index.html

<ul>
  <li class="big"></li>
  <li class="yellow"></li>
  <li class="pink"></li>
</ul>

KwrxrG.markdown

KwrxrG

A Pen by Captain Anonymous on CodePen.

License.

style.css

ul {
  margin: 0;
  padding: 0;
}

li {
  float: left;
  width: 33%;
  padding-bottom: 33%;
  list-style: none;
  background: blue;
}

li.big {
  width: 66%;
  padding-bottom: 66%;
  background: green;
}

li.pink {
  background: pink;
}