CodePen

HTML

            
              <h1>Find the Kitten!</h1>

<div class="container">
	<div tabindex=1></div>
	<div tabindex=2></div>
	<div tabindex=3></div>
	<div tabindex=4></div>
	<div tabindex=5></div>
	<div tabindex=6></div>
	<div tabindex=7></div>
	<div tabindex=8></div>
	<div tabindex=9></div>
	<div tabindex=10></div>
	<div tabindex=11></div>
	<div tabindex=12></div>
	<div tabindex=13></div>
	<div tabindex=14></div>
	<div tabindex=15></div>
	<div tabindex=16></div>
	<div tabindex=17></div>
	<div tabindex=18></div>
	<div tabindex=19></div>
	<div tabindex=20></div>
	<div tabindex=21></div>
	<div tabindex=22></div>
	<div tabindex=23></div>
	<div tabindex=24></div>
</div>
            
          
!

↑ Insert the most common viewport meta tag

CSS

            
              * {
  transition: all .3s linear;
  box-sizing: border-box;
}

h1 {
  font-family: Georgia, sans-serif;
  font-size: 44px;
  font-weight: normal;
  text-align: center;
  margin: 10px 0;
  font-style: italic;
  color: #444;
}

.container {
  overflow: hidden;
  text-align: center;
}

.container div {
  width: 150px;
  height: 150px;
  margin: 10px;
  background: goldenrod;
  cursor: pointer;
  display: inline-block;
}

:nth-child(1):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(2):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(3):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(4):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(5):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(6):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(7):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(8):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(9):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(10):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(11):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(12):focus {
  outline: none;
  background: url(http://placekitten.com/150/150);
}

:nth-child(13):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(14):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(15):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(16):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(17):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(18):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(19):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(20):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(21):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(22):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(23):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}

:nth-child(24):focus {
  outline: none;
  background: url(http://placeape.com/150/150);
}
            
          
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JS

            
              /*************************

Pure CSS "find game" using tabindex and :focus

See:

http://www.impressivewebs.com/using-tabindex-with-focus/

**************************/
            
          
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Must be a valid URL.
+ add another resource
via JS Hint
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