JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
	<li>1</li>
	<li>2</li>
	<li>3</li>
	<li>4</li>
	<li>5</li>
	<li>6</li>
</ul>

CSS

body {
  padding: 50px;
  background: white;
  text-align: center;
}

ul {
  max-width: 800px;
  margin: auto;
  background: grey;
  flex-wrap: wrap;
  position: relative;
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -webkit-flex;
  display: flex;
}

ul:before {
  content: "";
  border: 1px solid white;
  position: absolute;
  top: 0px;
  bottom: 0px;
  left: 0px;
  right: 0px;
}

li {
  padding: 100px 0px;
  background: white;
  margin: 0px 1px 1px 0px;
  -webkit-box-flex: 1 220px;
  -moz-box-flex: 1 220px;
  -webkit-flex: 1 220px;
  -ms-flex: 1 220px;
  flex: 1 220px;
}