    transform: rotate(7deg) translate(90px, 20px);
  }
}

.layer {
  position: relative;
  height: 100vh;
  margin-bottom: 30%;
  display: flex;
  justify-content: center;
  align-items: center;
  transition: all .3s ease;
}

.layer:last-child {
  margin-bottom: 0;
}

.layer .info {
  text-align: center;
  font-size: 2rem;
  margin-bottom: 20%;
  filter: blur(100px);
  transition: all 1s ease;
  z-index: 9;
}

.layer.active .info {
  filter: blur(0px);
  transition: all .4s ease;
}

.layer:after {
  content: "";
  position: absolute;
  width: 100%;
  height: 100%;
  padding-bottom: 30%;
  top: 0;
  left: 0;
  box-sizing: content-box;
}

#purple:after {
  background: linear-gradient(180deg, var(--purple) 70%, var(--green) 99%, var(--green));
}

#green:after {
  background: linear-gradient(180deg, var(--green) 70%, var(--yellow) 99%, var(--yellow));
}

#yellow:after {
  background: linear-gradient(180deg, var(--yellow) 70%, var(--orange) 99%, var(--orange));
}

#orange:after {
  background: linear-gradient(180deg, var(--orange) 70%, var(--red) 99%, var(--red));
}

#red:after {
  background: var(--red);
}

#red:after,
#red:before {
  padding-bottom: 0;
  border-radius: 0 0 95px 95px;
}
!

JS 

              
                console.clear()

var scrollSpy = new bootstrap.ScrollSpy(document.body, {
	target: '#navbar'
});

let navLinks = document.querySelectorAll('a.nav-link'); 
let sections = document.querySelectorAll('.layer');
const backtop = document.getElementById("backtop");

let sectionData = Array.from(sections).map(section => {
    return {
        element: section,
        id: section.getAttribute('id')
    };
});

const firstScrollSpyEl = document.querySelector('[data-bs-spy="scroll"]');
firstScrollSpyEl.addEventListener('activate.bs.scrollspy', (e) => {
    let activeSectionId = e.relatedTarget.getAttribute('href').substring(1);
    let activeSection = sectionData.find(section => section.id === activeSectionId);
    if (activeSection) {
        sectionData.forEach(section => section.element.classList.remove('active'));
        activeSection.element.classList.add('active');
    }
});

window.onscroll = function () {
	if (document.body.scrollTop > 50 || document.documentElement.scrollTop > 50) {
		backtop.classList.add("show");
	} else {
		backtop.classList.remove("show");
		sectionData.forEach(section => section.element.classList.remove('active'));
	}
}
!
999px

Console