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CSS

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CSS Base

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JavaScript

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HTML 

              
                <div>

  <ul class="tree">

    <!-- Root node (Category) -->
    <li><div class="sticky">Animals</div>
      <ul>

        <!-- Sub category -->
        <li><div class="sticky">Birds</div></li> <!-- Second level node -->
        <li><div class="sticky">Mammals</div>
          <ul>

            <!-- Skill -->
            <li><div>Elephant</div>
              <ul>

                <!-- Attribute -->
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
              </ul>
            </li>
            <li><div>Mouse</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
              </ul>
            </li>
          </ul>
        </li>
        <li><div class="sticky">Reptiles</div></li>
        <li><div class="sticky">Birds</div></li>
        <li><div class="sticky">Mammals</div>
          <ul>
            <li><div>Elephant</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
            <li><div>Mouse</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
          </ul>
        </li>
        <li><div class="sticky">Reptiles</div></li>
      </ul>
    </li>

    <li><div class="sticky">Plants</div>
      <ul>
        <li><div class="sticky">Flowers</div>
          <ul>
            <li><div>Rose</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
              </ul>
            </li>
            <li><div>Tulip</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
              </ul>
            </li>
          </ul>
        </li>
        <li><div class="sticky">Trees</li>
        <li><div class="sticky">Flowers</div>
          <ul>
            <li><div>Rose</div></li>
            <li><div>Tulip</div>
            </li>
          </ul>
        </li>
        <li><div class="sticky">Trees</li>
      </ul>
    </li>

    <li><div class="sticky">Animals</div>
      <ul>

        <li><div class="sticky">Birds</div></li>
        <li><div class="sticky">Mammals</div>
          <ul>
            <li><div>Elephant</div></li>
            <li><div>Mouse</div></li>
          </ul>
        </li>
        <li><div class="sticky">Reptiles</div></li>
      </ul>
    </li>

    <li><div class="sticky">Plants</div>
      <ul>
        <li><div class="sticky">Flowers</div>
          <ul>
            <li><div>Rose</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
            <li><div>Tulip</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
          </ul>
        </li>
        <li><div class="sticky">Trees</li>
      </ul>
    </li>

    <li><div class="sticky">Animals</div>
      <ul>

        <li><div class="sticky">Birds</div></li>
        <li><div class="sticky">Mammals</div>
          <ul>
            <li><div>Elephant</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
            <li><div>Mouse</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
          </ul>
        </li>
        <li><div class="sticky">Reptiles</div></li>
      </ul>
    </li>

    <li><div class="sticky">Plants</div>
      <ul>
        <li><div class="sticky">Flowers</div>
          <ul>
            <li><div>Rose</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
            <li><div>Tulip</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
          </ul>
        </li>
        <li><div class="sticky">Trees</li>
      </ul>
    </li>

    <li><div class="sticky">Animals</div>
      <ul>

        <li><div class="sticky">Birds</div></li>
        <li><div class="sticky">Mammals</div>
          <ul>
            <li><div>Elephant</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
            <li><div>Mouse</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
          </ul>
        </li>
        <li><div class="sticky">Reptiles</div></li>
      </ul>
    </li>

    <li><div class="sticky">Plants</div>
      <ul>
        <li><div class="sticky">Flowers</div>
          <ul>
            <li><div>Rose</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
            <li><div>Tulip</div>
              <ul>
                <li><div>List item 1</div></li>
                <li><div>List item 2</div></li>
                <li><div>List item 3</div></li>
                <li><div>List item 4</div></li>
                <li><div>List item 5</div></li>
                <li><div>List item 6</div></li>
                <li><div>List item 7</div></li>
                <li><div>List item 8</div></li>
              </ul>
            </li>
          </ul>
        </li>
        <li><div class="sticky">Trees</li>
      </ul>
    </li>
  </ul>


</div>
!

CSS 

              
                // typography
// ----------
$font-stack: normal normal 13px/1.4 Segoe,"Segoe UI",Calibri,Helmet,FreeSans,Sans-Serif;

// colors
// ------
$background-color: white;     // all backgrounds
$connector-color: #369;       // connectors and text
$shadow-color: #8F949A;
$box-color: #afafaf;          // box border

// major dimensions
// ----------------
$tree-margin-left: 0px;

$entry-height: 3em;           // total hight of entry: card + gap on top

$card-ident: 2em;             // card horizontal ident (to nearest connector)
$card-height: 1.5em;
$card-width: 15em;            // 'auto' is acceptable
$card-padding-top: ($card-height - 1em)/2;
$card-padding-side: $card-padding-top * 2;

$hconnector-top: 1em;

// =================== Don't edit below this line ========================

$card-margin-top: $entry-height - $card-height; // card vertical gap
$connector-vident: ($entry-height)/2 + $card-margin-top - 1.5em;
$tree-ident: $card-width;     // connector horizontal ident (to card's front)

$background-color-presudo: $background-color;   //for dev: beige;
$box-shadow: 1px 1px 4px $shadow-color;
$width-presudo: 0px;

body {
  background: $background-color;
  font:$font-stack;
}

// All of the magic inside .tree
.tree {
  & ul {
    margin:0 0 0 $tree-ident; // whole tree indentation TODO:
    padding:0;
    list-style:none;
    color:$connector-color;
    position:relative;
  }
  &:before, ul:before {
    content:"";
    display:block;
    width:$width-presudo;
    position:absolute;
    top:0;
    bottom:0;
    left:0;
    border-left:1px solid; // vertical connectors
    background: $background-color-presudo; // same with body background
    z-index: -1;
  }
  & li {
    font-weight:bold;
    line-height: $entry-height;
    margin:0;
    padding: $card-margin-top 0 0 $card-ident; // indentation + .5em
    position:relative;
    z-index: -1;
    &:before {
      border-top:1px solid;
      content:"";
      display:block;
      height:100%;
      left:0;
      margin-top: $hconnector-top; // border top width
      position:absolute;
      top: $connector-vident; // (line-height/2)
      width:$card-ident; // same with indentation
    }
    &:last-child:before {
      background: $background-color-presudo; // same with body background
      bottom:0;
      height:auto;
      top: $connector-vident; // (line-height/2)
    }
  }
  & > li:before {
    border-top:none !important;
  }
  & li div {
    border-radius: 5px;
    border: 1px solid $box-color;
    margin: 0;
    max-width: $card-width;
    min-width: $card-width;
    padding: $card-padding-top $card-padding-side;

    background: $background-color;
    box-shadow: $box-shadow;
    color:$connector-color;
    font-weight:bold;
  }

}

// Espasially for root level ul
ul.tree {
  margin-left:$tree-margin-left; // Set ident for the root ul (whole tree ident)
  &:before {
    border-left:none; // Remove left border for the root ul
  }
}

// Put it to sticky divs
.sticky {
  position: -webkit-sticky;
  position: sticky;
  top: 0px;
  width: 100%;
}
!

JS
!
999px

Console