function td_hover () {
    
    var elements = document.getElementsByTagName('td');
	
    for (var i = 0; i < elements.length; i++) {
        var element = elements[i];
	
        element.onmouseover = function () {
            this.className = 'hover';
        }
	
        element.onmouseout = function () {
            this.className = '';
        }
    }
}

window.onload = function () {
    td_hover();
}