var tableStr = '<table>';
var n = 5;
var nn = 5;
var count = 0;
for (var i = 0; i < nn; i++) {
    tableStr += '<tr>';
    for (var j = 0; j < nn; j++) {
        if (i == count && j == count) {
            tableStr += '<td class="mainD">' + n + '</td>';
            count++;
            n--;
        } else
            tableStr += '<td>0</td>';

    }
    tableStr += '</tr>';
}
tableStr += '</table>';
document.getElementById('tables').innerHTML = tableStr;