function linkDown(){
	var _links = document.getElementById('button');
	_links.onmousedown = function(){
		this.className += ' button-down';
	}
	_links.onmouseup = function(){
		this.className = this.className.replace('button-down','');
	}
}
if (window.addEventListener) window.addEventListener("load", linkDown, false);
else if (window.attachEvent && !window.opera) window.attachEvent("onload", linkDown);