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HTML 

              
                <div class="wrapper">
    <h1>dragons <span>vs</span> unicorns</h1>
    <div class="current-status" id="currentStatus">
      <img src="https://assets.codepen.io/2558758/unicorn.png" alt="" id="currentBeastImg">
      <p>'s turn</p>
    </div>
    <div class="board" id="board">
      <div class="cell" data-cell></div>
      <div class="cell" data-cell></div>
      <div class="cell" data-cell></div>
      <div class="cell" data-cell></div>
      <div class="cell" data-cell></div>
      <div class="cell" data-cell></div>
      <div class="cell" data-cell></div>
      <div class="cell" data-cell></div>
      <div class="cell" data-cell></div>
    </div>
    <div class="game-end-overlay" id="gameEndOverlay">
      <div class="winning-message" data-winning-message>
        <p></p>
      </div>
      <div class="btn-container">
        <button class="reset-button" id="resetButton">play again</button>
      </div>
    </div>
  </div>
!

CSS 

              
                /* --- Universal Box Sizing with Inheritance --- */
html {
  box-sizing: border-box;
}

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

$blue: #55acee;
$purple: #a186be;
$white: #f5f5f5;
$navy: #0d1021;

:root {
  --cell-size: 90px;
  --grid-gap: 12px;
  --h1-font-size: 32px;
  --h1-span-font-size: 24px;
  --current-status-font-size: 24px;
  --current-beast-img-height: 32px;
  --winning-beast-img-width: 100px;
  --winning-msg-font-size: 48px;
  --reset-btn-font-size: 30px;
}

@import url('https://fonts.googleapis.com/css2?family=Bungee+Inline&display=swap');

@mixin mq( $size ) {
  @if $size == tablet {
    @media ( min-width: 600px ) { @content; }
  } @else if $size == desktop {
    @media ( min-width: 1024px ) { @content; }
  }
}

@mixin pseduo-div( $w, $h ) {
  content: '';
  width: $w;
  height: $h;
  display: block;
  position: absolute;
  background-repeat: no-repeat;
}

@mixin align-center() {
  top: 50%;
  left: 50%;
  transform: translate3d( -50%, -50%, 0 );
}

@include mq( tablet ) {
  :root {
    --cell-size: 155px;
    --grid-gap: 15px;
    --h1-font-size: 56px;
    --h1-span-font-size: 40px;
    --current-status-font-size: 36px;
    --current-beast-img-height: 45px;
  }
}

@include mq( desktop ) {
  :root {
    --h1-font-size: 64px;
    --h1-span-font-size: 48px;
    --winning-msg-font-size: 72px;
    --reset-btn-font-size: 48px;
    --winning-beast-img-width: 150px;
  }
}

/* --- Styles --- */
body {
  margin: 0;
  padding: 0;
  background-image: url( 'https://raw.githubusercontent.com/anniebombanie/tic-tac-toe/master/img/fantasy-bg.jpg' );
  background-size: cover;
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  text-align: center;
  font-family: 'Bungee Inline', cursive;
  color: $white;
}

h1 {
  position: relative;
  max-width: 100%;
  margin: 15px auto 5%;
  text-align: center;
  font-size: var( --h1-font-size );
  text-shadow: 3px 3px $blue;

  &::before,
  &::after {
    @include pseduo-div( null, null );
    background-size: 100%;
    top: 50%;
    transform: translateY( -50% );
  }

  &::before {
    background-image: url( 'https://assets.codepen.io/2558758/fire.png' );
    left: 0;
    width: calc( 93px / 2.1 ) ;
    height: calc( 96px / 2.1 );

    @include mq( desktop ) {
      left: -64px;
    }
  }

  &::after {
    background-image: url( 'https://assets.codepen.io/2558758/stars.png' );
    right: 0;
    width: calc( 91px / 2 );
    height: calc( 88px / 2 );

    @include mq( desktop ) {
      right: -64px;
    }
  }

  span {
    display: block;
    font-size: var( --h1-span-font-size );

    @include mq( desktop ) {
      display: inline;
    }
  }
}

h1,
p {
  letter-spacing: 1px;
}

.current-status {
  display: flex;
  justify-content: center;
  align-items: center;
  margin-bottom: 25px;

  @include mq( desktop ) {
    margin-bottom: 35px;
  }

  p {
    margin: 0 5px 0 0;
    font-size: var( --current-status-font-size );
  }

  img {
    width: auto;
    height: var( --current-beast-img-height );
  }
}

.board {
  display: grid;
  grid-template-columns: repeat( 3, minmax( 90px, 1fr ));
  grid-template-rows: repeat( 3, minmax( 90px, 1fr ));
  grid-gap: var( --grid-gap );
  width: 100%;
  height: 100%;
  max-width: 495px;
  margin: 0 auto 15px;

  &.unicorn,
  &.dragon {
    .cell:not( .dragon ):not( .unicorn ):hover::before {
      @include pseduo-div( 70%, 70% );
      @include align-center();
      background-size: contain;
      opacity: 50%;
    }
  }

  &.unicorn {
    .cell:not( .dragon ):hover::before {
      background-image: url( 'https://assets.codepen.io/2558758/unicorn.png' );
    }
  }

  &.dragon {
    .cell:not( .unicorn ):hover::before {
      background-image: url( 'https://assets.codepen.io/2558758/dragon.png' );
    }
  }
}

