A snippet to open links in a background tab. The idea is to trigger a mouse click event with control key pressed set as true, to open the clicked link in a background tab.

<a class="bgLink" href="http://www.example.com">Example</a>
<a class="bgLink" href="http://foo.example.com">Example Foo</a>

// js
window.onload = function () {
    //event listeners
    var a = document.querySelectorAll(".bgLink");
    if (a != null) {
        var len = a.length, i=0;
        if (len > 0) {
            for (i = 0; i < len; i++) {
                a[i].addEventListener("click", openNewBackgroundTab, false);
            }
        }
    }

    // event handlers
    // open links in background tabs
    function openNewBackgroundTab(e){
        var a = document.createElement("a"), evt;
        a.href = this.getAttribute('href');
        evt = document.createEvent("MouseEvents");    
        evt.initMouseEvent("click", true, true, window, 0, 0, 0, 0, 0, true, false, false, false, 0, null);
        a.dispatchEvent(evt);
        e.preventDefault();
    }
};

Reference: initMouseEvent

Update: Stopped working with Google Chrome dev release 41.0.2224.0 after 2014-11-20.