JSFiddle - React, Tailwind, and code Playground

HTML

<div class="board">
  <div class="item" id="item1-">1</div>
  <div class="item" id="item2-">2</div>
  <div class="item" id="item3-">3</div>
  <div class="item" id="item4-">4</div>
  <div class="item" id="item5-">5</div>
  <div class="item" id="item6-">6</div>
  <div class="item" id="item7-">7</div>
  <div class="item" id="item8-">8</div>
  <div class="item" id="item9-">9</div>
</div>

CSS

html, body {
  margin: 0;
}

.board {
  display: flex;
  flex-wrap: wrap;
}

.item {
  width: 33vmin;
  height: 33vmin;
  box-shadow: inset 0 0 0 1px #999999;
}