//使いかた
//<a rel="external nofollow" href="あどれす">


function externalLinks() {
	if (!document.getElementsByTagName) return;
	var anchors = document.getElementsByTagName("a");
	for (var i=0; i<anchors.length; i++) {
		var anchor = anchors[i];
		if (anchor.getAttribute("href")) {
			if (anchor.getAttribute("rel")) {
				var rels = anchor.getAttribute("rel").split(" ");
				for (var j=0; j<rels.length; j++) {
					if (rels[j] == "external") anchor.target = "_blank";
					}
				}
			}
		}
	}

window.onload = externalLinks;

//Event.observe('window', 'load', externalLinks, false);　←元のソースコードにあったけれど動かない
//window.onload = externalLinks;　←動く