.cell {
  cursor: pointer;
  position: relative;
  background-color: $white;
  width: var( --cell-size );
  height: var( --cell-size );
  opacity: 0.5;
  transition: opacity 0.2s ease-in-out;

  &:hover {
    opacity: 1;
  }

  &.dragon,
  &.unicorn {
    opacity: 1;
    position: relative;
    cursor: not-allowed;

    &::before {
      @include pseduo-div( 70%, 70% );
      @include align-center();
      background-size: contain;
    }
  }

  &.dragon::before {
    background-image: url( 'https://assets.codepen.io/2558758/dragon.png' );
  }

  &.unicorn::before {
    background-image: url( 'https://assets.codepen.io/2558758/unicorn.png' );
  }
}

.game-end-overlay {
  display: none;
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: $navy;

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

.winning-message {
  margin: -50px 0 20px;

  img {
    width: var( --winning-beast-img-width );
  }

  p {
    font-size: var( --winning-msg-font-size );
    margin: 0;
  }
}

.btn-container {
  position: relative;
}

.reset-button {
  color: $white;
  font-family: 'Bungee Inline', cursive;
  font-size: var( --reset-btn-font-size );
  white-space: nowrap;
  border: none;
  padding: 10px 20px;
  background-color: $purple;
  box-shadow: 5px 5px 0 $blue;
  cursor: pointer;
  transition: transform 0.1s ease-in-out;
  position: relative;

  &:hover {
    transform: scale( 1.2 );
  }

  &:active {
    top: 6px;
    left: 6px;
    box-shadow: none;
    background-color: darken( $purple, 5 );
  }
}
!

JS 

              
                // Twitter @anniebombanie_

// -- HTML elements --
const board = document.getElementById( 'board' );
const cells = document.querySelectorAll( '[data-cell]' );
const currentStatus = document.getElementById( 'currentStatus' );
const resetButton = document.getElementById( 'resetButton' );
const gameEndOverlay = document.getElementById( 'gameEndOverlay' );
const currentBeastStatusImg = document.getElementById( 'currentBeastImg' );
const winningMessage = document.querySelector( '[data-winning-message]' );
const winningMessageText = document.querySelector( '[data-winning-message] p' );
const winningMessageImg = document.createElement( 'img' );

// -- Game Variables --
let gameIsLive = true;
let unicornTurn = true;
let winner = null;
const winningCombinations = [
  [0, 1, 2],
  [3, 4, 5],
  [6, 7, 8],
  [0, 3, 6],
  [1, 4, 7],
  [2, 5, 8],
  [0, 4, 8],
  [2, 4, 6]
];

// -- Functions --
const setBoardHoverClass = () => {
  board.classList.remove( 'unicorn' );
  board.classList.remove( 'dragon' );

  if ( unicornTurn ) {
    board.classList.add( 'unicorn' );
  } else {
    board.classList.add( 'dragon' );
  }
}

const placeBeastImg = ( cell, currentBeast ) => {
  cell.classList.add( currentBeast );
}

const swapTurns = () => {
  unicornTurn = !unicornTurn;
}

const updateCurrentStatus = () => {
  if ( unicornTurn ) {
    currentBeastStatusImg.src = 'https://assets.codepen.io/2558758/unicorn.png';
    currentBeastStatusImg.alt = 'unicorn';
  } else {
    currentBeastStatusImg.src = 'https://assets.codepen.io/2558758/dragon.png';
    currentBeastStatusImg.alt = 'dragon';
  }
}

const checkWin = ( currentBeast ) => {
  return winningCombinations.some( combination => {
    return combination.every( i => {
      return cells[i].classList.contains( currentBeast );
    })
  });
}

const isDraw = () => {
  return [...cells].every( cell => {
    return cell.classList.contains( 'unicorn' ) || cell.classList.contains( 'dragon' );
  })
}

const startGame = () => {
  cells.forEach( cell => {
    winningMessageImg.remove();
    cell.classList.remove( 'unicorn' );
    cell.classList.remove( 'dragon' );
    cell.removeEventListener( 'click', handleCellClick );
    cell.addEventListener( 'click', handleCellClick, { once: true });
  });

  setBoardHoverClass();
  gameEndOverlay.classList.remove( 'show' );
}

const endGame = ( draw ) => {
  if ( draw ) {
    winningMessageText.innerText = `draw!`;
  } else {
    winningMessageImg.src = unicornTurn ? 'https://assets.codepen.io/2558758/unicorn.png' : 'https://assets.codepen.io/2558758/dragon.png';
    winningMessageImg.alt = unicornTurn ? 'unicorn' : 'dragon';
    winningMessage.insertBefore( winningMessageImg, winningMessageText );
    winningMessageText.innerText = `wins!!!`
  }

  gameEndOverlay.classList.add( 'show' );
}

// -- Event Handler --
const handleCellClick = ( e ) => {
  const cell = e.target;
  const currentBeast = unicornTurn ? 'unicorn' : 'dragon';

  placeBeastImg( cell, currentBeast );
  if ( checkWin( currentBeast )) {
    endGame( false );
  } else if ( isDraw()) {
    endGame( true );
  } else {
    swapTurns();
    updateCurrentStatus();
    setBoardHoverClass();
  }
}

// -- Event Listener --
resetButton.addEventListener( 'click', startGame );

// -- Start Game --
startGame();
!
999px

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