<div class="container">
  <p class="column column-1">1</p>
  <p class="column column-2">2</p>
  <p class="column column-3">3</p>
  <p class="column column-4">4</p>    
  <p class="column column-5">5</p>    
  <p class="column column-6">6</p>    
</div>
.container {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(120px, 1fr));
  grid-auto-flow: dense;
  gap: 10px;
}

.column-3,
.column-5 {
  grid-column-end: span 2;
}

/* etc: */

body {
  margin: 1.5em;
  font-size: 130%;
}

.column {
  background: #b3e888;
  padding: 1em;
  margin-top: 0;
  margin-bottom: 0;
  font-family: monospace;
  text-align: center;
  border: 1px solid darkgreen;
  /* "border-collapse" hack */
  margin-left: -1px;
  margin-top: -1px;
}

.container {
  border: 5px solid black;
  padding: 1em;
  margin-bottom: 2em;  
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.