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HTML 

              
                <div class="container">
<header>
    <nav class="menu-container">
      <div class="menu-toggler" id="menu-toggler">
<div class="hamburger"></div>
      </div>
      <ul class="menu-list" id="menu-list">
<li><a href="/01">Element 01</a>
    <ul class="submenu-container">
<li><a href="#">Element 011</a></li>
<li><a href="#">Element 012</a></li>
<li><a href="#">Element 013</a></li>
<li><a href="#">Element 014</a></li>
<li><a href="#">Element 015</a></li>
    </ul>
</li>
<li><a href="/02">Element 02</a>
    <ul class="submenu-container">
        <li><a href="#">Element 021</a></li>
        <li><a href="#">Element 022</a></li>
        <li><a href="#">Element 023</a></li>
        <li><a href="#">Element 024</a></li>
        <li><a href="#">Element 025</a></li>
    </ul>
</li>
<li><a href="/03">Element 03</a>
    <ul class="submenu-container">
        <li><a href="#">Element 031</a></li>
        <li><a href="#">Element 032</a></li>
        <li><a href="#">Element 033</a></li>
        <li><a href="#">Element 034</a></li>
        <li><a href="#">Element 035</a></li>
      </ul>
</li>
<li><a href="/04">Element 04</a>
    <ul class="submenu-container">
        <li><a href="#">Element 041</a></li>
        <li><a href="#">Element 042</a></li>
        <li><a href="#">Element 043</a></li>
        <li><a href="#">Element 044</a></li>
        <li><a href="#">Element 045</a></li>
      </ul>
</li>
<li><a href="/05">Element 05</a>
    <ul class="submenu-container">
        <li><a href="#">Element 051</a></li>
        <li><a href="#">Element 052</a></li>
        <li><a href="#">Element 053</a></li>
        <li><a href="#">Element 054</a></li>
        <li><a href="#">Element 055</a></li>
      </ul>
</li>
     </ul>
   </nav>
</header>

<main>
    <h3>About</h3>
    <ul>
        <li>Minimal, easy to understand code.</li>
        <li>Dropdown menu with Hamburger in mobile resolution.</li>
        <li>In desktop resolution works only CSS, JS action is blocked or impossible.</li>
        <li>In mobile resolution all action is only on click, because of JS.</li>
        <li>Hamburger menu works like accordion, expanding any submenu collapses previously expanded.</li>
        <li>Second click on menu link of expanded submenu collapses it.</li>
        <li>Collapsing Hamburger collapses expanded submenu.</li>
        <li>Both previous action can be easily turned off.</li>
        <li>Resizing from mobile resolution to desktop one collapses expanded submenu (it also can be turned off, but IMO it's nonsense).</li>
        <li>TODO: find graceful way of hiding Hamburger. Except for that as for 2022-05-14 this is final version.</li>
    </ul>
  
<p>Illustation to blogpost <a href="https://dygresje.info/blog/webdev-menu">Frontend - menu RWD z Hamburgerem</a></p>
</main>
</div>
!

CSS 

              
                .container {
  max-width: 1100px;
  margin: 0 auto;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

main {
  flex: 1;
}

ul {
  padding-left: 0;
  list-style-type: none;
  margin: 0;
}

.menu-list > li > a {
  pointer-events: none;
}

.menu-list > li:hover > a {
    background-color: #ccc;
}

/* # MENU # */

.menu-container {
  position: relative;
  height: 2rem;
}

.menu-list {
  display: flex;
  position: absolute;
  flex-direction: column;
  left: -100%;
  width: 100%;
  /* transition: all 0.3s linear; */
}

.menu-list li {
  flex: 1;
}

.menu-list a {
  display: block;
  text-align: center;
  text-decoration: none;
  height: 1.2rem;
  padding: 5px 0;
  user-select: none;
}

.menu-list a:hover {
  text-decoration: underline;
  background-color: beige;
}

.submenu-container {
  display: none;
}

.menu-toggler {
  display: flex;
  height: 100%;
  cursor: pointer;
  padding: 0 10vw;
  padding-left: 1rem;
  background-color: #666;
}

/* # HAMBURGER # */

.hamburger {
  background-color: #ccc;
  width: 30px;
  height: 3px;
  transition: all 0.3s linear;
  align-self: center;
  position: relative;
}

.hamburger::before,
.hamburger::after {
  width: 30px;
  height: 3px;
  background-color: #ccc;
  content: "";
  position: absolute;
  transition: all 0.3s linear;
}

.hamburger::before {
  top: -10px;
}

.hamburger::after {
  top: 10px;
}

/* # OPEN # */

.open .hamburger {
  transform: rotate(-45deg);
}

.open .hamburger::before {
  transform: rotate(-90deg) translate(-10px, 0px);
}

.open .hamburger::after {
  opacity: 0;
  transform: rotate(90deg);
}

/* MENU ACTION HERE */
.open ~ .menu-list {
  left: 0;
  transition: all 0.3s linear;
}

/* SUBMENU ACTION HERE */
.menu-list li .open {
  display: block;
}

@media (min-width: 800px) {
  .menu-toggler {
    display: none;
  }

  .menu-list {
    flex-direction: row;
    left: 0;
    /* transition: all 0s; */
  }

  .menu-list li:hover .submenu-container {
    display: block;
  }
}
!

JS 

              
                const toggler = document.getElementById("menu-toggler");
const sublistToggler = document.getElementById("menu-list");

const sublistToggle = (e) => {
    if (window.visualViewport.width<800)  {
    const clickedItemClasslist = e.path[0].children[1].classList
    // COLLAPSE IF EXPANDED
    if (clickedItemClasslist.contains('open')) {clickedItemClasslist.remove("open"); return;}
    // ACCORDION
    for (let i = 0; i < sublistToggler.children.length; i++) {sublistToggler.children[i].children[1].classList.remove("open")}
    // EXPAND
    clickedItemClasslist.toggle("open");
    }
  };

const listToggle = () => {
    toggler.classList.toggle("open");
    console.log("WARNING")
    for (let i = 0; i < sublistToggler.children.length; i++) {sublistToggler.children[i].children[1].classList.remove("open")}
}

sublistToggler.addEventListener("click", (e) => sublistToggle(e));
toggler.addEventListener("click", () => listToggle());

// CLOSE EXPANDED IF RESIZED UP
const closeExpanded = () => {
      if (window.visualViewport.width>799) {
        toggler.classList.remove("open");
    for (let i = 0; i < sublistToggler.children.length; i++) {sublistToggler.children[i].children[1].classList.remove("open")}
         }
   }
window.addEventListener('resize', closeExpanded);
!
999px

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