Nested & Expandable Folders

Demo by Martin Ivanov utilizing hidden checkboxes and adjacent sibling combinators to set the "state" of each folder.

.css-treeview ul,
.css-treeview li
{
 padding: 0;
 margin: 0;
 list-style: none;
}

.css-treeview input
{
 position: absolute;
 opacity: 0;
}

.css-treeview
{
 font: normal 11px "Segoe UI", Arial, Sans-serif;
 -moz-user-select: none;
 -webkit-user-select: none;
 user-select: none;
}

.css-treeview a
{
 color: #00f;
 text-decoration: none;
}

.css-treeview a:hover
{
 text-decoration: underline;
}

.css-treeview input + label + ul
{
 margin: 0 0 0 22px;
}

.css-treeview input + label + ul
{
 display: none;
}

.css-treeview label,
.css-treeview label::before
{
 cursor: pointer;
}

.css-treeview input:disabled + label
{
 cursor: default;
 opacity: .6;
}

.css-treeview input:checked:not(:disabled) + label + ul
{
 display: block;
}

.css-treeview label,
.css-treeview label::before
{
 background: url("icons.png") no-repeat;
}

.css-treeview label,
.css-treeview a,
.css-treeview label::before
{
 display: inline-block;
 height: 16px;
 line-height: 16px;,
 vertical-align: middle;
}

.css-treeview label
{
 background-position: 18px 0;
}

.css-treeview label::before
{
 content: "";
 width: 16px;
 margin: 0 22px 0 0;
 vertical-align: middle;
 background-position: 0 -32px;
}

.css-treeview input:checked + label::before
{
 background-position: 0 -16px;
}

/* webkit adjacent element selector bugfix */
@media screen and (-webkit-min-device-pixel-ratio:0)
{
 .css-treeview
 {
  -webkit-animation: webkit-adjacent-element-selector-bugfix infinite 1s;
 }

 @-webkit-keyframes webkit-adjacent-element-selector-bugfix
 {
  from
  {
   padding: 0;
  }
  to
  {
   padding: 0;
  }
 }
}

<div class="css-treeview">
 <ul>
  <li><input type="checkbox" id="item-0" /><label for="item-0">This Folder is Closed By Default</label>
   <ul>
    <li><input type="checkbox" id="item-0-0" /><label for="item-0-0">Ooops! A Nested Folder</label>
     <ul>
      <li><input type="checkbox" id="item-0-0-0" /><label for="item-0-0-0">Look Ma - No Hands!</label>
       <ul>
        <li><a href="./">First Nested Item</a></li>
        <li><a href="./">Second Nested Item</a></li>
        <li><a href="./">Third Nested Item</a></li>
        <li><a href="./">Fourth Nested Item</a></li>
       </ul>
      </li>
      <li><a href="./">Item 1</a></li>
      <li><a href="./">Item 2</a></li>
      <li><a href="./">Item 3</a></li>
     </ul>
    </li>
    <li><input type="checkbox" id="item-0-1" /><label for="item-0-1">Yet Another One</label>
     <ul>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
     </ul>
    </li>
    <li><input type="checkbox" id="item-0-2" disabled="disabled" /><label for="item-0-2">Disabled Nested Items</label>
     <ul>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
     </ul>
    </li>
    <li><a href="./">item</a></li>
    <li><a href="./">item</a></li>
    <li><a href="./">item</a></li>
    <li><a href="./">item</a></li>
   </ul>
  </li>
  <li><input type="checkbox" id="item-1" checked="checked" /><label for="item-1">This One is Open by Default...</label>
   <ul>
    <li><input type="checkbox" id="item-1-0" /><label for="item-1-0">And Contains More Nested Items...</label>
     <ul>
      <li><a href="./">Look Ma - No Hands</a></li>
      <li><a href="./">Another Item</a></li>
      <li><a href="./">And Yet Another</a></li>
     </ul>
    </li>
    <li><a href="./">Lorem</a></li>
    <li><a href="./">Ipsum</a></li>
    <li><a href="./">Dolor</a></li>
    <li><a href="./">Sit Amet</a></li>
   </ul>
  </li>
  <li><input type="checkbox" id="item-2" /><label for="item-2">Can You Believe...</label>
   <ul>
    <li><input type="checkbox" id="item-2-0" /><label for="item-2-0">That This Treeview...</label>
     <ul>
      <li><input type="checkbox" id="item-2-2-0" /><label for="item-2-2-0">Does Not Use Any JavaScript...</label>
       <ul>
        <li><a href="./">But Relies Only</a></li>
        <li><a href="./">On the Power</a></li>
        <li><a href="./">Of CSS3</a></li>
       </ul>
      </li>
      <li><a href="./">Item 1</a></li>
      <li><a href="./">Item 2</a></li>
      <li><a href="./">Item 3</a></li>
     </ul>
    </li>
    <li><input type="checkbox" id="item-2-1" /><label for="item-2-1">This is a Folder With...</label>
     <ul>
      <li><a href="./">Some Nested Items...</a></li>
      <li><a href="./">Some Nested Items...</a></li>
      <li><a href="./">Some Nested Items...</a></li>
      <li><a href="./">Some Nested Items...</a></li>
      <li><a href="./">Some Nested Items...</a></li>
     </ul>
    </li>
    <li><input type="checkbox" id="item-2-2" disabled="disabled" /><label for="item-2-2">Disabled Nested Items</label>
     <ul>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
      <li><a href="./">item</a></li>
     </ul>
    </li>
   </ul>
  </li>
 </ul>
</div>