<header role="banner">
  <nav class="nav" role="navigation">
    <ul class="nav__list">
      <li>
        <input id="group-1" type="checkbox" hidden />
        <label for="group-1"><span class="fa fa-angle-right"></span> First level</label>
        <ul class="group-list">
          <li><a href="#">1st level item</a></li>
          <li>
            <input id="sub-group-1" type="checkbox" hidden />
            <label for="sub-group-1"><span class="fa fa-angle-right"></span> Second level</label>
            <ul class="sub-group-list">
              <li><a href="#">2nd level nav item</a></li>
              <li><a href="#">2nd level nav item</a></li>
              <li><a href="#">2nd level nav item</a></li>
              <li>
                <input id="sub-sub-group-1" type="checkbox" hidden />
                <label for="sub-sub-group-1"><span class="fa fa-angle-right"></span> Third level</label>
                <ul class="sub-sub-group-list">
                  <li><a href="#">3rd level nav item</a></li>
                  <li><a href="#">3rd level nav item</a></li>
                  <li><a href="#">3rd level nav item</a></li>
                </ul>
              </li>
            </ul>
          </li>
        </ul>
      </li>
      <li>
        <input id="group-2" type="checkbox" hidden />
        <label for="group-2"><span class="fa fa-angle-right"></span> First level</label>
        <ul class="group-list">
          <li>
            <li><a href="#">1st level item</a></li>
            <li><a href="#">1st level item</a></li>
            <input id="sub-group-2" type="checkbox" hidden />
            <label for="sub-group-2"><span class="fa fa-angle-right"></span> Second level</label>
            <ul class="sub-group-list">
              <li><a href="#">2nd level nav item</a></li>
              <li><a href="#">2nd level nav item</a></li>
              <li>
                <input id="sub-sub-group-2" type="checkbox" hidden />
                <label for="sub-sub-group-2"><span class="fa fa-angle-right"></span> Third level</label>
                <ul class="sub-sub-group-list">
                  <li><a href="#">3rd level nav item</a></li>
                </ul>
              </li>
            </ul>
          </li>
        </ul>
      </li>
      <li>
        <input id="group-3" type="checkbox" hidden />
        <label for="group-3"><span class="fa fa-angle-right"></span> First level</label>
        <ul class="group-list">
          <li>
            <li><a href="#">1st level item</a></li>
            <li><a href="#">1st level item</a></li>
            <input id="sub-group-3" type="checkbox" hidden />
            <label for="sub-group-3"><span class="fa fa-angle-right"></span> Second level</label>
            <ul class="sub-group-list">
              <li><a href="#">2nd level nav item</a></li>
              <li><a href="#">2nd level nav item</a></li>
              <li><a href="#">2nd level nav item</a></li>
              <li>
                <input id="sub-sub-group-3" type="checkbox" hidden />
                <label for="sub-sub-group-3"><span class="fa fa-angle-right"></span> Third level</label>
                <ul class="sub-sub-group-list">
                  <li><a href="#">3rd level nav item</a></li>
                  <li><a href="#">3rd level nav item</a></li>
                  <li><a href="#">3rd level nav item</a></li>
                </ul>
              </li>
            </ul>
          </li>
        </ul>
      </li>
      <li>
        <input id="group-4" type="checkbox" hidden />
        <label for="group-4"><span class="fa fa-angle-right"></span> First level</label>
        <ul class="group-list">
          <li>
            <li><a href="#">1st level item</a></li>
            <input id="sub-group-4" type="checkbox" hidden />
            <label for="sub-group-4"><span class="fa fa-angle-right"></span> Second level</label>
            <ul class="sub-group-list">
              <li><a href="#">2nd level nav item</a></li>
              <li><a href="#">2nd level nav item</a></li>
            </ul>
          </li>
        </ul>
      </li>
    </ul>
  </nav>
  <footer>
    <ul class="soc-media">
      <li><a href="https://twitter.com/alexdevero" target="_blank"><span class="fa fa-twitter"></span></a></li>
      <li><a href="https://www.facebook.com/deveroalex" target="_blank"><span class="fa fa-facebook"></span></a></li>
      <li><a href="https://plus.google.com/+AlexDevero" target="_blank"><span class="fa fa-google-plus"></span></a></li>
      <li><a href="http://blog.alexdevero.com" target="_blank"><span class="fa fa-globe"></span></a></li>
    </ul>
  </footer>
</header>
// Remy function
@function remy($value) {
  @return ($value / 16px) * 1rem;
}
body {
  margin-top: 2rem;
  font: 100% "Open sans", "Trebuchet MS", sans-serif;
}
a {text-decoration: none;}
/**
 * Hidden fallback
 */
[hidden] {
  display: none;
  visibility: hidden;
}
/**
 * Styling navigation
 */
header {
  margin-right: auto;
  margin-left: auto;
  max-width: remy(360px);
  box-shadow: 0 3px 12px rgba(0,0,0,.25);
}
/**
 * Styling top level items
 */
.nav a,
.nav label {
  display: block;
  padding: .85rem;
  color: #fff;
  background-color: #151515;
  box-shadow: inset 0 -1px lighten(#151515, 3%);
  transition: all .25s ease-in;
  &:focus,
  &:hover {
    color: rgba(255,255,255,.5);
    background: darken(#151515, 7%);
  }
}
.nav label {cursor: pointer;}
/**
 * Styling first level lists items
 */
.group-list a,
.group-list label {
  padding-left: 2rem;
  background: #252525;
  box-shadow: inset 0 -1px lighten(#252525, 7%);
  &:focus,
  &:hover {background: darken(#252525, 7%);}
}
/**
 * Styling second level list items
 */
.sub-group-list a,
.sub-group-list label {
  padding-left: 4rem;
  background: #353535;
  box-shadow: inset 0 -1px lighten(#353535, 7%);
  &:focus,
  &:hover {background: darken(#353535, 7%);}
}
/**
 * Styling third level list items
 */
.sub-sub-group-list a,
.sub-sub-group-list label {
  padding-left: 6rem;
  background: #454545;
  box-shadow: inset 0 -1px lighten(#454545, 7%);
  &:focus,
  &:hover {background: darken(#454545, 7%);}
}
/**
 * Hide nested lists
 */
.group-list,
.sub-group-list,
.sub-sub-group-list {
  height: 100%;
  max-height: 0;
  overflow: hidden;
  transition: max-height .5s ease-in-out;
}
 
.nav__list input[type=checkbox]:checked + label + ul {
	/* reset the height when checkbox is checked */
  max-height: 1000px;
}
/**
 * Rotating chevron icon
 */
label > span {
  float: right;
  transition: transform .65s ease;
}
.nav__list input[type=checkbox]:checked + label > span {transform: rotate(90deg);}
/**
 * Styling footer
 */
footer {
  padding-top: 1rem;
  padding-bottom: 1rem;
  background-color: #050505;
}
.soc-media {
  display: flex;
  justify-content: center;
}
.soc-media li:nth-child(n+2) {margin-left: 1rem;}
.soc-media a {
  font-size: 1.25rem;
  color: rgba(255,255,255,.65);
  transition: color .25s ease-in;
  &:focus,
  &:hover {color: rgba(255,255,255,.2);}
}

External CSS

  1. https://maxcdn.bootstrapcdn.com/font-awesome/4.4.0/css/font-awesome.min.css
  2. https://fonts.googleapis.com/css?family=Open+Sans:400,700,300

External JavaScript

This Pen doesn't use any external JavaScript resources.