<div class="container">
  <p class="item">1</p>
  <p class="item">2</p>
  <p class="item item--head">Head</p>
</div>
.container {
  display: grid;
  grid-template-columns: [head-start] 50% 50% [head-end];
  grid-template-rows: [head-start] auto [head-end] auto;  
}

.item--head {
  grid-area: head;
}

/* etc: */

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

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

.item--head {
  background: #b3e888;
  border-color: darkgreen;  
}

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

External CSS

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External JavaScript

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