target-blank

target-blank.html

<a href="document.html" rel="ext">в новое окно</a><br />
<script>
  function externalLinks() {
    if (!document.getElementsByTagName) return;
    var anchors = document.getElementsByTagName("a");
    for (var i=0; i < anchors.length; i++) {
      if (anchors[i].getAttribute("href") && anchors[i].getAttribute("rel") == "ext") {
        anchors[i].target = "_blank";
      }
    }
  }
  window.onload = externalLinks;
</script>
<a href="link" onclick="this.target='_blank'">способ 2</a><br />
<a href="link" onclick="return !window.open(this.href)">способ 3</a><br />