<div class="grid">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>
body {
  background: #000;
}

.grid {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-auto-rows: 100px;
  border-top: 1px solid transparent;
  border-left: 1px solid transparent;
  position: relative;
}

.grid:after {
  content: '';
  position: absolute;
  height: 100%;
  width: 1px;
  background: #000;
  top: 0;
  right: 0;
}

.grid:before {
  content: '';
  position: absolute;
  height: 1px;
  width: 100%;
  background: #000;
  bottom: 0;
  right: 0;
}

.item {
  border-right: 1px solid #fff;
  border-bottom: 1px solid #fff;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.