gistfile1.txt

<style>
.clients {
  li {
    width: 4.5em;
    margin: 0 .5em .5em 0;
    padding: 0;
    line-height: 1em;
    list-style: none;
    background-color: #ccc;
  }

  .column2 {
    margin-left: 5em;
  }

  .column3 {
    margin-left: 10em;
  }

  .break {
    margin-top: -4.5em;
  }
}
</style>

<ul class="clients">
  <li class="column1">a</li>
  <li class="column1">b</li>
  <li class="column1">c</li>
  <li class="column2 break">d</li>
  <li class="column2">e</li>
  <li class="column2">f</li>
  <li class="column3 break">g</li>
  <li class="column3">h</li>
  <li class="column3">i</li>
</ul>