"collapsing" borders - cred to Jesper

by jonahe

HTML

<section>
  <div>
  1
  </div>
  <div>
  2
  </div>
  <div>
  3
  </div>
  <div>
  4
  </div>
  <div>
  5
  </div>
</section>

CSS

section {
  display: flex;
  width: 100%;
  flex-direction: row;
  flex-wrap: wrap;
}

section > div {
  display: flex-inline;
  width: calc(33.3%);
  height: 100px;

  outline: dashed 10px blue;
  outline-offset: -5px;
  box-sizing: border-box;
}