A valid way to open a link in a new window

Switching from an HTML transitional DOCTYPE to an XHTML strict DOCTYPE is fairly straightforward although there are quite a few elements and attributes not allowed in documents with Strict DOCTYPEs.

One of these is the target attribute so, if you decide that opening a link in a new window is appropriate, you'll need to use an alternative method.

First, we add the class "external" to any links that we want to open in a new window and also inform the user that this is going to happen by using the title attribute of the link.

<a href="http://www.url.com/" class="external" title="Explanatory text for link (link opens in a new window)">

Next, we use JavaScript to grab all links with the class "external" and open them in a new window:

Save the following code as "external_links.js"

<!--
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") &&
       anchor.className == "external")
   anchor.target = "_blank";
 }
}
window.onload = externalLinks;
//-->

Then we include the JavaScript file in the head of our documents:

<script type="text/javascript" src="external_links.js"></script>

To make users more aware that the link will be opening in a new window we will use CSS to add a background image to the link. I've used two 14px X 12 px gifs: New WIndow Icon and New WIndow Icon

CSS:

a.external {
padding-right:18px;background: url(new-window.gif) 100% 60% no-repeat;
}
a.external:focus, a.external:hover, a.external:active {
background: url(new-window-hover.gif) 100% 60% no-repeat;
}

A couple of points:
If JavaScript is disabled, the links will open in the same window as normal.  
You should only open links in new windows when absolutely necessary - it can be annoying!
Although this method works, it's really just a cheat to get pages to validate.

You can see this script in action if you look at the Links area to the right of this site.