<div class="container">
  <div class="item">
    <div class="subitem">A</div>
    <div class="subitem">B</div>
  </div>
  <div class="item">2</div>
  <div class="item">3</div>
</div>
body {
  padding: 20px;
  font: 1em Helvetica Neue, Helvetica, Arial, sans-serif;
}

* {box-sizing: border-box;}

p {
  margin: 0 0 1em 0;
}

.container {
  width: 500px;
  border: 5px solid rgb(111,41,97);
  border-radius: .5em;
  padding: 10px;
  display: flex;
}

.item {
  flex: 1 1 200px;
  padding: 10px;
  background-color: rgba(111,41,97,.3);
  border: 2px solid rgba(111,41,97,.5);
}

.subitem {
  padding: 10px;
  background-color: rgba(193,225,237,.3);
  border: 2px solid rgba(193,225,237,.5);
}

.container .item:first-child {
  display: contents;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.