var myrules = {
	'.popup' : function(el){
		var href = el.getAttribute('href');
		if (href){
			el.onclick    = function(){ window.open(href); return false;}
			el.onkeypress = function(){ window.open(href); return false;}
		}
	}
};
Behaviour.register(myrules);

<script type="text/javascript" src="behaviour.js"></script>

<a href="index.html">Normal</a>
<a class="popup" href="index.html">Pop up</a>