<script>
function relWindow(container, relval, atarget) {
	if (!document.getElementById) return;
	if (!document.getElementsByTagName) return;
	var section = (document.getElementById(container) ? document.getElementById(container) : 'document'); // check if container exists or use the whole document
	if ( !section.getElementsByTagName('a') ) return;
	var links = section.getElementsByTagName('a');
	for (var i = 0; i < links.length; i++) {
		var link = links[i];
		if (link.getAttribute('href') && link.getAttribute('rel') == relval) {
			var re = RegExp(relval, 'gi');
			if ( re.test( link.getAttribute('rel') ) ) {
				link.className = relval; // add a class with the rel value
				link.target = atarget;
				if ( !link.getAttribute('title') ) {
					link.setAttribute('title', 'link opens in another window');
				}
			}
		}
	}
}

relWindow('page-holder', 'external', 'outside');
</script>

<style>
.external { /* set by JavaScript on links with rel="external" */
	padding-right: 12px;
	background: url('external.png') right center no-repeat;
}
</style>

http://www.sitepoint.com/article/standards-compliant-world/

Based on the linked SitePoint article but with options to limit the container (eg body copy) and set the target. Use '_blank' for a new window for each link. Also adds a class (based on the rel value) for styling. Call the 'relWindow' on window load.

relWindow - New window based on 'rel' attr

class, window

JavaScript