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CSS

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HTML 

              
                <!-- Note works with or without JS -->
<nav id='accordion' class='dropdown'>
  <input
    type='checkbox'
    id='toggle-01'
    class='toggle burger-toggle'
    hidden
  >
  <label for='toggle-01' class='toggle-button'>
    <div class='burger'>
      <div class='line1'></div>
      <div class='line2'></div>
      <div class='line3'></div>
    </div>
  </label>
  <ul class='dropdown-menu lvl-1'>
    <li class='item'><a><i class='bi bi-house-fill'></i> Home</a></li>
    <li class='item'><a>About</a></li>
    <li class='dropdown'>
      <input type='checkbox' id='toggle-02' class='toggle' hidden>
      <label for='toggle-02' class='toggle-button caret'><i class='bi bi-image'></i> Gallery </label>
      <ul class='dropdown-menu lvl-2'>
        <li class='dropdown'>
          <input type='checkbox' id='toggle-03' class='toggle' hidden>
          <label for='toggle-03' class='toggle-button caret'>3D Graphics </label>
          <ul class='dropdown-menu lvl-3'>
            <li class='item'><a>Characters</a></li>
            <li class='item'><a>Landscapes</a></li>
          </ul>
        </li>
        <li class='item'><a>2D Illustration</a></li>
      </ul>
    </li>
    <li class='dropdown'>
      <input type='checkbox' id='toggle-04' class='toggle' hidden>
      <label for='toggle-04' class='toggle-button caret'><i class='bi bi-code-slash'></i> Web Development </label>
      <ul class='dropdown-menu lvl-2'>
        <li class='item'><a>Articles</a></li>
        <li class='dropdown'>
          <input type='checkbox' id='toggle-05' class='toggle' hidden>
          <label for='toggle-05' class='toggle-button caret'>Projects </label>
          <ul class='dropdown-menu lvl-3'>
            <li class='item'><a>JS</a></li>
            <li class='item'><a>CSS</a></li>
          </ul>
        </li>
        <li class='item'><a>Tutorials</a></li>
      </ul>
    </li>
    <li class='item'><a><i class='bi bi-envelope-fill'></i> Contact</a></li>
  </ul>
</nav>
!

CSS 

              
                @import "https://assets.codepen.io/3351103/__reset.css";
@import url('https://fonts.googleapis.com/css2?family=Poppins:wght@200;300;400;500&display=swap');

:root {
  --nav-bg-color: rgb(21, 21, 21);
  --toggle-color: rgb(221, 221, 221);
  --toggle-bg-color: rgb(40 40 40);
  --nav-link-bg-color-01: rgb(50 50 50);
  --nav-link-bg-color-02: rgb(30 30 30);
  --nav-link-bg-color-03: rgb(15 15 15);
  --nav-link-color: hsl(0 0% 90%);
  --nav-secondary-color: rgb(255, 193, 7);
  --nav-caret-color: var(--nav-secondary-color);
  --nav-hover-color: hsl(0 0% 90%);
  --nav-bg-hover-color-01: rgb(209, 59, 45);
  
  --ff-body: 'Poppins', sans-serif;
  --ff-nav: 'Poppins', sans-serif;
  
  --fw-200: 200;
  --fw-300: 300;
  --fw-400: 400;
  --fw-500: 500;
  
  --fs-300: .9rem
  --fs-400: .95rem;
  --fs-500: 1.2rem;
  
  --calc-height: auto;
  --calc-transition-delay: 500ms; 
  --dropdown-indent: 0px;
  --burger-width: 40px;
  --mobile-break: 480px;
}

body {
  display: grid;
  justify-items: center;
  height: 100vh;
  padding: 1rem;
  font-family: var(--ff-body);
  font-weight: var(--fw-300);
  font-size: var(--fs-400);
  background-color: var(--nav-bg-color);
}

ul {
  list-style: none;
  margin: 0;
}

a {
  cursor: pointer;
}

#accordion {
  width: 100%;
  max-width: var(--mobile-break);
  height: fit-content;
  color: var(--nav-link-color);
  font-size: var(--fs-300);
  
  [hidden] {
    margin: 0;
  }
  
  a {
    display: block;
    padding: 0.75rem 0;
    font-family: var(--ff-nav);
    text-align: center;
    color: var(--nav-link-color);
    transition: color 150ms, background-color 150ms;
  }
  
  /* dropdown links level 1 */
  ul > li > a {
    background-color: var(--nav-link-bg-color-01);
  }

  /* dropdown links level 2 */
  ul ul > li > a {
    background-color: var(--nav-link-bg-color-02);
  }

  /* dropdown links level 3 */
  ul ul ul > li > a {
    background-color: var(--nav-link-bg-color-03);
  }
  
  ul > li > a:hover {
    color: var(--nav-hover-color);
    background-color: var(--nav-bg-hover-color-01);
  }
  
  .toggle-button {
    display: block;
    padding: 0.75rem 1rem;
    color: var(--toggle-color);
    background-color: var(--toggle-bg-color);
    cursor: pointer;
    user-select: none;
  }

