JSFiddle - React, Tailwind, and code Playground

HTML

<html lang="en">
<head>
    <meta charset="utf-8">
    <title>Memory Game</title>
    <meta name="description" content="">
    <link rel="stylesheet prefetch" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.6.1/css/font-awesome.min.css">
    <link rel="stylesheet prefetch" href="https://fonts.googleapis.com/css?family=Coda">
    <link rel="stylesheet" href="css/app.css">
</head>
<body>

    <div class="container">
        <header>
            <h1>Memory Game</h1>
        </header>

        <section class="score-panel">
        	<ul class="stars">
        		<li><i class="fa fa-star"></i></li>
        		<li><i class="fa fa-star"></i></li>
        		<li><i class="fa fa-star"></i></li>
        	</ul>

        	<span class="moves">0</span> Moves

          <div id="timer">00:00:00</div>

            <div class="restart">
              <button type="button" class="btn btn-info btn-sm">
        		<i class="fa fa-repeat"></i>
            </button>
        	</div>
        </section>

        <ul class="deck">
            <li class="card">
                <i class="fa fa-diamond"></i>
            </li>
            <li class="card">
                <i class="fa fa-paper-plane-o"></i>
            </li>
            <li class="card">
                <i class="fa fa-anchor"></i>
            </li>
            <li class="card">
                <i class="fa fa-bolt"></i>
            </li>
            <li class="card">
                <i class="fa fa-cube"></i>
            </li>
            <li class="card">
                <i class="fa fa-anchor"></i>
            </li>
            <li class="card">
                <i class="fa fa-leaf"></i>
            </li>
            <li class="card">
                <i class="fa fa-bicycle"></i>
            </li>
            <li class="card">
                <i class="fa fa-diamond"></i>
            </li>
            <li class="card">
                <i class="fa fa-bomb"></i>
            </li>
            <li class="card">
              ...

CSS

html {
    box-sizing: border-box;
}

*,
*::before,
*::after {
    box-sizing: inherit;
}

html,
body {
    width: 100%;
    height: 100%;
    margin: 0;
    padding: 0;
}

body {
    font-family: 'Coda', cursive;
}

.container {
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
}

h1 {
    font-family: 'Open Sans', sans-serif;
    font-weight: 300;
    margin-bottom: 20px;
}

/*
 * Styles for the deck of cards
 */

.deck {
    width: 660px;
    min-height: 680px;
    background: linear-gradient(160deg, #ddbf4f 0%, #d33a32 100%);
    padding: 32px;
    border-radius: 10px;
    box-shadow: 12px 15px 20px 0 rgba(46, 61, 73, 0.65);
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    align-items: center;
    margin: 0 0 3em;
    z-index: 2;
    pointer-events: inherit;
}

.deck .card {
    height: 125px;
    width: 125px;
    background: #2e3d49;
    font-size: 0;
    color: #ffffff;
    border-radius: 8px;
    cursor: pointer;
    display: flex;
    justify-content: center;
    align-items: center;
    box-shadow: 5px 2px 20px 0 rgba(46, 61, 73, 0.65);
    pointer-events: inherit;
}


.deck .card.open {
    transform: rotateY(180deg);
    transition: 0.3s;
    background: #2f91f9;
    cursor: default;
    pointer-events: none;
}

.deck .card.close {
	transform: rotateY(360deg);
	transition: 0.2s;
	transform-style: preserve-3d;
	background: #2e3d49;
}

.deck .card.show {
    font-size: 33px;
}

.deck .card.match {
    cursor: default;
    background: #1bc45f;
    font-size: 33px;
}

/*
 * Styles for the Score Panel
 */

.score-panel {
    text-align: left;
    width: 345px;
    margin-bottom: 10px;
    z-index: 2;
}

.score-panel .stars {
    margin: 0;
    padding: 0;
    display: inline-block;
    margin: 0 5px 0 0;
}

.score-panel .stars li {
    list-style: none;
    display: inline-block;
    color: #3572a8;
}

.score-panel .restart {
    float: right;
    cursor:...

JavaScript

let card1 = ''; // The former
let card2 = ''; // The latter
let card1Parent = '';
let card2Parent = '';
let ready = true;
let stopTimer = false;
let cardCounter = 0;

document.querySelector(".restart").addEventListener("click", restart);
document.querySelector(".deck").addEventListener("click", function() {stopTimer = false; timerStart()});
document.querySelector(".deck").addEventListener("click", cardOpen);
document.querySelector(".playAgain").addEventListener("click", function() {
  document.querySelector(".winPage").className = "winPage closed"; restart()});

// Unlocking clicked cards and comparing them

function cardOpen(evt) {
  if (evt.target.className == "card" && cardCounter != 2) {
	    evt.target.className += " open show";

    // Determines which card comes first in a unlocked pair of cards

    if (card1 == false) {
      card1 = evt.target.firstElementChild.className;
      card1Parent = evt.target;
      cardCounter = 1;
    } else {

      // Increasing the amount of moves

      document.querySelector(".moves").innerText = +document.querySelector(".moves").innerText + 1;

      // Rating system. Stars decrease depending on how many moves you've made

      if (document.querySelector(".moves").innerText == '16' || document.querySelector(".moves").innerText == '22') {
        document.querySelector(".fa-star").parentNode.removeChild(document.querySelector(".fa-star"));
      }

      card2 = evt.target.firstElementChild.className;
      card2Parent = evt.target;
      cardCounter = 2;

      // Card matching

      if (card1 == card2) {
        card1Parent.className = "card open show match";
        card2Parent.className = "card open show match";
        card1 = '';
        card2 = '';
        cardCounter = 0;
        win();
      } else {
        setTimeout(function () {
          evt.target.className = "card close"; card1Parent.className = "card close"}, 700);
        setTimeout(function () {
          evt.target.className = "card";...