custom Twitter button function

Twitter Button CSS

#custom-tweet-button {
	width: 60px;
}
#custom-tweet-button a {
  background: url('http://a4.twimg.com/images/favicon.ico') 1px center no-repeat;
  border: 1px solid #ccc;
  color: #437792;
  display: block;
  padding: 2px 5px 2px 20px;
  text-decoration: none;
}

Twitter Button HTML

<div id="custom-tweet-button">
	<a href="#" id="tweet">Tweet</a>
</div>

Twitter Button Javascript

function tweetWindow(url) {
	window.open("http://twitter.com/share?url=http%3A%2F%2Fwww.google.com", "tweet", "location=0,toolbar=0,directories=0,scrollbars=0,status=0,menubar=0,resizable=0,width=600,height=500");
}

function window_onload() {
	var tweetbutton = document.getElementById("tweet");
	if (tweetbutton.addEventListener) {
		tweetbutton.addEventListener("click", tweetWindow, false);	
	} else if (tweetbutton.attachEvent) {
		tweetbutton.attachEvent('onclick', tweetWindow);
	}
}