Организация миниатюр в галерею с помощью CSS

HTML

<div>
   <ul>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>
    <li><a href="#"><img src="http://placehold.it/100x100"></a></li>    
  </ul>        
</div>

CSS

body {
  background: #eee;
} 
div {
  max-width: 600px;
  margin: 0 auto; 
} 
ul {
  margin: 0;
  padding: 0;  
  list-style-type: none;
} 
li img {
  float: left;
  margin: 10px;
  border: 5px solid #fff;
  width: 100px;
  height: 100px;
  -webkit-transition: box-shadow 0.5s ease;
  -moz-transition: box-shadow 0.5s ease;
  -o-transition: box-shadow 0.5s ease;
  -ms-transition: box-shadow 0.5s ease;
  transition: box-shadow 0.5s ease;
}
 
li img:hover {
  -webkit-box-shadow: 0px 0px 7px rgba(0,0,0,0.7);
  box-shadow: 0px 0px 7px rgba(0,0,0,0.7);
}