[bes]

<button onclick="opacity(this, 2000)">click</button>
<script>
function opacity(element, speed){
	var op = 100;
		setInterval(function(){
			if(!element.style.opacity){
				element.style.opacity = 1;
			}
			if(!element.style.filter){
				element.style.filter = 'alpha(opacity = 100)';	
			}
			element.style.opacity -= 0.08;
			element.style.filter = 'alpha(opacity =' + op + ')';
			op = op - 8;// Как присвоить минус?
		}, speed / 30)
	}
</script>