JSFiddle - React, Tailwind, and code Playground

HTML

<div id="tetris">
  <span id="start-btn">START</span>
</div>

CSS

#tetris {
  display: inline-block;
  width: 264px;
  height: 440px;
  border: 2px solid #000;
  line-height: 0px;
  background: #000;
}
#tetris b, #tetris i {
  display: inline-block;
  width: 20px;
  height: 20px;
  background: #000;
  border: 1px solid #333;
}
#tetris i {
  background: #fff;
  border-color: #999;
}
#start-btn {
  display: block;
  width: 30%;
  margin: 70% auto 0;
  padding: 20px;
  background: #eee;
  text-align: center;
  cursor: pointer;
}
#start-btn:hover {
  background: #fff;
}

JavaScript

var Tetris = {
  config: {
    pitchID: "tetris",
    freeBrick: "<b></b>",
    filledBrick: "<i></i>"
  },
  pitch: {
    width: 12,
    height: 20,
    bricks: []
  },
  startBtn: document.getElementById('start-btn')
};
for (var i = 0; i < Tetris.pitch.height; i++) {
  Tetris.pitch.bricks[i] = [];
  for (var j = 0; j < Tetris.pitch.width; j++) {
    Tetris.pitch.bricks[i][j] = 0;
  }
}
Tetris.startBtn.onclick = function () {
  var tetrisDom = document.getElementById(Tetris.config.pitchID);
  tetrisDom.innerHTML = '';
  for (var i = 0; i < Tetris.pitch.bricks.length; i++) {
    for (var j = 0; j < Tetris.pitch.bricks[i].length; j++) {
      tetrisDom.innerHTML += Tetris.pitch.bricks[i][j]
                           ? Tetris.config.filledBrick
                           : Tetris.config.freeBrick;
    }
  }
}