function alternate(container, element, elementClass) {	
	if (!document.getElementById) return;
	var ol=document.getElementsByTagName(container);
	var i, x; 
	for (i=0; i<ol.length; i++) {
		lis=ol[i].getElementsByTagName(element);	
		for (x=0; x<lis.length; x++) {
			if (x%2 != 0) { 
				lis[x].setAttribute('class', elementClass);
				lis[x].setAttribute('className', elementClass);
			} 
		}	
	}
}

window.onload=function() { alternate('table', 'tr', 'even'); }