a11y-nav.css

.a11y-nav {
  display: flex;
  justify-content: space-between;
  align-items: center;
  list-style: none;
  margin: 0;
  padding: 0;
}

/* Global/Reset Styles */

.a11y-nav ul {
  list-style: none;
  margin: 0;
  padding: 0;
}

.a11y-nav a,
.a11y-nav button {
  display: flex;
  justify-content: flex-start;
  align-items: center;
  border: none;
  background: none;
  padding: 0;
  font-size: 1em;
  cursor: pointer;
}

.a11y-nav a:focus,
.a11y-nav button:focus {
  outline: 3px solid dodgerblue;
}

/* Show arrow for dropdowns */

.a11y-nav button[aria-expanded]::after {
  content: "";
  display: block;
  border-bottom: 1px solid #000;
  border-right: 1px solid #000;
  height: 0.35em;
  width: 0.35em;
  margin-left: 0.5em;
  transform: rotate(45deg);
  flex: 0 0 auto;
}

.a11y-nav button[aria-expanded="true"]::after {
  transform: rotate(-135deg);
}

/* Hide/Show Menus */

.a11y-nav [aria-expanded][aria-controls] ~ [id] {
  display: none;
}

.a11y-nav [aria-expanded][aria-controls] ~ .a11y-nav-active {
  display: block;
}

.a11y-nav [aria-expanded][aria-controls] ~ .a11y-nav-animate-in {
  animation: a11y-nav-show 0.3s ease-in-out both;
}

.a11y-nav [aria-expanded][aria-controls] ~ .a11y-nav-animate-out {
  animation: a11y-nav-hide 0.3s ease-in-out both;
}

/* 1st Level Styling */

.a11y-nav > li {
  position: relative;
  display: flex;
  justify-content: center;
  flex: 1 1 auto;
}

/* 2nd Level Styling */

.a11y-nav li ul {
  position: absolute;
  top: calc(100% + 1em);
  left: 0;
  background: white;
  width: 100%;
  border: 1px solid black;
}

.a11y-nav li ul li {
  padding: 5px;
  justify-content: flex-start;
}

/* 3rd Level Styling */

.a11y-nav li ul ul {
  left: 100%;
  top: 0;
}

.a11y-nav li:last-child ul ul {
  left: auto;
  right: 100%;
}

@keyframes a11y-nav-show {
  from {
    opacity: 0;
    transform: translateY(5%);
  }

  to {
    opacity: 1;
    transform: translateY(0%);
  }
}

@keyframes a11y-nav-hide {
  from {
    opacity: 1;
    transform: translateY(0%);
  }

  to {
    opacity: 0;
    transform: translateY(5%);
  }
}