With pure HTML you can't influence this - every modern browser (= the user) has complete control over this behavior because it has been misused a lot in the past...

HTML option

You can open a new window (HTML4) or a new browsing context (HTML5). Browsing context in modern browsers is mostly "new tab" instead of "new window". You have no influence on that, and you can't "force" modern browsers to open a new window.

In order to do this, use the anchor element's attribute target^[1]. The value you are looking for is _blank^[2].

<a href="www.example.com/example.html" target="_blank">link text</a>

JavaScript option

Forcing a new window is possible via javascript - see Ievgen's excellent answer below for a javascript solution.

(!) However, be aware, that opening windows via javascript (if not done in the onclick event from an anchor element) are subject to getting blocked by popup blockers!

[1] This attribute dates back to the times when browsers did not have tabs and using framesets was state of the art. In the meantime, the functionality of this attribute has slightly changed (see MDN Docu)

[2] There are some other values which do not make much sense anymore (because they were designed with framesets in mind) like _parent, _self or _top.

It shouldn't be your call to decide whether the link should open in a new tab or a new window, since ultimately this choice should be done by the settings of the user's browser. Some people like tabs; some like new windows.

Using _blank will tell the browser to use a new tab/window, depending on the user's browser configuration and how they click on the link (e.g. middle click, Ctrl+click, or normal click).

Additionally, some browsers don't have a tabs feature and therefore cannot open a link in a new tab, only in a new window.

This works in theory but it will depend on the preferences set in the browser. Now a days you can fake a new window by using div's and Layers. Is there anyway to implement a layer that hides what is behind it.

JavaScript:

function getElementsByClass( searchClass, domNode, tagName) { 
    if (domNode == null) domNode = document;
    if (tagName == null) tagName = '*';
    var el = new Array();
    var tags = domNode.getElementsByTagName(tagName);
    var tcl = " "+searchClass+" ";
    for(i=0,j=0; i<tags.length; i++) { 
    var test = " " + tags[i].className + " ";
    if (test.indexOf(tcl) != -1) 
        el[j++] = tags[i];
} 
return el;
}
// paste getElementsByClass function (see above) here 

function showtab(tabname) 
{ 
// hide every element with class 'box1'  
var tabs = getElementsByClass('box1');
for(i=0; i<tabs.length; i++) {
    tabs[i].style.display = 'none';
    //tabs[i].style.visibility = 'hidden';
}
// hide every element with class 'box1'      
var tabs2 = getElementsByClass('myStyle');
for(i=0; i<tabs2.length; i++) {
    tabs2[i].style.display = 'none';
    //tabs2[i].style.visibility = 'hidden';
}

document.getElementById(tabname).style.display='block';
//document.getElementById(tabname).style.visibility='visible';
// show element with given tabname 
} 

function showsubtab(tabname)
{
//hide every element with class 'myStyle'
var tabs = getElementsByClass('myStyle');
for(i=0; i<tabs.length; i++) {
    tabs[i].style.display = 'none';
    //tabs[i].style.visibility = 'hidden';
}
document.getElementById(tabname).style.display='block';
//document.getElementById(tabname).style.visibility='visible';
}

if (window.addEventListener) { // Mozilla, Netscape, Firefox
window.addEventListener('load', showmessage, false);
} else if (window.attachEvent) { // IE
window.attachEvent('onload', showmessage);
}

function showmessage() {
document.getElementById('box').style.display='block';
document.getElementById('div1').style.backgroundColor='grey';
document.getElementById('div1').style.opacity = 0.2;
document.documentElement.style.overflow = "hidden"; //firefox, chrome
document.body.scroll = "no"; // ie only
var bodyLayer = document.getElementById('div1');
DisableLinks(bodyLayer);
}
function hidemessage() {
document.getElementById('box').style.display='none';
document.getElementById('div1').style.backgroundColor='transparent';
document.getElementById('div1').style.opacity = 1.0;
document.documentElement.style.overflow = 'auto';  // firefox, chrome
    document.body.scroll = "yes"; // ie only
var bodyLayer = document.getElementById('div1');
EnableLinks(bodyLayer);
}

function DisableLinks(dom) {
if(undefined != dom){
    links=dom.getElementsByTagName('A');
} else {
    links=document.getElementsByTagName('A');
}

for(var i=0; i<links.length; i++) {
    links[i].style.pointerEvents="none";
}
}

function EnableLinks(dom) {
if(undefined != dom){
    links=dom.getElementsByTagName('A');
} else {
    links=document.getElementsByTagName('A');
}

for(var i=0; i<links.length; i++) {
    links[i].style.pointerEvents="auto";
}
}

HTML

<div id="box">
<section id="close">
<section id="title">Important Site Message</section>
<section id="button"><a href="#" onClick="hidemessage();">[X]</a>&nbsp;</section>
</section>
<!--Body of the Message-->
</div>

CSS #box {position: absolute; top: 50%; left: 50%; height: 15.625em; width: 25em; background-color:#FFF; margin-top: -7.8125em; margin-left: -12.5em; display: none; overflow: auto; border-color:#000; border-style:ridge; border-width:medium; z-index: 3; color: #000;} #close {border-bottom: inset thick #CCC; background-color: #000; width: inherit; height: 1.2em; color: #FFF; position: fixed;} #close a:visited {color: #FFF;} #close a:hover {color: red; text-decoration:none;} #close #title {text-align: center; font-weight:bold; width: 90%; padding: 1 1 1 1; clear: left; float: left; background-color:#000; color:#FFF;} #close #button {text-align: right; padding: 1 1 1 1; width: 10%; clear: right; float: right; background-color: #000; color: #FFF;}

Use one of these as per your requirements.

Open the linked document in a new window or tab:

 <a href="xyz.html" target="_blank"> Link </a>

Open the linked document in the same frame as it was clicked (this is default):

 <a href="xyz.html" target="_self"> Link </a>

Open the linked document in the parent frame:

 <a href="xyz.html" target="_parent"> Link </a>

Open the linked document in the full body of the window:

 <a href="xyz.html" target="_top"> Link </a>

Open the linked document in a named frame:

 <a href="xyz.html" target="framename"> Link </a>

See MDN

Since I fell into this old question and then found that it is now possible (maybe this css option wasn't available then), I just want to add an update on how it can be done:

<a href="[yourlink]" target="_blank" style="target-new: tab;">Google</a>

Here are the options for the target-new style:

target-new: window | tab | none 

Didn't test the none option, maybe it uses the default browser setting.

I confirmed this for Firefox and IE7-9.

Nothing an author can do can choose to open in a new tab instead of a new window; it is a user preference. (Note that the default user preference in most browsers is for new tabs, so a trivial test on a browser where that preference hasn't been changed will not demonstrate this.)

CSS3 proposed target-new, but the specification was abandoned.

The reverse is not true; by specifying certain window features for the window in the third argument of window.open(), you can trigger a new window when the preference is for tabs.