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Código HTML:
<html>
<head>
<title></title>
<style type="text/css">
*{
margin: 0;
padding: 0;
}
body{
background: darkgray;
}
nav{
text-align: center;
margin-top: 50px;
}
ul{
list-style-type: none;
}
li{
display: inline-block;
margin: 10px;
width: 150px;
background: gray;
}
li img{
height: 150px;
opacity: 0.7;
}
li img:hover{
opacity: 1;
}
</style>
</head>
<body>
<nav>
<ul>
<li><a href="#"><img src="http://www.masquewordpress.com/wp-content/uploads/2011/10/jquery-logo.png"></a></li>
<li><a href="#"><img src="http://www.w3.org/html/logo/downloads/HTML5_Logo_512.png"></a></li>
<li><a href="#"><img src="http://www.blog.pc-serveis.es/wp-content/uploads/2012/08/css3.png"></a></li>
<li><a href="#"><img src="http://2.bp.blogspot.com/_3SNLgcPiF9c/R-Fb024PZjI/AAAAAAAAAHk/lQ2SLmi49yo/s320/drupal.gif"></a></li>
</ul>
</nav>
</body>
</html>