  .dropdown-menu {
    position: relative;
    height: 0;
    overflow: hidden;
    padding-left: var(--dropdown-indent);
    transition: height var(--calc-transition-delay);
  }

  .toggle:checked ~ .dropdown-menu {
    height: var(--calc-height);
  }

  .caret:after {
    content: '\25BC';
    font-size: .9rem;
    line-height: 1;
    display: inline-block;
    color: var(--nav-caret-color);
    transform: rotatez(-90deg);
    transition: transform 150ms linear;
  }

  .toggle:checked + .caret:after {
    transform: rotatez(0deg);
  }
}

/* ---- burger toggle ----- */
nav {
  .burger {
    display: grid;
    place-content: center;
    width: var(--burger-width);
    height: calc(var(--burger-width) * .8);
    border: 2px solid var(--nav-secondary-color);
    border-radius: 4px;
    cursor: pointer;

    [class^='line'] {
      width: 26px;
      height: 2px;
      margin-bottom: 5px;
      background-color: var(--nav-secondary-color);
      transition: transform .2s ease-out;
    }
    
    .line3 {
      margin-bottom: 0px;
    }
  }
  
  .burger:hover {
    .line1{
      transform: translate(0px, 2px);
    }

    .line3 {
      transform: translate(0px, -2px);
    }
  }

  .burger-toggle:checked + .toggle-button {
    .burger {
      .line1 {
        transform: rotate(45deg) translate(5px, 5px);
      }

      .line2 {
        opacity: 0;
      }

      .line3 {
        transform: rotate(-45deg) translate(5px, -5px);
      }
    }
  }
}
/* ---- END burger toggle ----- */

/* ---- some custom styling ---- */
#accordion {
  --nav-link-bg-color-01: hsl(200 80% 13%);
  --nav-link-bg-color-02: hsl(200 80% 11%);
  --nav-link-bg-color-03: hsl(200 80% 9%);
  --nav-link-color: hsl(0 0% 90%);
  --nav-secondary-color: hsl(11 100% 65%);
  --nav-caret-color: hsl(11 100% 65%);
  --toggle-bg-color: hsl(200 80% 15%);
  
  --fs-300: .95rem;
  
  --burger-width: 36px;
  --mobile-break: 400px;
  
  .burger {
    margin: .15rem 0;
  }

  a {
    text-align: left;
  }
  
  .bi {
    margin-right: 5px;
  }
  
  .toggle:checked + .toggle-button .bi {
    color: var(--nav-secondary-color);
  }
  
  .lvl-1 a, .lvl-1 .toggle-button {
    padding-left: 1.5rem;
  }
  
  .lvl-2 a, .lvl-2 .toggle-button {
    padding-left: 2.5rem;
  }
  
  .lvl-3 a {
    padding-left: 3.5rem;
  }
}
!

JS 

              
                window.CP.PenTimer.MAX_TIME_IN_LOOP_WO_EXIT = 6000;

// searches next siblings for a target element
const findNextSibling = (elem, selector) => {
  while (elem !== null) {
    elem = elem.nextElementSibling
    if (elem.matches(selector)) return elem
  }
  return elem
}

// if unable to toggle between a and b or b and a
// will return false
const toggleBetween = (elem, a, b) => {
  return elem.classList.replace(a, b) || elem.classList.replace(b, a)
}

// A recursive function to set the total height of an open dropdown
// based on the heights of its child elements
const calcHeights = function (elems) {
  let height = 0
  
  // argument for first call will be a single root element
  // recursive calls will be children
  if (elems instanceof Element) elems = [elems]

  for (const elem of elems) {
    
    // only include calculations for opened dropdowns
    if (elem.matches('.dropdown, .open')) {
      const parentHeight = calcHeights(elem.children)

      if (elem.matches('.open')) {
        elem.style.setProperty('--calc-height', parentHeight + 'px')
      }

      height += parentHeight     
    } else {
      // getBoundingClientRect gives a more accurate height
      const elemHeight = elem.getBoundingClientRect().height
      
      height += (elem.matches('.closed')) ? 0 : elemHeight
    }
  }
  return height
}

const toggleMenu = (dropdown, rootElem) => { 
  if (!toggleBetween(dropdown, 'open', 'closed')) {
    // first time clicking on this dropdown
    dropdown.classList.add('open')
  }
  calcHeights(rootElem)
}

window.addEventListener('DOMContentLoaded', () => {
  const rootElem = document.querySelector('#accordion .dropdown-menu')
  
  // change --calc-height from auto to 0 for initial transition
  document
    .documentElement
    .style.setProperty('--calc-height', '0px')
  
  document
    .querySelectorAll('#accordion .toggle')
    .forEach((toggle) => {
      const dropdown = findNextSibling(toggle, '.dropdown-menu')
      
      toggle.addEventListener('click', (event) => {
        toggleMenu(dropdown, rootElem)
      })
    })
  
    document.querySelector('#toggle-01').click()
})
!
999px

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