JS: Opening a random video, image or link in overlay

I am trying to make a random video, image or link open in an overlay using js and css, but I cannot get the overlay part correct.

<a id="overlay" href="javascript:openSite()">Click to go to a random site</a>
<script>
var links = [
              "google.com",
              "youtube.com",
              "stackoverflow.com",
              "apple.com"]

           var openSite = function() {
              // get a random number between 0 and the number of links
              var randIdx = Math.random() * links.length;
              // round it, so it can be used as array index
              randIdx = parseInt(randIdx, 10);
              // construct the link to be opened
              var link = 'http://' + links[randIdx];

     var win = window.open(link, '_blank');
     win.focus();

    };
</script>

And the css:

.overlay {
  position: fixed; /* Sit on top of the page content */
  display: none; /* Hidden by default */
  width: 100%; /* Full width (cover the whole page) */
  height: 100%; /* Full height (cover the whole page) */
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: rgba(0,0,0,0.5); /* Black background with opacity */
  z-index: 2; /* Specify a stack order in case you're using a different order for other elements */
  cursor: pointer; /* Add a pointer on hover */
}

Here is a jsfiddle