Should work on a modern browser (Firefox 4, Safari 5/Chrome (Webkit), IE9/8).
The HTML:
<ul id="nav">
<li class="active"><a href="#1">nav item</a></li>
<li><a href="#2">nav item2</a></li>
<li><a href="#3">nav item3</a></li>
<li><a href="#4">nav item3</a></li>
</ul>
The CSS:
#nav {
margin: 0; padding: 0;
display: block;
overflow: hidden;
}
#nav li {
float: left;
margin: 0; padding: 0 10px 10px;
position: relative;
}
#nav li a {
display: block;
}
#nav li.active a:after,
#nav li a:hover:after {
content: "";
display: block;
margin: 0 auto; width: 0;
border: 5px solid red;
border-color: transparent transparent red transparent;
}