The 'target' attribute is deprecated and should not be used. Here is a quick and easy way to replicate that behaviour with javascript. I got the idea - well, I guess I nicked the idea - from Kevin Yank's article on Sitepoint. My script is a little bit shorter and I've cut out the explanation.
HTML
<a rel="external" href="http://tinyurl.com/rdefj">Kevin Yank's article</a>
javascript
function external() {
var as=document.getElementsByTagName('a');
for (i=0; i<as.length; i++) {
if (as[i].getAttribute('rel')=='external') {
as[i].onclick=function() {
!window.open(this.href);
return false;
}
}
}
}
window.onload=external;